We want to have a 75% chance of the battery lasting through that length of time.
By choosing the upper quartile, we have a 25% chance of the battery lasting LONGER than that time, and a 75% chance of the battery lasting LESS than that time.
By choosing the lower quartile, we have a 25% chance of the battery lasting LESS than that time, and a 75% chance of the battery lasting LONGER than that time.
Mathematically, P (t > film length) = 75%, so we need 75% of the values to be greater than that time, therefore choose the lower quartile value as 75% of values are greater than it.
Hope that helps. =)
Last edited by a moderator: Sep 30, 2016