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385 lines
22 KiB
385 lines
22 KiB
\section{Literature Review}
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% To better understand which metrics and methods are meaningful in the domain of keyboards and especially when
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% To investigate whether or not solely the actuation force of individual keys can make a difference in terms of efficiency or satisfaction an ....
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\subsection{Keyboards and key switches}
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\begin{figure}[ht]
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\centering
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\includegraphics[width=1.0\textwidth]{images/keyboard_models.jpg}
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\caption{Different keyboard models}
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\label{fig:keyboard_models}
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\end{figure}
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Keyboards are well known input devices used to operate a computer. There are a
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variety of keyboard types and models available on the market, some of which can
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be seen in Figure \ref{fig:keyboard_models}. The obvious difference between
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those keyboards in Figure \ref{fig:keyboard_models} is their general
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appearance. What we see is mainly the shape of the enclosure and the keycaps,
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which are the rectangular pieces of plastic on top of the actual keyswitches
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which sometimes indicate which letter, number or symbol, also known as
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characters, a keypress should send to the computer. These keycaps are mainly
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made out of the two plastics \gls{ABS} and \gls{PBT} which primarily differ in
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feel, durability, cost and sound \parencite[8]{bassett_keycap}.
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\begin{figure}[ht]
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\centering
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\includegraphics[width=1.0\textwidth]{images/keyboard_layouts.png}
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\caption{The three major physical keyboard layouts all labeled with the
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alphanumeric characters of the most popular layout―\gls{QWERTY}
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\cite{wiki_kb_layouts}}
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\label{fig:keyboard_layouts}
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\end{figure}
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Nowadays, there are three main standards for the physical layout of keyboards
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namely ISO/IEC 9995 \cite{iso9995-2}, ANSI-INCITS 154-1988
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\cite{ansi-incits-154-1988} and JIS X 6002-1980 \cite{jis-x-6002-1980}, that
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propose slightly different arrangements of the keys and some even alter the
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shape of a few keys. Figure TODO\ref{fig:keyboard_layouts} shows the layouts
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defined by the three standards mentioned and shows the main differences. In
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addition to the physical layout, there are also various layouts concerning the
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mapping of the physical key to a character that is displayed by the
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computer. Most of the time, the mapping happens on the computer via software and
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therefore the choice of layout is not necessarily restricted by the physical
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layout of the keyboard but rather a personal preference. As seen in Figure
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\ref{fig:keyboard_models}, there are also non standard physical layouts
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available which are often designed to improve the posture of the upper extremity
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while typing to reduce the risk of injury or even assist in recovering from
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previous \gls{WRUED} \cite{ripat_ergo}. Those designs often split the keyboard
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in two halves to reduce ulnar deviation and some designs also allow tenting of
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the halves or provide a fixed tent which also reduces forearm pronation
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\cite{baker_ergo, rempel_ergo}. Besides the exterior design of the keyboard,
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there is another part of interest—the keyswitch. This component of a keyboard
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actually sends the signal that a key is pressed. There are different types of
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keyswitches available to date. The more commonly available ones are scissor
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switches and rubber dome switches which are both subsets of the membrane
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switches. Scissor switches are often found in keyboards that are integrated into
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notebooks while rubber dome switches are mostly used in workplace
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keyboards. Both variants use a rubber membrane with small domes underneath each
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key. When a key is pressed, the corresponding dome collapses and because the
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dome's inner wall is coated with a conductive material, closes an electrical
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circuit \cite{ergopedia_keyswitch, peery_3d_keyswitch}. Another type of switches
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are mechanical keyswitches. These switches are frequently used in gaming and
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high quality workplace keyboards as well as by enthusiast along with prosumers
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which build and then sell custom made keyboards to the latter audience
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\cite{bassett_keycap, ergopedia_keyswitch}. These keyswitches are composed of
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several mechanical parts which can be examined in Figure
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\ref{fig:mech_keyswitches_dissas}. The housing is made up of two parts, the
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bottom and top shell. The actual mechanism consists of two conductive plates,
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which when connected trigger a keypress, a stainless steel spring which defines
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how much force has to be applied to the switch to activate it and a stem which
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sits on top of the spring and separates the two plates. The shape of the stem,
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represented by the enlarged red lines in Figure
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\ref{fig:mech_keyswitches_dissas}, defines the haptic feedback produced by the
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keyswitch. When pressure is applied to the keycap, which is connected to the
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stem, the spring gets contracted and the stem moves downwards and thereby stops
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separating the two plates which closes the electrical circuit and sends a
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keypress to the computer. After the key is released, the spring pushes the stem
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back to its original position \cite{bassett_keycap, peery_3d_keyswitch,
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ergopedia_keyswitch, chen_mech_switch}. Usually, mechanical keyswitches are
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directly soldered onto the \gls{PCB} of the keyboard but there are also
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keyboards where the \gls{PCB} features special sockets where the keyswitches can
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be hot-swapped without soldering at all \cite{gmmk_hot_swap}. It is also
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possible to equip an already existing \gls{PCB} with sockets to make it
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hot-swappable \cite{te_connect}.
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\begin{figure}[ht]
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\centering
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\includegraphics[width=1.0\textwidth]{images/mech_keyswitches_dissas.jpg}
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\caption{Disassembled tactile, clicky and linear mechanical keyswitchs. The
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red lines resemble the shape of the stem which is responsible for the haptic
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feedback and thus, in combination with the strength of the spring, the
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required actuation force}
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\label{fig:mech_keyswitches_dissas}
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\end{figure}
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Mechanical keyswitches also have three main subcategories. Those categories
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primarily define if and how feedback for a keypress is realised:
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\begin{enumerate}
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\item \textbf{Tactile Switches} utilize a small bump on the stem to slightly
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increase and then instantly collapse the force required immediately before the
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actual actuation happens \cite{cherry_mx_brown}. This provides the typist with
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a short noticeable haptic feedback and which should encourage a premature
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release of the key. An early study by Brunner and Richardson suggested, that
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this feedback leads to faster typing speeds and a lower error rate in both
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experienced and casual typists (n=24) \cite{brunner_keyswitch}. Contrary, a
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study by Akagi yielded no significant differences in terms of speed and error
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rate between tactile and linear keyswitches and links the variation found in
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error rates to differences in actuation force (n=24)
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\cite{akagi_keyswitch}. Tactile feedback could still assist the typist to
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prevent \gls{bottoming}.
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\item \textbf{Tactile and audible Switches (Clicky)} separate the stem into
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two parts, the lower part also features a small bump to provide tactile
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feedback and is also responsible for a distinct click sound when the actuation
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happens \cite{cherry_mx_blue}. Gerard et al. noted, that in their study
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(n=24), keyboards with audible feedback increased typing speed and decreased
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typing force. This improvement could have been due to the previous experience
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of participants with keyboards of similar model and keyswitch characteristic
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\cite{gerard_keyswitch}.
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\item \textbf{Linear Switches} do not offer a distinct feedback for the
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typist. The activation of the keyswitch just happens after approximately half
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the total travel distance \cite{cherry_mx_red}. The only tactile feedback that
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could happen is the impact of \gls{bottoming}, but with enough practice,
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typist can develop a lighter touch which reduces overall typing force and
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therefore reduces the risk of \gls{WRUED} \cite{gerard_keyswitch,
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peery_3d_keyswitch, fagarasanu_force_training}.
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\end{enumerate}
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The corresponding force-displacement curves for one exemplary keyswitch of each
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category by the manufacturer Cherry are shown in Figure
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\ref{fig:ks_fd_curves}. The Operational position indicates the activation of the
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keyswitch.
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\begin{figure}[ht]
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\centering
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\includegraphics[width=1.0\textwidth]{images/ks_fd_curves.jpg}
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\caption{Actuation graphs for Cherry MX BROWN \cite{cherry_mx_brown} | BLUE
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\cite{cherry_mx_blue} | RED \cite{cherry_mx_red} switches. Tactile position marks the point where a haptic feedback happens, operational position marks the activation of the keyswitch and reset position is the point where the keyswitch deactivates again}
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\label{fig:ks_fd_curves}
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\end{figure}
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All types of keyswitches mentioned so far are available in a myriad of actuation
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forces. Actuation force, also sometimes referred to as make force, is the force
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required to activate the keyswitch \cite{radwin_keyswitch,
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ergopedia_keyswitch}. That means depending on the mechanism used, activation
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describes the closing of an electrical circuit which forwards a signal, that is
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then processed by a controller inside of the keyboard and finally send to the
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computer. The computer then selects the corresponding character depending on the
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layout used by the user. Previous studies have shown, that actuation force has
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an impact on error rate, subjective discomfort, muscle activity and force
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applied by the typist \cite{akagi_keyswitch, gerard_keyswitch,
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hoffmann_typeright} and as suggested by Loricchio, has a moderate impact on
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typing speed, which could be more significant with greater variation of
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actuation force across tested keyboards \cite{loricchio_force_speed}.
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\subsubsection{Relevance for this thesis}
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Since this thesis is focused around keyboards and especially the relation
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between the actuation force of the keyswitch and efficiency (speed, error rate)
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and also the differences in satisfaction while using keyswitches with varying
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actuation forces, it was important to evaluate different options of keyswitches
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that could be used to equip the keyboards used in the experiment. The literature
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suggested, that the most common switch types used in the broader population are
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rubber dome and scissor switches \cite{ergopedia_keyswitch,
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peery_3d_keyswitch}. Naturally, those keyswitches should also be used in the
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study, but one major problem due to the design of those keyswitches arises. It
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is not easily possible to alter the actuation force of individual keyswitches
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\cite{peery_3d_keyswitch}. This will be necessary to create a keyboard where
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each key should have an adjusted actuation force depending on the finger that
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normally operates it. It should be mentioned, that it is theoretically possible
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to exchange individual rubber dome switches on some keyboards, e.g. keyboards
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with \gls{Topre} switches, but the lacking availability of compatible keyboards
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and especially the limited selection of actuation forces (30g to 55g for
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\gls{Topre} \cite{realforce_topre}) makes this not a viable option for this
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thesis \cite{keychatter_topre}. Therefore, we decided to use mechanical
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keyswitches for our experiment, because these keyswitches are broadly available
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in a variety of actuation forces and because the spring which mainly defines the
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actuation force can be easily replaced with any other compatible spring on the
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market, the selection of actuation forces is much more appropriate for our use
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case (30g to 150g) \cite{peery_3d_keyswitch}. We also decided to use linear
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switches because they closest resemble the feedback of the more wide spread
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rubber dome switches. Further, linear switches do not introduce additional
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factors beside the actuation force to the experiment. In addition, based on the
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previous research we settled on using a keyboard model with hot-swapping
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capabilities for our experiment to reduce the effort required to equip each
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keyboard with the required keyswitches and in case a keyswitch fails during
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the experiment, decrease the time required to replace the faulty switch.
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\subsection{Measurement of typing related metrics}
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\label{sec:metrics}
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Nowadays, a common way to compare different methods concerning alphanumeric
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input in terms of efficiency is to use one of many typing test or word
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processing applications which are commercially available. Depending on the
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software used and the experimental setup, users have to input different kinds of
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text, either for a predefined time or the time is measured till the whole text
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is transcribed \cite{chen_typing_test, hoffmann_typeright,
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fagarasanu_force_training, akagi_keyswitch, kim_typingforces,
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pereira_typing_test}. Text used should be easy to read for typists
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participating in studies that evaluate their performance and are therefore is
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chosen based on a metric called the \gls{FRE} which indicates the
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understandability of text \cite{fagarasanu_force_training,
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kim_typingforces, flesch_fre}. The score ranges from 0 which implies very poor reading
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ease to 100 suggesting that the style of writing used causes the text to be very
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easy to comprehend \cite{flesch_fre}. Immel proposed an adjusted formula of the
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\gls{FRE} that is suitable for German text \cite{immel_fre} and can be seen in
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(\ref{eq:fre_german}).
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\begin{equation}\label{eq:fre_german}
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FRE_{deutsch} = 180 - \underbrace{ASL}_{\mathclap{\text{Average Sentence Length}}} - (58,5 * \overbrace{ASW}^{\mathclap{\text{Average Syllables per Word}}})
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\end{equation}
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According to Flesch, the values retrieved by applying the formula to text can be
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classified according to the ranges given in Table \ref{tbl:fre_ranges} \cite{flesch_fre}.
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\begin{table}
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\centering
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\caption{Categories for different FRE scores to classify the understandability
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of text \cite{flesch_fre}}
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\label{tbl:fre_ranges}
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\begin{tabular}{l|c}
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\hline\hline
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\multicolumn{1}{c|}{FRE} & Understandability \\
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\hline
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\multicolumn{1}{c|}{0 - 30} & Very difficult \\
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30 - 50 & Difficult \\
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50 - 60 & Fairly difficult \\
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60 - 70 & Standard \\
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70 - 80 & Fairly easy \\
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80 - 90 & Easy \\
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\multicolumn{1}{r|}{90 - 100} & Very easy \\
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\hline
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\end{tabular}
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\end{table}
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There are several metrics to measure the performance of typists. Typical methods
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to measure speed are
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\begin{enumerate}
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\item \textbf{\Gls{WPM}}
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\begin{equation}\label{eq:wpm}
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WPM = \frac{|T| - 1}{S} * 60 * \frac{1}{5}
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\end{equation}
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In Eq. (\ref{eq:wpm}), $|T|$ is the length of the transcribed text, $S$ the
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time in seconds taken to transcribe $T$, $\frac{1}{5}$ the average word length
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and $60$ the conversion to minutes. $|T| - 1$ counteracts the first input
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which starts the timer in many typing tests \cite{mackenzie_metrics}.
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\item \textbf{\Gls{AdjWPM}} is especially useful if participants are allowed to
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make mistakes and at the same time not forced to correct them. This method adds
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an adjustable factor to lower the \gls{WPM} proportionally to the uncorrected
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error rate $UER := [0;1]$ defined in Eq. (\ref{eq:uer}). The exponent $a$ in
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Eq. (\ref{eq:cwpm}) can be chosen depending on the desired degree of correction
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\cite{mackenzie_metrics}.
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\begin{equation}\label{eq:cwpm}
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AdjWPM = WPM * (1 - UER)^{a}
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\end{equation}
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\item \textbf{\Gls{KSPS}} measures the raw input rate of a typist by capturing
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the whole input stream including backspaces and deleted characters ($IS$)
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\cite{mackenzie_metrics}.
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\begin{equation}\label{eq:ksps}
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KSPS = \frac{|IS| - 1}{S}
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\end{equation}
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\end{enumerate}
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In addition to speed, the error rate of typists can be measured with the
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following two methods
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\begin{enumerate}
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\item \textbf{\gls{CER}} is the ratio of erroneous input that got fixed
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($IF$) to any character typed during transcription, including $IF$
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\cite{soukoreff_metrics}.
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\begin{equation}\label{eq:cer}
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CER = \frac{|IF|}{|T| + |IF|}
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\end{equation}
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\item \textbf{\gls{UER}} is the ratio of erroneous input that was \textbf{not}
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fixed ($INF$) to any character typed during transcription, including $IF$
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\cite{soukoreff_metrics}.
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\begin{equation}\label{eq:uer}
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UER = \frac{|INF|}{|T| + |IF|}
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\end{equation}
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\end{enumerate}
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In several other studies, in addition to the metrics mentioned so far, \gls{EMG}
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data was captured to evaluate the muscle activity or applied force while typing
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on completely different or modified hardware \cite{kim_typingforces,
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fagarasanu_force_training, gerard_audio_force, gerard_keyswitch, martin_force,
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rose_force, rempel_ergo, pereira_typing_test}. \gls{EMG} signals, are captured
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with the help of specialized equipment that utilize electrodes which are either
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placed onto the skin above the muscles of interest (non-invasive) or inserted
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directly into the muscle (invasive). The disadvantage of non-invasive surface
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level electrodes is the lacking capability to capture the distinct signal of one
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isolated muscle \cite{reaz_emg}. Nevertheless, because this type of electrode is
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more likely to find acceptance among participants and is also easier to apply by
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non-medically trained researchers, it finds wide adoption among studies
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\cite{takala_emg}. To make \gls{EMG} data comparable across subjects, it is
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necessary to conduct initial measurements where each individual participant is
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instructed to first completely relax and then fully contract (\gls{MVC}) the
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muscles of interest. These values are used to normalize further data obtained in
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an experimental setting. The mean signal yielded by complete relaxation is
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subtracted to reduce noise and the \gls{MVC} is used to obtain the individuals
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percentage of muscle activity (\%MVC or EMG\%) during tests \cite{halaki_emg,
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takala_emg, rempel_ergo}. Muscles typically measured during typing exercises
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are the \gls{FDS}, \gls{FDP} and \gls{ED}. The main function of the \gls{FDS}
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and \gls{FDP} is the flexion of the medial four digits, while the \gls{ED}
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mainly extends the medial four digits. Therefore, these muscles are primarily
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involved in the finger movements required for typing on a keyboard
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\cite{netter_anatomy, kim_typingforces, gerard_keyswitch, gerard_audio_force}.
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A method frequently used to measure applied force is to place one or multiple
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load cells under the keyboard \cite{gerard_keyswitch, rempel_ergo,
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bufton_typingforces}. Load cells are electronic components that are able to
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convert applied force to an electrical signal. This signal usually gets
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amplified by specialized circuits and then further processed by a micro
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controller, computer or other hardware \cite{johnson_loadcell}.
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Lastly, subjective metrics e.g., comfort, usability, user experience, fatigue
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and satisfaction, are evaluated based on survey data collected after
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participants used different input methods \cite{kim_typingforces,
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bell_pauseboard, bufton_typingforces, pereira_typing_test, iso9241-411}. In
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their study, Kim et al. used a survey provided by the \gls{ISO} which is
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specifically designed to evaluate different keyboards in terms of user
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satisfaction, comfort and usability \cite{kim_typingforces, iso9241-411}. This
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survey poses a total of twelve questions concerning e.g., fatigue of specific
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regions of the upper extremity, general satisfaction with the keyboard,
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perceived precision and uniformity while typing, etc., which are presented on a
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seven-point Likert-scale \cite{iso9241-411}. Further, studies concerning the
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usability and user experience of different text entry methods used the \gls{UEQ}
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or \gls{UEQ-S} to evaluate the differences in those categories \cite{nguyen_ueq,
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olshevsky_ueq, gkoumas_ueq}. While the full \gls{UEQ} provides a total of 26
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questions divided into six scales (attractiveness, perspicuity, efficiency,
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dependability, stimulation and novelty), the \gls{UEQ-S} only features 8
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questions and two scales (pragmatic and hedonic quality). Because of the limited
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explanatory power of the \gls{UEQ-S}, it is recommended to only use it, if there
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is not enough time to complete the full \gls{UEQ} or if the participants of a
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study are required to rate several products in one session
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\cite{schrepp_ueq_handbook}.
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\subsubsection{Relevance for this thesis}
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Measuring metrics related to data entry tasks can be performed with the help
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several commercially available tools and equipment. Especially muscle activity
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has to be measured with appropriate tools and accurate placement of the
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electrodes is important to ensure quality results \cite{takala_emg, halaki_emg,
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kim_typingforces, gerard_keyswitch}. Metrics related to performance such as
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\gls{WPM}, \gls{CER} and \gls{UER} are well defined and can be applied in almost
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any experimental setup concerning the transcription of text
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\cite{soukoreff_metrics, mackenzie_metrics}. In addition to the measured data,
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questionnaires can help to gather subjective feedback about the keyboards and
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thereby reveal differences that cannot be easily acquired by a device or formula
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\cite{rowley_surveys}.
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\subsection{Crowdsourcing / Observer Bias}
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As shown by the previous research in Section \ref{sec:metrics}, it is common
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practice in research related to typing to present a text that has to be
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transcribed by the participant. Usually, the text was chosen by the researcher
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or already available through the used typing test software. If the
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understandability of text is of concern, the binary choice of, is understandable
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or not, made by the researcher could lead to a phenomenon called the observer
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bias \cite{hrob_observer, berger_observer}. Thus, the text could potentially be
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to difficult to understand for the participants if not evaluated with e.g. the
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\gls{FRE} or other adequate formulas. Further, if there is previous knowledge
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about the requested participants, the researcher could subconsciously select
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text that is familiar to, or well received by some of the subjects and could
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thereby conceivably influence the outcome \cite{hrob_observer, berger_observer}.
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The same problem arises, if the typing test software already provides such texts
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but the researcher has to select some of them for the experiment. Further, the
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difficulty of the provided texts should be verified to ensure accurate results
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across multiple treatments. A possible solution for this problem is
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crowdsourcing.
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Howe CONTINUE
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\cite{howe_crowdsource}. If there are automated checks for text
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difficulty in place, this method completely excludes the researcher from the
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text selection process.
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\subsubsection{Relevance for this thesis}
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% \subsection{Keyboard usage}
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% \subsection{Finger strength}
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% \subsection{Traditional methods}
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% \subsection{Alternative methodology}
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% - Available Methods (Impact vs load)
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% - Load cells |