The online writing software Calibrated Peer Review (CPR) is a useful tool for assigning writing assignments in large college classes. In this system, students submit essays online and are guided in how to rate essays using criteria written by the instructor. The instructor does not have to grade the essays, and CPR has educational benefits that make it desirable. I used the CPR system for four essays in the largest section of my undergraduate astronomy class, and to create a comparison group, I assigned the same essays in the sections of the course that did not use the CPR system. I surveyed students at the beginning and end of the semester about confidence in evaluating their own essays and confidence in their abillity to write a good essay. I found that students who were not already confident in the ability to evaluate their own writing tended to increase their confidence after using the CPR system. There was no increase in confidence in the comparison group, indicating that the CPR system helps students become more confident at assessing their own writing. (Contains 5 tables.)