ISO 09241-11:1998 pdf download – Ergonomic requirements for office work with visual display terminals (VDTs)一 Part 11: Guidance on usability.
5.4 Usability measures
5.4.1 Choice of measures
It is normally necessary to provide at least one measure for each of effectiveness, efficiency and satisfaction. Because the relative importance of components of usability depends on the context of use and the purposes for which usability is being described, there is no general rule for how measures should be chosen or combined
The choice of measures and the level of detail of each measure. is dependent on the objectives of the parties involved in the measurement. The relative importance of each measure to the goals should be considered For example where usage is infrequent, high importance may be given to measures of learning and relearning
If it is not possible to obtain oblective measures of effectiveness and efficiency. subjOctive measures based on the users perception can provide an indication of effectiveness and efficiency.
5.4.2 Effectiveness
Measures of effectiveness relate the goals or subgoals of the user to the accuracy and completeness with which these goals can be achieved
For example if he desired goal is to accurately reproduce a two-page document in a specified formal, then accuracy could be specified or measured by the number of spelling mistakes and the number of deviations from the specified format, and completeness by the number of words of the document transcribed divided by the number of words in the source document.
5.4.3 Efficiency
Measures of efficiency relate the level of elfectiveness achieved to the expenditure of resources. Relevant resources can include mental or physical etforl, time, materials or financial cost. For exame, human efficiency could be measured as elf ectiveness divided by human effort, temporal efficiency as effectiveness divided by time, or economic efficiency as effectiveness divided by cost.
If the desired goal is to print copies of a report, then efficiency could be specified or measured by the number of usable copies of (he report printed, divided by the resources spent on the task such as labour hours, process expense and materials consumed.
5.4.4 Satisfaction
Satisfaction measures the extent to which users are tree from discomfort, and their altitudes towards the use of the product.
Satisfaction can be specified and measured by subjective rating on scales such as discomfort experienced, liking for the product, satisfaction with product use, or acceptability of the workload when carrying out different tasks, or the extent to which particular usability objectives Isuch as efficiency or learnabilityf have been met. Other measures of satisfaction might include the number of positive and negative comments recorded during use. Additional information can be obtained from longer-term measures such as rate of absenteeism, observation of overloading or underloading of (he user’s cognitive or physical workload, or from health problem reports, or the frequency with which users request transfer to another lob.
5.4.5 Further examples
Further examples of measures that can be used for assessing usability are included in Annexes B and C.
5.5 Interpretation of measures
Care should be taken in generalizing the results of any measurement of usability to another context which may have significantly different types of users, tasks or environments- If measures of usability are obtained over short periods of time, the values may not take account of infrequent events which could have a significant impact on usability. for example intermittent system errors.
For a general-purpose product, it will generally be necessary to specify or measure usability in several different representative contexts, which will be a subset of the possible contexts and of the tasks which can be performed There may be differences between usability in these contexts.