# Differences from logistic regression and mixed effects logistic regression - rounding error or conceptual mistake?

I'm a bit confused. To my understanding, the standard logistic regression should be equivalent to a mixed effect logistic regression where the statistical unit is defined as random effect - but I found the results of the two analysis to be different.

As example, let's say that I have an experiment with 20 participants. To each participant I ask x questions, and their answer is scored either as either "right" or "wrong".

I simulated the dataset with:

data0 = cbind(data.frame(matrix(sample(1:10, 40, replace=TRUE), nrow = 20, byrow = TRUE)), rep(c('a', 'b'), each=10)) names(data0) = c('s','f','c')

Let's say that the column s has the number of right answers, the column f has the number of wrong answer, and the column c represents a between-subject experimental condition. The subjects are organized per rows.

To test the effect of c I can write:

anova(glm ( cbind( s, f) ~ 1, data = data0, family = binomial), glm ( cbind( s, f) ~ 1 + c, data = data0, family = binomial), test="Chisq")

which gives me:

Model 1: cbind(s, f) ~ 1 Model 2: cbind(s, f) ~ 1 + c Resid. Df Resid. Dev Df Deviance Pr(>Chi) 1 19 49.144 2 18 42.978 1 6.1659 0.01302 *

So I tried to model the same data with a logistic mixed effect model.

First I converted it to a long format with:

data1=NULL for (i in 1 : nrow( data0) ) { tt = c(rep(1, data0$s[i]), rep(0, data0$f[i])) t = cbind(tt, rep(i, length(tt)), rep(data0$c[i], length(tt))) data1 = rbind( data1, t) } data1 = data.frame(data1) names(data1) = c("r","s", "c") data1$s = factor(data1$s) data1$c = factor(data1\$c)

Where s is the subject ID. Then I tested the effect of c with:

library(lme4) anova(glmer( r ~ 1 + ( 1| s), data = data1, family = binomial), glmer( r ~ 1 + c + ( 1| s), data = data1, family = binomial), test="Chisq")

Now the same analysis gives me a different result!

glmer(r ~ 1 + (1 | s), data = data1, family = binomial): r ~ 1 + (1 | s) glmer(r ~ 1 + c + (1 | s), data = data1, family = binomial): r ~ 1 + c + (1 | s) Df AIC BIC glmer(r ~ 1 + (1 | s), data = data1, family = binomial) 2 290.72 297.45 glmer(r ~ 1 + c + (1 | s), data = data1, family = binomial) 3 289.97 300.07 logLik deviance glmer(r ~ 1 + (1 | s), data = data1, family = binomial) -143.36 286.72 glmer(r ~ 1 + c + (1 | s), data = data1, family = binomial) -141.98 283.97 Chisq Chi Df glmer(r ~ 1 + (1 | s), data = data1, family = binomial) glmer(r ~ 1 + c + (1 | s), data = data1, family = binomial) 2.7548 1 Pr(>Chisq) glmer(r ~ 1 + (1 | s), data = data1, family = binomial) glmer(r ~ 1 + c + (1 | s), data = data1, family = binomial) 0.09696 .

Is it because of rounding errors in the implementation of the mathematical functions, or there is some fundamental concept I'm missing here?

PS: the results of the two analys can be closer that in the case I reported - I cherry picked an extreme example for the sake of a clear explanation.