From 9.1 section in the paper,

> Each student watched one video, took the corresponding quiz, watched the second video, and took the second quiz. About half of the participants did the Paxos portion first and the other half did the Raft portion first in order to account for both individual differences in performance and experience gained from the first portion of the study. We compared participants’ scores on each quiz to deter- mine whether participants showed a better understanding of Raft.