\section{Goals}
There are three main goals of this evaluation:
\begin{itemize}
\item To compare SDAZ to rate-based scrolling.
\item To model SDAZ using Fitts' law.
\item To provide further insight into the accuracy of the previous calibration of the system.
\end{itemize}
The evaluation also has a three secondary aims:
\begin{itemize}
\item To explore whether people more naturally use the dragging or the modal version of the SDAZ and rate-based systems, and how this choice affects their performance.
\item We are interested in seeing how previous experience with SDAZ and previous experience at using computers affects ability with the system.
\item To investigate the idea that a large document might adversely affect performance with scrollbars while making no difference to the rate-based systems. 
\end{itemize}
The evaluation is based on the experimental paradigm of \citeasnoun{hcbm:1}. They proposed that Fitts' law be used to model different scrolling systems. Participants perform what is known as a Fitts' reciprocal tapping task, where they scroll back and forth several times between two targets. This is repeated several times with targets different distances apart and with the different scrolling systems. Each scrolling system can then be modelled using Fitts' law (see Section \ref{fittslaw}).

Our experimental method differs from that employed by \citename{hcbm:1} in two minor ways. \citename{hcbm:1} tested their scrolling systems over different levels of accuracy --- for example, for some tasks the users had to position the target in the centre half of the screen, while getting the target on the visible screen was sufficient in other tasks. Thus they varied the level of accuracy they were requiring from the users. We saw no point in doing this and therefore simply asked the user to get the target on the screen in all our tasks. Our reasoning for this is threefold. One, if Fitts' Law does accurately model scrolling, as has been shown (\citeasnoun{hcbm:1} and \citeasnoun{gbm:1}), then there is little to be gained by testing different accuracies as the model accounts for the accuracy and can therefore be used to predict the results for different accuracies. Two, the purpose of scrolling is to get information into the visible screen space, once it is visible it is a fairly trivial task for the user to get it to the precise location on the screen that they happen to desire and such minor pagination is not the concern of long distance scrolling systems. Three, it would increase the time required from our participants. Our experiment took participants between 20 and 40 minutes and we did not want to greatly increase this by adding further conditions.

The second way in which our method differed from that of \citename{hcbm:1} was our lack of line numbers. \citename{hcbm:1} numbered every line of the document up the margin hoping that this would simulate user familiarity with the document. For example, a user familiar with a document would know in which direction and approximately how far their desired target was compared to their current position in the document. Users were therefore given the line numbers of their targets so they could use that as a check against the line numbers in the margin of their current location. There were two reasons we did not use such line numbering. One, an integral part of SDAZ is that it allows the user to visually identify the target as it passes. If the users simply focused at the line numbers on the side of the screen there would be absolutely no difference between SDAZ and rate-based scrolling. Since it is precisely a comparison of those two systems that we are attempting, such an experimental condition would be entirely unacceptable. Two, there is the problem of deciding what to do with the line numbers in the SDAZ system --- do the line numbers zoom out too, or do they bunch together so none can be clearly read, or are only some of them shown? We decided that the best solution was to use no line numbers. We instead made sure that the documents both had normal page numbers so that the users could use those if desired (none did) and the chapter and section headings of the larger document were numbered (which several participants did use for reference).

\section{Participants and Apparatus}
Fifteen volunteer university students (eleven male, four female) took part in the experiment. Eight regularly played computer games involving rapid information flow. Nine were full time computer science students. Five had used SDAZ before, and nine had used rate-based scrolling before.

The evaluation was done using the SDAZ system discussed at length in Chapter \ref{calSD}, which was modified to handle rate-based scrolling and a standard scrollbar. This system provided frame rates of approximately 150 frames per second during the experiment and was running on a 2.4 GHz Intel Pentium 4 computer with 512MB RAM and a Geforce 4 MX video card, running Redhat Linux 8. The 19-inch display was set to 1280x1024 pixels and the interface ran in full-screen mode.

\section{User Tasks}
The experiment was a 3x6 repeated measures design. The dependent measure was time taken to tap between the targets. The two factors were as follows:
\begin{itemize}
\item \emph{Scrolling system}

The levels of this factor were: rate-based, SDAZ, and scrollbar.
\item \emph{Target set}

There were six pairs of targets that participants scrolled back and forth between eight times. That is, each target in each pair was acquired four times by the user.
\end{itemize}

Participants were given a print out of the pages of the document that their current target pair were on, and the targets themselves were highlighted on the hard copy. Participants had as much time as they desired between each target set to familiarise themselves with the document and with the location of their targets. Participants were then asked to scroll back and forth between the two targets as quickly as possible, pressing the space bar when they had reached each target (when the highlighted portion of the page was on screen). The data from their first acquisition of each target was discarded as the user was still learning the location of the target.

The target order remained the same for every participant and every interface. The targets were spread across two documents, the first a 47 page honours thesis and the second an 8 page conference paper. Information about the target sets and documents is displayed in Table \ref{targetsets}. Participants performed all the tasks in one interface, then filled out a NASA Task Load Index subjective evaluation sheet \cite{NASA:1}, then repeated the process with the two remaining interfaces. Participants were order-balanced in their exposure to the three scrolling systems, using a Latin square, to avoid order effects. After performing all the experimental tasks, users were asked to rank the three systems from best to worst.

\begin{table}[!btp]
\begin{center}
\begin{tabular}{c|c|clc|c|c}
Target Set \# & Distance & Doc \# & \hspace{1cm} & Doc \# & Length & Reference\\
\cline{1-3} \cline{5-7}
1 & 35 & 1 & & & & \\
2 & 20 & 1 & & 1 & 47 & \citeasnoun{mm:1}\\
3 & 5 & 1 & & & & \\
4 & 5 & 2 & & & & \\
5 & 3 & 2 & & 2 & 8 & \citeasnoun{hcbm:1}\\
6 & 1.5 & 2 & & & & \\
\end{tabular}
\caption{The target sets and documents used in the experiments. The target ``Distance'' and and document ``Length'' are reported in units of pages.}
\label{targetsets}
\end{center}
\end{table}
