\section{Literature Review}
To better understand which metrics and methods are meaningful in the domain of keyboards and especially when

To investigate whether or not solely the actuation force of individual keys can make a difference in terms of efficiency or satisfaction an ....
\subsection{Keyboards and key switches}

Keyboards are well known input devices used to operate a computer. There are a
variety of keyboard types and models available on the market, some of which can
be seen in Figure \ref{fig:keyboard_models}. The obvious difference between
those keyboards in Figure \ref{fig:keyboard_models} is their general
appearance. What we see is mainly the general shape of the enclosure and the
keycaps, which are the rectangular pieces of plastic on top of the actual
keyswitches which indicate which letter, number or symbol, also known as
characters, a keypress should send to the computer. These keycaps are mainly
made out of the two plastics \gls{ABS} and \gls{PBT} which mainly differ in
feel, durability, cost and sound \parencite[8]{bassett_keycap}. Nowadays, there
are three main standards for the physical layout of keyboards namely ISO/IEC
9995 \cite{iso9995-2}, ANSI-INCITS 154-1988 \cite{ansi-incits-154-1988} and JIS
X 6002-1980 \cite{jis-x-6002-1980}, that propose slightly different arrangements
of the keys and some even alter the shape of a few keys. Figure
TODO\ref{fig:keyboard_ISO_ANSI_JSP} shows the layouts defined by the three
standards mentioned and shows the main differences. In addition to the physical
layout, there are also various layouts concerning the mapping of the physical
key to a character that is displayed by the computer. Most of the time, the
mapping happens on the computer via software and therefore the choice of layout
is not necessarily restricted by the physical layout of the keyboard but rather
a personal preference. As seen in Figure TODO \ref{fig:keyboard_models}, there
are also non standard physical layouts available which are often designed to
improve the posture of the upper extremity while typing to reduce the risk of
injury or even assist in recovering from previous \gls{WRUED}
\cite{ripat_ergo}. Those designs often split the keyboard in two halves to
reduce ulnar deviation and some designs also allow tenting of the halves or
provide a fixed tent which also reduces forearm pronation \cite{baker_ergo,
  rempel_ergo}. Besides the exterior design of the keyboard, there is another
part of interest—the keyswitch. This component of a keyboard actually sends the
signal that a key is pressed down. There are different types of keyswitches
available to date. The more commonly available ones are scissor switches and
rubber dome switches which are both subsets of the membrane switches. Scissor
switches are often found in keyboards that are integrated into notebooks while
rubber dome switches are mostly used in workplace keyboards. Both variants use a
rubber membrane with small domes underneath each key. When a key is pressed, the
corresponding dome collapses and because the dome's inner wall is coated with a
conductive material, closes an electrical circuit \cite{ergopedia_keyswitch,
  peery_3d_keyswitch}. Another type of switches are mechanical
keyswitches. These switches are frequently used in gaming and high quality
workplace keyboards as well as by enthusiast along with prosumers which build
and then sell custom made keyboards to the latter audience \cite{bassett_keycap,
  ergopedia_keyswitch}. These keyswitches are composed of several mechanical
parts which can be examined in Figure TODO\ref{fig:mech_keyswitch_dissas}. The
housing is made up of two parts, the bottom and top shell. The actual mechanism
consists of two conductive plates, which when connected trigger a keypress, a
stainless steel spring which defines how much force has to be applied to the
switch to activate it and a stem which sits on top of the spring and separates
the two plates. When pressure is applied to the keycap, which is connected to
the stem, the spring gets contracted and the stem moves downwards and thereby
stops separating the two plates which closes the electrical circuit and sends a
keypress to the computer. After the key is released, the spring pushes the stem
back to its original position \cite{bassett_keycap, peery_3d_keyswitch,
  ergopedia_keyswitch, chen_mech_switch}. Usually, mechanical keyswitches are
directly soldered onto the \gls{PCB} of the keyboard but there are also
keyboards where the \gls{PCB} features special sockets where the keyswitches can
be hot-swapped without soldering at all \cite{gmmk_hot_swap}. It is also
possible to equip an already existing \gls{PCB} with sockets to make it
hot-swappable \cite{te_connect}.

Mechanical keyswitches also have three main subcategories. Those categories
primarily define if and how feedback for a keypress is realised:
\begin{enumerate}
  \item \textbf{Tactile Switches} utilize a small bump on the stem to slightly
  increase the force required immediately before and a collapse of force right
  after the actual actuation happens. This provides the typist with a short
  noticeable haptic feedback and which should encourage a premature release of
  the key. An early study by Brunner and Richardson suggested, that this
  feedback leads to faster typing speeds and a lower error rate in both
  experienced and casual typists (n=24) \cite{brunner_keyswitch}. Contrary, a
  study by Akagi yielded no significant differences in terms of speed and error
  rate between tactile and linear keyswitches and links the variation found in
  error rates to differences in actuation force (n=24)
  \cite{akagi_keyswitch}. Tactile feedback could still assist the typist to
  prevent \gls{bottoming}.
  \item \textbf{Tactile and audible Switches (Clicky)} separate the stem into
  two parts, the lower part also features a small bump to provide tactile
  feedback and is also responsible for a distinct click sound when the actuation
  happens. Gerard et al. noted, that in their study (n=24), keyboards with
  audible feedback increased typing speed and decreased typing force. This
  improvement could have been due to the previous experience of participants
  with keyboards of similar model and keyswitch characteristic
  \cite{gerard_keyswitch}.
  \item \textbf{Linear Switches} do not offer a distinct feedback for the
  typist. The activation of the keyswitch just happens after approximately half
  the total travel distance. The only tactile feedback that could happen is the
  impact of \gls{bottoming}, but with enough practice, typist can develop a
  lighter touch which reduces overall typing force and therefore reduces the
  risk of \gls{WRUED} \cite{gerard_keyswitch, peery_3d_keyswitch, fagarasanu_force_training}.

\end{enumerate}
The corresponding force-displacement curves for one exemplary keyswitch of each
category are shown in Figure TODO\ref{fig:ks_fd_curves}.

All types of keyswitches mentioned so far are available in a myriad of actuation
forces. Actuation force, also sometimes referred to as make force, is the force
required to activate the keyswitch \cite{radwin_keyswitch,
  ergopedia_keyswitch}. That means depending on the mechanism used, activation
describes the closing of an electrical circuit which then forwards a signal,
that is then processed by a controller inside of the keyboard and then forwarded
to the computer. The computer then registers the character depending on the
layout used by the user. Previous studies have shown, that actuation force has
an impact on error rate, subjective discomfort, muscle activity and force
applied by the typist \cite{akagi_keyswitch, gerard_keyswitch,
  hoffmann_typeright} and as suggested by Loricchio, has a moderate impact on
typing speed, which could be more significant with greater variation of
actuation force across tested keyboards \cite{loricchio_force_speed}.


\subsubsection{Relevance for this thesis}
Since this thesis is focused around keyboards and especially the relation
between the actuation force of the keyswitch and efficiency (speed, error rate)
and also the differences in satisfaction while using keyswitches with varying
actuation forces, it was important to evaluate different options of keyswitches
that could be used to equip the keyboards used in the experiment. The literature
suggested, that the most common switch types used in the broader population are
rubber dome and scissor switches \cite{ergopedia_keyswitch,
  peery_3d_keyswitch}. Naturally, those keyswitches should also be used in the
study, but one major problem due to the design of those keyswitches arises. It
is not easily possible to alter the actuation force of individual keyswitches
\cite{peery_3d_keyswitch}. This will be necessary to create a keyboard where
each key should have an adjusted actuation force depending on the finger that
normally operates it. It should be mentioned, that it is theoretically possible
to exchange individual rubber dome switches on some keyboards, e.g. keyboards
with \gls{Topre} switches, but the lacking availability of compatible keyboards
and especially the limited selection of actuation forces (30g to 55g for
\gls{Topre} \cite{realforce_topre}) makes this not a viable option for this
thesis \cite{keychatter_topre}. Therefore, we decided to use mechanical
keyswitches for our experiment, because these keyswitches are broadly available
in a variety of actuation forces and because the spring which mainly defines the
actuation force can be easily replaced with any other compatible spring on the
market, the selection of actuation forces is much more appropriate for our use
case (30g to 150g) \cite{peery_3d_keyswitch}. We also decided to use linear
switches because they closest resemble the feedback of the more wide spread
rubber dome switches. Further, linear switches do not introduce additional
factors beside the actuation force to the experiment. In addition, based on the
previous research we settled on using a keyboard model with hot-swapping
capabilities for our experiment to reduce the effort required to equip each
keyboard with the required keyswitches and in case a keyswitch fails during
the experiment, decrease the time required to replace the faulty switch.

\subsection{Measurement of keyboard related metrics}
A common way to compare different methods concerning alphanumeric input in terms
of efficiency is to use one of many typing test applications which are
commercially available. Depending on the software used and the experimental
setup, users have to input different kinds of text, either for a predefined time
or the time is measured till the whole text is transcribed \cite{chen_typing_test}.

\subsection{Satisfaction while using a keyboard}
\subsection{Text understandability / FRE}
\subsection{Crowdsourcing / Observer Bias}
\subsection{Keyboard usage}
\subsection{Keyswitch types}
- Rubber dome
- Mechanical switches (Why linear -> rubberdome is not tactile nor has audible feedback)
\subsection{Muscle activity / EMG measurements}
\subsection{Finger strength}
\subsection{Traditional methods}
\subsection{Alternative methodology}
- Available Methods (Impact vs load)
- Load cells