\label{C411:2}

The contents of this chapter have been taken from earlier work we performed this year. (See Section \ref{C411:ex} for further explanation.)

\section{Introduction to Pilot Evaluation}

A fundamental question regarding SDAZ is whether a zoom-out actually solves the problem of blurring. An major assumption of SDAZ is that a zoom-out means a greater rate of progress through the document can be achieved without the information flow becoming intolerable. But if the tolerable information flow is a function of magnification then it is possible, depending on the nature of the relationship, that the idea behind SDAZ is flawed. If the maximum comprehensible rate of flow decreases proportionally to the zoom level, there will not be any gain obtained by zooming-out.

Furthermore, in order to calibrate an SDAZ system, a value needs to be set for the threshold. \citename{ih:1} refer to this threshold value as one of several ``predefined constants'' used in their equations, but do not provide the values of these constants. \citeasnoun{cs:1} in their analysis of SDAZ also mention that they use a threshold, but similarly say nothing about the value used for it.

\subsection*{A Theoretical Estimation}
Let us assume that the user will use major headings and images as cues about where they are in the document when scrolling. A few informal calculations suggest that the tolerable scrolling velocities are somewhere around 20 deg/s. Hence, it will be necessary that the smooth-pursuit system activates to track the target. Thus, we can predict the time required as follows:
\begin{itemize}
\item The time taken to recognise a portion of the document as being an important landmark in the document. $\approx$200ms according to \citename{cmn:1}'s model, as it requires one cycle of both the visual system and the cognitive system.
\item The time taken for the smooth-pursuit system to activate and reduce the retinal flow to an acceptable level. $\approx$150ms according to \citeasnoun{rashbass:1}.
\item The time taken to identify the image ($\approx$100ms according to \citename{spence:1}) or read the heading. The normal reading rate is approximately 280 words per minute \cite{km:1}. Thus, given a one to three word section heading, it will take 200-600ms to read.
\end{itemize}
Therefore, a crude approximation would say that images need to be on-screen for about 450 milliseconds to be recognised properly, and headings for between 550 and 950 milliseconds to be read. However, we believe that when the user is familiar with the document that the pages themselves are treated as images --- they shape of the layout of the text forms an image recognisable to the user.

\subsection*{Reflections on units of speed}

\label{SpeedUnits}
The introduction of the idea of different levels of magnification (in such systems as SDAZ, for example) means that two distinct measurements of speed need to be separated.

Let us define \emph{document speed} as the rate at which navigation through the information space is occurring. This will be measured primarily in units such as pages/second or lines/second. It is this value that it is desirable to maximise in order to acquire the target more rapidly.

Let us define \emph{viewer speed} as the rate at which information moves across the display. We believe it is most convenient to measure this value in terms of \emph{screen time}. That is, using units of seconds. Alternative units that might be used for this include cm/s, pixels/s and degrees/s. These alternative units can be calculated as functions of screen resolution, physical screen size, and viewer distance from the screen. Since, we are dealing with image velocities of significantly greater than 3 deg/s, the smooth-pursuit system is reducing the actual retinal image velocities to manageable levels, rendering the actual image velocity in deg/s somewhat irrelevant. The more generalisable unit therefore, should be \emph{screen time}, which is in agreement with our theoretical analysis.

Given these definitions there will be a relationship between document speed and viewer speed as a function of magnification:
\[
Viewerspeed = Magnification / Documentspeed
\]

\section{Evaluation}
The object of the experiment was to obtain some empirical data on the rate of information flow that humans find tolerable when scrolling rapidly.

\subsection*{Participants}
Twelve volunteer participants (ten male, two female) took part in the experiment. Eleven were Computer Science students and one was a Computer Science lecturer.

\subsection*{Apparatus}
The evaluation was done using a basic document display interface written in OpenGL/C which was capable of rendering the document at different magnifications and providing a high frame rate ($>300$ frames per second).
The experiment was conducted on a 2.4 GHz Intel Pentium 4 computer with 512Mb RAM and a Geforce 4 MX Video card. The 19-inch display was set to 1280x1024 pixels, though the interface only took up 690x810 pixels. The software ran under Redhat Linux 8.

\subsection*{User interface}
Initially the document was displayed stationary with page-one showing. Pressing pg-down caused the document to begin scrolling slowly to wards the end. Further pressing of pg-down caused the scrolling of the document to speed up, while pressing pg-up reduced the scrolling speed. The up- and down-arrows could also be used to fine-tune the scrolling speed, with each arrow having only a tenth of the effect of the pg-buttons. The pg-buttons provided sufficiently fine adjustments ($\approx$0.074 pages/s) that most participants found the use of the arrows largely unnecessary. When the end of the document was reached, the document started scrolling from the beginning again.

\begin{figure}[!tbh]
\begin{center}
\includegraphics[width=0.5\textwidth]{graphs/SSSystem.eps}
\caption{The user interface displaying a document at one-third magnification.}
\label{dv}
\end{center}
\end{figure}

The document could be scrolled in one direction only (from top to bottom). Thus, continual pressing of pg-up or the up-arrow eventually reduced the scrolling to a halt, and further pressing of these keys had no effect.

Once an appropriate speed had been acquired, pressing the escape key recorded the speed and moved on to the next condition.

\subsection*{Experimental design}
The experiment was a 2x2 repeated-measures design. The dependent measure was scrolling velocity. (This scrolling velocity will be reported in terms of both document speed and viewer speed.) The two factors were as follows:
\begin{itemize}
\item \emph{Magnification}. The levels of this factor were full magnification (one document page on the screen at once) and one-third magnification (three pages on the screen at once).
\item \emph{Task Type}. The two levels of this factor were ``most comfortable'' and ``maximum tolerable''.
\end{itemize}

Participants were order-balanced in their exposure to the four conditions, using a Latin square, to avoid order effects.

The document used was a report \cite{js:1} with blank pages removed, which resulted in it being a 28 page document (see Figure \ref{dv}).

\begin{figure*}[!t]
\begin{center}
\subfigure[Mean Document Speed versus Magnification.]{
\label{DSGraph}
\includegraphics[width=0.45\textwidth]{graphs/DSGraphOld.eps}
}%note that ___Old.eps as opposed to ___.eps is referring to the old and new pictures. ``Old'' are the ones used in the original 411 assign with gridlines. ``New'' are ones with out gridlines but which are low quality due to rdesktop.
\subfigure[Mean Viewer Speed versus Magnification.]{
\label{VSGraph}
\includegraphics[width=0.45\textwidth]{graphs/VSGraphOld.eps}
}
\caption{Results.}
\label{Graphs}
%note that this actually gets put on the next page
\end{center}
\end{figure*}

\subsection*{Procedure}
An important part of the experiment lay in the participants' understanding of the meaning of the two different task types. Participants were advised to consider the task of scrolling through a familiar document with the goal of target acquisition in mind. They were instructed not to try and read the body text, but rather look for the locations of significant features such as pictures and section headings. They were asked to imagine they were scrolling through the document with the intention of navigating to a known heading or image.

The ``most comfortable'' speed was defined to be the speed at which they felt provided the optimal trade-off between acquiring the target quickly and maintaining an understanding of their location in the document. It was emphasised that this speed was to be the one at which they were most subjectively happy using to scroll through the document. They were advised to consider the question that ``If this was your document and you were scrolling through it to get to another part of it, how fast would you go?''

The ``maximum tolerable'' speed was explained as the speed at which any faster was considered to be ``too fast''. Participants were told that this was the point at which they considered that if the document was scrolled any faster, they would no longer be able to see their target as it went by.

Each participant was questioned after the explanation of these two terms in order to ensure that they had fully understood the ideas behind them. (All participants had done so accurately.) By our understanding of the definition of these terms we expected that for any given participant, the ``maximum tolerable'' speed would be greater than the ``most comfortable'' speed. An analysis of the results showed that all participants had similarly understood the definitions.

Participants were strongly advised to rest their eyes regularly and were informed that there was no time limit. Most participants completed the four conditions in about five minutes. Three complained of eye strain, one commenting ``my eyes hurt''. Though another added, after expressing a similar complaint, ``but if you look away for a bit, it's okay.''

\section{Results}
We first present the results of the evaluation in terms of both viewer speed and document speed. We then discuss the practical applications of these results and their limitations.

\subsection*{Document speed}

A summary of the data obtained for document speed is given in Table \ref{DSTable} and shown in Figure \ref{DSGraph}.

\begin{table}[!b]
\begin{center}
\begin{tabular}{llrr}
%\hline
Magnification & Task type & Mean & SD\\%\hline
full & max & 2.6753 & 1.1649\\%\hline
full & comfort & 1.5160 & 0.9893\\%\hline
third & max & 5.2574 & 2.0945\\%\hline
third & comfort & 2.6963 & 1.6941\\%\hline
\end{tabular}
\caption{Document Speed (pages/s).}
\label{DSTable}
\end{center}
\end{table}

An analysis of variance revealed a significant main effect for magnification ($F_{1,11}=54.5, p<0.001$). The effect of magnification on document speed is good news for SDAZ. It indicates (see Figure \ref{DSGraph}) that reducing the magnification allows for much more rapid navigation through the document space while maintaining the same level of user comfort. Using the mean ``most comfortable'' speed, it would take 13.2 seconds to scroll 20 pages through a document at full magnification and 7.4 seconds to scroll the same distance at one-third magnification.

There was also a significant main effect for task-type ($F_{1,11}=43.6, p<0.001$) and a significant interaction ($F_{1,11}=13.1, p<0.005$). These will be discussed later.

\begin{table}[!b]
\begin{center}
\begin{tabular}{llrr}
%\hline
Magnification & Task type & Mean & SD\\%\hline
full & max & 0.4426 & 0.1835\\%\hline
full & comfort & 0.9277 & 0.5502\\%\hline
third & max & 0.6532 & 0.2451\\%\hline
third & comfort & 1.5208 & 0.8047\\%\hline
\end{tabular}
\caption{Viewer Speed (screen time, in seconds).}
\label{VSTable}
\end{center}
\end{table}

\subsection*{Viewer speed}

A summary of the data obtained for viewer speed is given in Table \ref{VSTable} and shown in Figure \ref{VSGraph}.

We had hoped there would be no main effect of magnification on viewer speed. This would have meant SDAZ systems could be implemented using the simple method used by \citename{cs:1} in which viewer speed is kept constant over all magnifications. However, we found a significant main effect for magnification ($F_{1,11}=36.0, p<0.001$), which implies a more complex relationship. We hypothesise two possible causes for this interrelationship:
\begin{itemize}
\item The reduced text size decreases reading rate, subsequently increasing the amount of time required to read the headings.
\item The lower magnification results in more information being on the screen at once, which means the user is being presented with more data to process, thus requiring more time.
\end{itemize}

There was also a significant main effect for task type ($F_{1,11}=24.4, p<0.005$) and a significant interaction ($F_{1,11}=8.7, p<0.05$). These will be discussed in the next section.

\section{Discussion}
Our theoretical estimation suggested that 450--950ms would be an appropriate length of time for the information to remain on the screen. Our results at full magnification agreed strongly with this estimation. However, our results at one-third magnification did not.

In both result sets there was a significant main effect for magnification. This was contrary to our theoretical estimation which took no account of the magnification and thus predicted results independent of magnification.

The main effect for task type makes application of the results more difficult. It shows that viewer comfort begins to degrade prior to the point at which maximum performance is reached. Any calibration of a scrolling system will therefore have to consider the trade-offs being made between viewer comfort and performance.

The significant interactions measured also serve to reemphasise this point by showing that viewer comfort and maximum performance degrade differently with magnification. This makes the task of calibration difficult, because even if an optimal speed is found at one level of magnification, there is no easy way to determine the optimal speed at other magnifications.

The standard deviations obtained in our data were very large when compared to the means (see Tables \ref{DSTable} and \ref{VSTable}). This indicates that the acceptable speeds varied significantly from user to user. The high variation suggests it may not necessarily be possible to find a speed that is acceptable to all users.

\subsection*{Confounding factors}
We identified seven potentially problematic factors in our evaluation:
\begin{enumerate}
\item All except one of the participants were reasonably young (early twenties) and all were experienced computer users. As such, they could be expected to be better at scrolling and more able to tolerate high rates of information flow than the general population.
\item It is possible that people who regularly deal with rapid information flow rates may develop a higher toleration of it. Thus such people such as computer games players (of which our evaluation probably contained a disproportionately high amount), or perhaps sports players, may be expected to perform better than average.
\item Our experiment only involved document scrolling in one direction (top to bottom). Given that humans are used to reading from top to bottom it is entirely possible that different results would be obtained if we had examined scrolling in the other direction.
\item We only used one document for the experiment. Perhaps if different documents were used (especially if they contained a different amount of graphics, different font sizes or different amounts of white-space) differing results would be obtained.
\item We asked participants to \emph{imagine} they were scrolling through a document rather than having them perform the actual task of scrolling. It is possible that this caused a systematic error as participants may not have accurately estimated their scrolling speeds.
\item Our experiment involved constant velocities over a relatively long period. In real scrolling the velocities are only sustained for a relatively short period of time. It is possible that people can tolerate a higher rate of visual flow if it is only sustained for a short period.
\item Several of our participants experienced eye strain despite our regular reminders to rest their eyes. It is possible that this fatigue meant participants were not able to tolerate flow rates that they might have otherwise found acceptable.
\end{enumerate}

\section{Conclusions}
The task of scrolling involves a rapid movement through the information space. This movement can cause blurring if it occurs too rapidly. \citename{ih:1} proposed Speed Dependent Automatic Zooming as a solution to this, which invokes an automatic zoom-out once information flow passes a predefined threshold. However, no work has been done to investigate an appropriate value for that threshold, neither is there conclusive proof that a lower magnification actually allows a greater navigation rate.

We conducted an evaluation to determine an appropriate threshold value and whether magnification had an effect on the navigation rate. We found that magnification had a marked effect on the navigation rate and that zooming-out allows a more rapid movement through the information space.

Our data suggests that information should be on screen for between 0.4 and 0.9 seconds at full magnification and for between 0.6 and 1.5 seconds at one-third magnification.
