I'm trying to analyze data between 2 timed trials and come up with an average percent change between Trial A and Trial B. The mean time of trial A is 12.87 seconds, and the mean time of trial B is 18.49 seconds. The percent change of those means is 43.67 seconds (using B-A/A *100). However, when I figure the individual %change for each participant in the two trials, add those percentages, and divide by the number of participants, I get a completely different answer-- 64.1%. Why is this so, and what is the best way to analyze such data?