Classification of types of Information
Buck (1983) provides a useful classification of types of information that can be displayed to users.
Each of these types of information can, in theory, be provided on most types of displays. However, some lend themselves better to one form of display rather than another. The characteristics of each of these types can now be briefly discussed. The particular forms of technology that can be used to implement them will be discussed in more detail in a later section.
1. Instructions refer to information that guides behaviour in a particular way. In other words, it supports performance to carry out a task by prompting on what to do and when to do it. A simple sign telling people to enter or not enter a door would be one example. Other simple cases include the dialogue messages that are provided on automated cash machines (ACM). More complex instructions will appear in printed form on the packaging or the instructional manuals for pieces of equipment.
2. Command messages give a very straightforward statement on what is or what is not permitted. 'Do not enter', 'do not smoke', 'do not eat or drink', are examples of command messages. Sometimes they are similar to instructions, but are much more focused on simple statements that refer to high priority items.
3. Advisory messages are somewhat watered down versions of command messages. In some cases, these will be recommendations to avoid a situation, at other times they would be information allowing for the preparation or planning of particular activities. For example, we might be advised that our train is late by a spoken message and we might, possibly, be given an accurate time estimate for when the train will be available.
4. Answers information may be provided in response to a particular enquiry that has been made. This is typical of an interactive information-handling situation, where we have a particular question in mind or degree of uncertainty and we seek information from a source with regard to removing that uncertainty. It turns out that most of the information that is sought from displays is of the answer kind. If we want to know what the time of day is, we look at our watches and clocks to find the answer. If we want to know what speed we are doing in our cars, or what level of fuel we have, we look at the gauges.
5. Historical displays are used to look back at the state of a variable over a period of minutes, hours, days or even years. A graphical representation of road accidents over the last century would be a historical display of information. If we want to know what the temperature fluctuation has been in an office on a daily basis, then specialist devices can be brought in and placed in the office that will give a pen recording over a fixed period of time. It is much easier to see if there is a trend in information if it is displayed in this way; the alternative is to hold in memory a general impression of what the temperature readings have been at a number of points during the day or record them manually on a chart. Gauging the temperature in an office concerns a relatively low risk situation. However, if the concern is with the temperature in a critical vessel in a chemical process, then the temperature trends exhibited over the time are quite important. If the current value is near a safety value, it may well be that it has been near that value for several weeks and is not a critical event. On the other hand, it may have reached that value in the last few hours; looking back at the trend in the information will indicate the rate of change of that variable and whether it constitutes a particular risk to the system.
6. Predictive displays are much more specialised, but increasingly found in complex processes. In the same way that historical data support performance in making a judgment based on the current value, predictive information enables examination of the current value and indicates any likely change in the future. Predictor displays enable better control over vehicles, typically at sea or airborne, and enable smoother transitions from one state to another. They are used in slow response systems where it is difficult to see the immediate effect of an action that has been carried out. Predictive displays will enable a variable to be plotted into the future. The same graphs that are used as historical displays can also be used as predictive displays. If a steady decline in road accidents over time is seen, then the best prediction of the future would be a continued decline. However, it may be that this does not turn out to be the case because of some other factor that can enter into the situation. A predictive value is based on the best evidence available. But in the case of control of dynamic situations, such displays have much to offer in extending the human skill.
Some of these points can now be illustrated by reference to particular areas of display design and the discussion begins with visual displays.