A study is conducted to determine the effect of the frequency of fires on forest diversity. Fifteen 1-ha plots are randomly selected and subjected to controlled burns at a frequency of once every 5, 10, or 15 years over a 30-year period. The number of tree species (= tree species richness) in each plot at the end of the study are as follows:

Burned Burned Burned every 5 yr every 10 yr every 15 yr 10 -+- 13 7 11 5 14 8 9 7 10 + 9 10 mean = 8.200 11.200 7.400 variance = 4.700 5.200 3.300 Grand mean = 8.933\f