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I am a biologist, considering whether or not it is correct to carry out a Bonferonni procedure on my data, and would be very grateful for any advice. >From DNA analysis I have measured the extent of genetic differentiation between pairs of populations of my study animal. Therefore I have a half matrix of values. The significance of these were calculated using a permutation procedure, using a well established method (using a custom written software package). I have 36 values, some of which are significant at the 0.001 level, some at 0.01, some at 0.05 and some not significant. The question is: should I carry out a Bonferroni Procedure, dividing the critical values by 36. If i do that very few are significant. From the literature some people seem to look at the number of values which you would expect to be significant by chance (eg. 5%) at the 0.05 level, (and then if there are >5% this is used to discount a type 1 error in all cases), while others carry out a Bonferonni correction. However in a theoretical situation, if I had all 36 pairwise comparisons yielding significant values (P<0.01) and then adjust my critical value of 0.01 by dividing by 36, I get no significant results. Is this a type 2 error, or not?. I'd like to hear from anyone who can put me right on this! Thanks, Steve Rossiter ---------------------------------------------------------------------------- ----- Stephen Rossiter Bat Ecology and Bioacoustics Lab School of Biological Sciences University of Bristol Woodland Rd Bristol BS8 1UG United Kingdom [log in to unmask] www.bio.bris.ac.uk/research/bats/ %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%