When you notice an increase in the error rate for an operation, you can use the Comparison view to compare a point of time when the error rate was stable (called the baseline) to the time when the rate was high (regression). You then use analysis tools to identify what caused the increase in errors.
The Comparison view opens to show data from both time periods. The color blue represents the baseline time window and red represents the regression time window.
Read all about it here!