Bonferroni, a technique for dealing with the problem of increasing the
chance of making Type I errors when multiple comparisons are made, works
by changing the alpha-level. I'll use the symbol & for alpha.
Step 1. Find n, the number of possible comparisons when the means of k
groups are to be compared. n = k(k-1)/2
For example, if 4 groups are to be compared, n = 6
Step 2. Find &*, the adjusted alpha level:
&* = &/n
For example, if & = .05 and n = 6, &* = .008
Then make multiple comparisons using the t-test, look up tables of
significance at the &* level (you may need to interpolate or
approximate) and then claim significance only at the & level. In this
example. .008 is approximately equal to .01.
Other techniques for dealing with the multiple comparisons problem are
the Scheffe procedure and the Tukey HSD (Honestly Significant
Difference) test.
Paul Gardner
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