Usability Testing 13 Usability Problems If a user encounters a problem while attempting a task and it can be associated with the interface, it's a user interface problem (UI problem). UI problems, typically organized into lists, have names, a description, and often a severity rating that takes into account the observed problem frequency and its impact on the user. The usual method for measuring the frequency of occurrence of a problem is to divide the num- ber of occurrences within participants by the number of participants. A common technique (Rubin, 1994; Dumas and Redish, 1999) for assessing the impact of a problem is to assign impact scores according to whether the problem (1) prevents task completion, (2) causes a significant delay or frustration, (3) has a relatively minor effect on task performance, or (4) is a suggestion. When considering multiple types of data in a prioritization process, it is necessary to combine the data in some way. One approach is to combine the data arithmetically. Rubin (1994) described a procedure for combining four levels of impact (using the criteria previously described with 4 assigned to the most serious level) with four levels of frequency (4: frequency 90%; 3: 51­89%; 2: 11­50%; 1: 10%) by adding the scores. For example, if a problem had an observed frequency of occurrence of 80% and had a minor effect on performance, its priority would be 5 (a frequency rating of 3 plus an impact rating of 2). With this approach, priority scores can range from a low of 2 to a high of 8.