A comparison helps to reveal points of an interface and high severity problems as well as the strengths of the solutions that are better utilized.
We create a list of user tasks that are performed in compared products. During the test, respondents go through the tasks in the lab. The results of different products are then analyzed.
Another variation of a comparative test is a current product vs. new prototype analysis. Such an analysis provides with the last chance to reveal the problems in prototype and solve them early. Starting from this point the expenses for changes will increase exponentially.