I have noticed that when Graphpad Prism does a sphericity correction using the Geisser-Greenhouse correction, it halves the P value for this source of variation (typically Treatments). For example, in the example I am working on, it calculates an F value of 20.25, with DF(num) = 1.391, and DF(den) = 2.783. The P value for this F-ratio should be 0.046. However, Prism gives 0.023. On the other hand, for the Individual or Between Rows source, there is no correction for sphericity, and the F-ratio is 0.25, with DF(num) = 2, and DF(den) = 4. It gives a P-value of 0.7901 for this F-ratio, and as you can verify, it is the 'full" P-value. Why does it halve it when there is a Geisser-Greenhouse correction for sphericity? I redid the analysis with no correction for sphericity; it now gives an F-ratio of 20.25, with DF(num) = 2, and DF(den) = 4. And the P-value is the "full" value of 0.0081.
Excel gives the one-tail P-value when the formula FDIST(x, df_num, df_den) is used. Using the above values, it gives a P-value of 0.046 for FDIST(20.25, 1.391, 2.783). To get the two-tail value, we need to double this P-value, not halve it. That is why I am confused.
Any insights? Thank you.
sgadag
Excel gives the one-tail P-value when the formula FDIST(x, df_num, df_den) is used. Using the above values, it gives a P-value of 0.046 for FDIST(20.25, 1.391, 2.783). To get the two-tail value, we need to double this P-value, not halve it. That is why I am confused.
Any insights? Thank you.
sgadag