What kind of customer research do you do with people that leave after the trial? Also, many companies that I know don't count trials as part of "customers" so

What kind of customer research do you do with people that leave after the trial? Also, many companies that I know don't count trials as part of "customers" so they don't impact churn (higher churn among people who have paid you in the past is a more urgent cause for alarm than increased amount of canceled trials)