Principles of Good Interface and Screen Design 269

Step 3: Principles of Good Interface and Screen Design 269

A scale is a set of measurement points or markers. There are three major types of scales: category, quantitative, and sequence. A category scale, also called qualitative or nominal, consists of an unordered or unsequenced series of words or numbers. A quan- titative scale, also referred to as value, interval, numeric, or amount, consists of sequential numbers with uniform spacing. A sequence scale consists of words or numbers in a cus- tomary and standard order, such as the months of the year.

Scaling is the positioning of data in relation to these points or markers. Choose an appropriate scale for both graph axes. If the scale is too expanded, the effect may be exaggerated, if too small, the effect may be underreported. Standard scaling practices are these:

Tick marks. Place ticks to mark scales on the outside edge of each axis. Placing them outside will prevent a tick from interfering with data located near the axis. Linear scale. Employ a linear scale. Most people are more familiar with linear scales

than with logarithmic or other nonlinear scales. These latter kinds are often inter- preted inaccurately. If necessary or desirable, two X or Y scales can be used, one displaying a linear scale, the other a logarithmic scale.

Scale markings. Mark scales at standard or customary intervals to aid comprehen-

sion. Familiar standard intervals are 1, 2, 5, 10, and multiples of 10. Familiar cus- tomary intervals include the days of the week and the months of the year. Construct scales with tick marks at these intervals. To aid visual comprehension, it may be necessary to provide intermediate marks as well. Intermediate marks should be consistent with the scale intervals shown.

Grid cells. Create squarish grid cells by using the same distance between ticks on both axes. Distorting the axes will distort the data as well. Start a numeric scale at zero. Using zero as the starting point on a scale aids visual

comparisons because zero is an expected starting point. If a zero point is omitted because of the nature of the data, this omission should be clearly indicated in the graphic.

Minimization of scale digits. Keep the number of digits in a scale to a minimum.

Smaller numbers aid understanding. Round off all numbers to two digits or less. Scales containing decimals. Place zeros in front of decimal numbers so the decimal point is not missed. Data points on zero line. If a substantial number of data points are on the zero (0) line, offset the scale to enhance readability. Otherwise, they may not be seen. Single scale on each axis. Display only a single scale on each axis. Avoid multiple

scales associated with a single axis. For all but the most experienced people, mul- tiple scales can be confusing and can lead to interpretation errors. Meanings can also be greatly distorted. If multiscale graphs must be used, permit the user to select any data curve individually and have the computer highlight its corre- sponding scale.

Duplicate axes for large matrices. For large data matrices, consider displaying

duplicate axes. The readability of large data matrices is improved if the X-axis scale appears at the top as well as the bottom of the graph, and the Y-axis scale at

270 Part 2: The User Interface Design Process

Scale interpretation. Provide aids for scale interpretation. When reading accuracy is extremely critical, provide computer aids for interpretation, such as the following:

Displaying a fine grid upon request.

Vertical and horizontal rules that the user can move to the intersection point.

Letting the user point at a data item and having the computer then provide the exact values.

Consistency. Provide scaling consistency across two or more related graphics. If comparisons must be made between multiple graphs or charts, use the same scale for each. Data sets scaled differently lead to interpretation errors.

Labeling. Each scale axis should be clearly labeled in a conventional left-to-right reading orientation. A complete description of the values, with measurement units, should be provided. Do not, however, overwhelm the graph with tick labels. Default to a reasonable number of tick marks and labels.