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Posts
• #8756
Nice2U
Participant

Hi,
I have used several online HW calculators, but I have noticed differences between them. How can I calculate the p value if I only know the Chi square?.
Thanks a lot

• #79133
blcr11
Participant

There may be some confusion of terms here. Which “p” are you talking about? The allele frequency, p, used in the HW expression, is determined by observation, not derived from the chi-squared statistic. From the observations, you can calculate the expected genome frequencies and calculate a chi-squared statistic to see if the observed distribution of genomes is consistent with HW equilibrium or not. If chi-square is larger than some critical chi-square value (determined by the df for the test—for two alleles the df = 1 for the chi-squared test), then HW equilibrium is NOT taking place—for whatever reason; or if the chi-square is less than the critical chi-squared value, then the data are said to be consistent with being in HW equilibrium. The p-value of the chi-squared test is the level at which you either accept or reject the presence of HW equilibrium, typically p=0.1, 0.05, or 0.01. (Chi-square critical for 1df is 2.71, 3.84, 6.64, 10.83 for p=0.1, 0.05, 0.01, and 0.001, respectively.) If your program won’t tell you the significance level of the test, then you have to go look up the critical values for chi-square on a table somewhere. You can look here if you don’t have a table handy: http://www.richland.edu/james/lecture/m170/tbl-chi.html