How do you find out which queries need the most improvement? Look at the ones that are underperforming compared to their expected number of clicks. If you look for low click-through rate (CTR), you’ll find underperforming queries, but they’ll almost all be in the long tail. Improving those won’t make an overall improvement. Click residual is a metric that combines CTR with overall traffic to give a useful number.
To find the queries with the most impact, start with the click count. “Click residual” is the difference between the expected number of clicks and the actual number of clicks. When that is negative, you can see how many times a customer did a search, but wasn’t satisfied enough with the results to click, relative to the overall performance of the search system.