The measures taken during a usability test depend on the test purpose and design. Some common measures include:

• The proportion of participants affected by a problem
• Severity of each problem event (e.g., blocking, impairing, or nuisance)
• Number of participants completing a task successfully, without assistance, divided by the number of participants who attempted the task
• The average time required for successful task completion

Other possible measures include:

• Frequencies of user error
• Frequencies of positive and negative comments
• Time required to complete each step of a procedure
• The number of participants who used each of several documentation features

Often, qualitative data from a usability study is more important than the quantitative measures. Examples of qualitative data include the following:

• Comments and suggestions from the participants
• Participants' interpretations of interactive controls or information in the UI, especially where those interpretations differ markedly from what was intended
• A participant's frame of reference or "mental model" which forms the basis of his or her interpretations of the UI and decisions made during task attempts