Hikers completed their trips in an average of 15.2 days (SD 7.6) .
15.2 days with an SD of 7.6? This seems like a very unusual cohort of JMT thru-hikers. The hiking plans data on the Yahoo JMT group looks more like an average of at least 20 days to me.
With an SD of 7.6 a normal distribution would suggest 15% of the hikers surveyed did the JMT in a week or less, and similarly only 15% take more than 23 days.
