\section{Methodology}
\label{sec:methodology}
\subsection{Research Approach}
Because of the controversial findings about the impact of key actuation forces
on speed \cite{akagi_keyswitch, loricchio_force_speed} and the fact, that
keyboard related work can increase the risk for \gls{WRUED} \cite{ccfohas_wrued,
  pascarelli_wrued}, we decided to further investigate possible effects of
different actuation forces and even a keyboard equipped with non-uniform
actuation forces on speed, error rate and satisfaction. To our best knowledge,
to this date, there is no published work about the influence of a keyboard with
non-uniform actuation forces on these metrics. Therefore, we first asked
seventeen people about their preferences, experiences and habits related to
keyboards to get a better understanding on what people might prefer as a
baseline for the design of the adjusted keyboard (keyboard with non-uniform
actuation forces) and to complement the findings obtained through our literature
review. Further, we collected information about available mechanical keyswitches
on the market. Additionally, we conducted a small preliminary experiment with 6
subjects, where we measured the peak forces each individual finger of the right
hand was able to apply to distinct keys in different locations. We then created
the design for the adjusted keyboard based on those measurements. Lastly, an
experiment with twenty-four participants was conducted, where we compared the
performance and user satisfaction while using four different keyboards,
including our adjusted keyboard, to values obtained with the participant's own
keyboards.

\subsection{Preliminary telephone interview}
Some of the studies we found that researched implications of actuation force on
speed, preference or other metrics were published between 1984 and 2010. That is
why we wanted to ascertain if and how, with the advance of technology in recent
years and especially the capabilities modern smartphones offer, keyboard usage
has changed. Further, we wanted to gather information about the preference of
key resistance, keyswitch type and experiences with \gls{WRUED}. Therefore, we
conducted a structured interview with seventeen volunteers (59\% females) via
telephone. The age of the subjects ranged between 22 and 52 with a mean age of
29 years. The professions of subjects were distributed among medical workers,
students, office employees, computer engineers and community workers. The first
question we asked was \textit{``Which keyboard in terms of actuation force would
  be the most satisfying for you to use in the long run?''}. Thirteen (76\%) out
of the seventeen subjects mentioned, that they would prefer a keyboard with
light actuation force over a keyboard with higher resistance. The next question
\textit{``Have you ever had pain when using a keyboard and if so, where did you
  have pain?''} yielded, that 41\% of those polled experienced pain at least
once while using a keyboard. The areas affected described by the seven who
already experienced pain were the wrist \underline{and} forearm (3 out of 7),
wrist only (2 out of 7), fingers (1 out of 7) and forearm only (1 out of 7). The
results for the third question \textit{``Which keyboard are you currently using
  and for how many hours a day on average?''} were in line with the statements
we found during our literature review \cite{ergopedia_keyswitch,
  peery_3d_keyswitch}. Nine answered that they use a notebook (scissor-switches,
membrane), six stated that they use an external keyboard with rubber dome
switches and only two responded that they use a keyboard featuring mechanical
keyswitches. The average, self-reported, usage ranged between half an hour and
10 hours with a mean of 4.71 hours. It is important to note, that a study by
Mikkelsen et al. found, that self-reported durations related to computer work
can be inaccurate \cite{mikkelsen_duration}. The last question \textit{``Which
  tasks do you still prefer to perform with a keyboard rather than your mobile
  phone?''} revealed, that all of the subjects preferred to use a keyboard when
entering greater amounts of data (emails, applications, presentations,
calculations, research), but also surprisingly 41\% preferred to use a keyboard
to write instant messages (chatting via Whatsapp
Web\footnote{\url{https://web.whatsapp.com/}}, Signal
Desktop\footnote{\url{https://signal.org/download/}}, Telegram
Desktop\footnote{\url{https://desktop.telegram.org/}}).

\subsection{Market analysis of available mechanical keyswitches}
To gather information about available actuation forces, we collected the product
lines of keyswitches for all well known manufacturers, namely
Cherry\footnote{\url{https://www.cherrymx.de/en/mx-original/mx-red.html}},
Kailh\footnote{\url{https://www.kailhswitch.com/mechanical-keyboard-switches/}},
Gateron\footnote{\url{http://www.gateron.com/col/58459?lang=en}},
Matias\footnote{\url{http://matias.ca/switches/}},
Razer\footnote{\url{https://www.razer.com/razer-mechanical-switches}} and
Logitech\footnote{\url{https://www.logitechg.com/en-us/innovation/mechanical-switches.html}}. Since
some of the key actuation forces listed on the manufacturers or resellers
websites were given in cN and most of them in g or gf, the values were adjusted
to gram to reflect a trend that is within a margin of ± 2 g of accuracy. The
results shown in Figure \ref{fig:keyswitches_brands} are used to determine the
minimum, maximum and most common actuation force for broadly available
keyswitches. According to our findings, the lowest commercially available
actuation force is 35 g ($\approx$ 0.34 \gls{N}) the most common one is 50 g
($\approx$ 0.49 \gls{N}) and the highest resistance available is 80 g ($\approx$
0.78 \gls{N}).

\begin{figure}[ht]
  \centering
  \includegraphics[width=1.0\textwidth]{images/keyswitches_brands}
  \caption{Available actuation forces for keyswitches of major keyswitch manufacturers}
  \label{fig:keyswitches_brands}
\end{figure}

\subsection{Preliminary study of finger strength}
To evaluate the impact of an adjusted keyboard (keyboard with non-uniform
actuation forces) on performance and satisfaction we first needed to get an
understanding on how to distribute keyswitches with different actuation forces
across a keyboard. Our first idea was to use a similar approach to the keyboard
we described in Section \ref{sec:lr_sum}, were the force required to activate
the keys decreased towards the left and right ends of the keyboard. This rather
simple approach only accounts for the differences in finger strength when all
fingers are in the same position, but omits possible differences in applicable
force depending on the position a finger has to enter to press a certain key.
To detect possible differences in peak force depending on the position of the
fingers, we conducted an experiment with six volunteers (50\%
females). Subject's ages ranged from 20 to 26 with a mean age of 24 years. The
subjects were all personal contacts. Subjects professions were distributed as
follows: computer science students (3/6), physiotherapist (1/6), user experience
consultant (1/6) and retail (1/6). All Participants were given instructions to
exert maximum force for approximately one second onto the key mounted to the
measuring device described in Section \ref{sec:force_meas_dev}. The order of
positions in which the participants had to press the key was complete counterbalanced
