What performance indicator most reliably demonstrates that optimized thumbnails drove incremental engagement with existing content?

An increase in the click-through rate (CTR) on a title, measured immediately after implementing optimized thumbnails, most reliably demonstrates that those thumbnails drove incremental engagement with existing content. The click-through rate is the percentage of users who view a thumbnail and then click to watch the content. If the CTR increases significantly after the thumbnail is changed, it indicates that the new thumbnail is more effective at attracting viewers. For example, if a title had a CTR of 2% with its original thumbnail, and the CTR increases to 4% after implementing an optimized thumbnail, this indicates a doubling of viewer engagement due to the new thumbnail. This increased engagement demonstrates that the optimized thumbnail successfully captured user attention and motivated them to learn more about the content. This metric is more reliable than simple view counts, as it directly measures the thumbnail's effectiveness in converting impressions into actual views. It should be measured against a control group or a baseline period before the change to ensure accuracy.