To study the correlations between common usability metrics used in typical usability tests, Sauro and Lewis made an analysis focusing on summative usability studies made in practice. They were able to collect data from 90 usability tests in which very diverse systems and products had been evaluated. The results showed strong correlation between common usability metrics, such as task time, completion rate, number of errors, and post-task and post-test satisfaction. The correlations were especially strong when analysed at the task-level, but analyses on the test-level also showed at least medium correlations. On the basis of these findings, Sauro and Lewis recommend, among other things, to use both post-task and post-test satisfaction measurements in usability tests.