Choose the Proper Screen-Based Controls 529

Step 7: Choose the Proper Screen-Based Controls 529

Directional or scroll arrows. To indicate the direction in which scrolling may be performed, directional or scroll arrows are also included. They are constructed of variously shaped arrows in small boxes with backgrounds that contrast with the scroll area/container. They are most often located at each end of the scroll bar, but some systems locate them adjacent to one another within the scroll area/con- tainer itself.

Placing directional arrows at opposite ends of the scroll bar is conceptually the clearest. The mouse pointer is moved in the same direction, away from the current position, when either the scroll arrow or scroll handle is manipulated. The distance that the directional arrows are separated by, however, causes increased effort when a window’s contents must be adjusted by scrolling in opposite directions.

One platform solved the direction-switching problem by positioning the directional arrows adjacent to one another at one end of the scroll bar. While the forward-backward scrolling is made more efficient, the spatial correspondence between the beginning, middle, and end of the data is lost.

Another platform took another approach, placing the directional arrows at opposite ends of the slider box/handle to maintain the desirable spatial corre- spondence while at the same time minimizing their separation. Since during a continuous scrolling operation the directional arrows move as the slider box/handle moves, this platform automatically moves the mouse pointer to keep it aligned with the scroll arrow. This eliminates the need for the user to move the pointer during the continuous scrolling operation, but it requires that the user relinquish control of the mouse operation, and may be disorienting.

Using a scroll bar, the scrolling movement can be performed in several ways. The most common actions involve grabbing the slider box/handle and moving it in the desired direction, or selecting the proper directional arrow. Clicking a mouse button while selecting a directional arrow moves the contents of a win- dow one line. Pressing the mouse button scrolls the window’s contents continu- ously until the button is released. One platform provides another more efficient process. A region of the scroll area/container can also be selected, automatically moving the slider box/handle to that point and displaying the proper window contents.

Based upon early scrolling research (Bury et al., 1982), movement of the win- dow data usually follows the window-up or telescope approach, whereby the window moves around over data that appears fixed in location. This causes the data in a window to move in the direction opposite the one indicated by the directional arrow or the direction of movement of the scroll container/handle. Scrolling using window systems, however, seems to be especially mistake-prone, users often assuming the arrows will move the data in the same direction as the directional arrow or scroll container/handle. In other words, it is sensed that the data moves under the window, not the window over the data (Billingsley, 1988). Why this happens is open to conjecture. Billingsley speculates that, because win- dows seem to move on screens, when data scrolls or moves in a window, people may conclude the data must be moving because the window remains still during the scrolling operation. Or, because of the close physical proximity of the direc-

530 Part 2: The User Interface Design Process

on the data, not the window. The implication is that the scrolling procedure should be rethought and restudied. Some recent applications have devised scrolling methods through actually point at the window data.

Selection. When the slider box/handle has been selected, highlight it in some visu- ally distinctive way. Most systems do provide some visual feedback of this kind. Location. While, again, no universal agreement exists, the majority of systems locate the vertical (top-to-bottom) scroll bar to the right of the window and the horizontal (left-to-right) scroll bar at the bottom of the window.

Size.

A vertical scroll bar should be the height of the scrollable portion of the win- dow body. A horizontal scroll bar should be at least one-half the width of the scrollable portion of the window body.

Current state indication. Whenever the window’s size or the position of informa- tion changes, the scroll bar components must also change, reflecting the current state of the scrolling process. Providing accurate information about the scrolling location facilitates user navigation and makes it easier to reposition the slider box/container. Include scroll bars in all sizable windows.

If scrolling cannot be performed in a particular direction, the relevant arrow box should be reduced in contrast or grayed out. If all the information in a win- dow is displayed and no information is available for scrolling, both directional arrows should be reduced in contrast or grayed out.