# Logistic regression yields very small p-values for any/every explanatory variable in my dataset

I'm investigating whether a federal district court judge's ABA rating (rating given to the judge by the American Bar Association when he/she is nominated) significantly correlates to the rate that his/her opinions are reversed on appeal. (I'm in the USA.) There are 645 judges total. My data looks like so:

judge gender president president_party aba_rating cases_tot aff_tot rev_tot rev_rate
Judge1 Male Clinton Democrat Qualified 272 211 61 0.22426471
Judge2 Male Obama Democrat Not Qualified 279 215 64 0.22939068
Judge3 Female Obama Democrat Well Qualified 348 310 38 0.1091954
Judge4 Female Bush II Republican Well Qualified 129 97 32 0.24806202
Judge5 Male Trump Republican Not Qualified 6 6 0 0
... ... ... ... ... ... ...

From the very helpful Cross Validated community, I believe I should be doing logistic regression using the glm() function. I am using the following model:

model_X <- glm(cbind(aff_tot, rev_tot) ~ [explanatory variable(s)], data = dct_data, family = binomial)


(I'm using cbind(aff_tot, rev_tot) as my dependent variable instead of a simple rev_rate because I want to account for the fact that judges have had different numbers of cases appealed.)

I'm sure this is just because I'm new to R/new to stats, but no matter what explanatory variable(s) I used, I almost always get really small p-values (examples below), even when it obviously doesn't make sense to have such small p-values. Am I setting my dataset up incorrectly? Am I using the glm() function incorrectly? I've asked several stats friends, and they couldn't figure it out.

...

[If it's helpful, I'm here are some examples...]

Example 1: Gender

All judges are either Female or Male. The average reversal rate for Females is 12.2%, and the average for Males is 13.5%. But when I use the below model, I get a p-value of 2x10-16 for the coefficient on Females and 2.5x10-8 for Males.

model_gender <- glm(cbind(aff_tot, rev_tot) ~ gender, data = dct_data, family = binomial)


Example 2: President Party

All judges were nominated either by a Democrat or a Republican. The average reversal rate for Democrat-appointed judges is 13.5%, and the average for Republican-appointed judges is 13.0%. But when I use the below model, I get a p-value of 2x10-16 for the coefficient on Democrats and 0.0203 for Republicans.

model_president_party <- glm(cbind(aff_tot, rev_tot) ~ president_party, data = dct_data, family = binomial)


With 645 judges, my instinct is that it's impossible that a ~1% difference in reversal rates for Female vs. Male should yield such a low p-value. (Ditto for the 0.5% reversal rate difference for Democrats vs. Republicans.) But whenever I look for a correlation between reversal rate and any of the explanatory variables, or whenever I combine multiple variables in the same model, I almost always get really low p-values for each explanatory variable. Again, I'm sure I'm making a dumb mistake, but I'd really appreciate it if someone could show me what I'm doing wrong.

• Please share the output you are using to calculate the p-values. Mar 9 at 15:51
• How many cases in total? Also please include the model outputs. Since you 645 judges and from the excerpt several hundred cases per judge, the sample size is quite large which will lead to small p values Mar 9 at 16:01
• Why do you get two p-values for binary regressors? It is a test for difference between the two groups, so should only yield one p-value Mar 9 at 16:36
• @AdamO I'm using summary(model_gender) to calculate p-values. Mar 10 at 1:29
• @RobertLong: There are 113,007 cases in the dataset. Mar 10 at 1:30

That said, the issue here is that the sample size is not 645. It is 113,007, and your glm model is treating the data as if they are independent. With such a sample size, even a very small effect can lead to a small p value. Maybe the data are independent, however we must not rule out the possibility that the probability of a reversal for a particular decision made by one judge is more likely to be similar to the probability of a reversal for a anothe decision made by the same judge, than that of a different judge. I would therefore consider fitting either a Generalised Estimating Equations model, or a mixed effects model, with random intercepts for judge. If there is considerably intra-class correlation, then the p values will probably be higher.