There are two test formats available: Classic and Consecutive.

Classic will run a different thumbnail each day/hour (A,B,C,D,A,B,C,D,A,B,C,D).

If you run a consecutive test with 2 thumbnails for 24 hours, thumbnail A will show for 12 hours, then thumbnail B will show for 12 hours. (A,A,A,B,B,B,C,C,C,D,D,D).

We currently recommend classic tests for recent videos, and consecutive tests for old videos.