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If I have a binary variable, say sex, and I want to test whether multiple other variables are associated with it. To do this, I run a logistic regression of the form

\begin{equation} logit(probability(sex = male)) = \beta X1 + \beta X2 ... + \beta Xk \end{equation}

Once I do this, I calculate the pseudo R-squared, and it is 0.45 meaning that the regression explained 45% of the variance in sex.

My question is, is it also fair/correct to interpret this as sex explained 45% of the variance in the regressors?

Likewise, if the odds ratio for X1 is 2.0, can one claim that being male increased the odds of X1 occurring by 100%?

Basically, can the equal sign in the regression equation truly be treated as an equal sign (i.e., bi-directional equivalence) if I have no claim to directionality?

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  • $\begingroup$ The linear equation estimated by the model is an equality, but that doesn't apply to the inferences made by the model. $\endgroup$ – david25272 Aug 21 '17 at 0:58
  • $\begingroup$ Is there any specific reason you tagged "causality"? $\endgroup$ – Carlos Cinelli Aug 21 '17 at 6:00
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    $\begingroup$ Adding to other answers: we can talk about "explained variance" only when conducting linear regression. For GLMs $R^2$ has nothing to do with explained variance. $\endgroup$ – Tim Aug 21 '17 at 6:25
  • $\begingroup$ @carloscinelli I tagged causality because, if interferences are directionally constrained, that is a sort of causal inference. For example, if we can claim that X has an effect on Y but not the inverse, we at least know that Y doesn't cause X. $\endgroup$ – JRF1111 Aug 21 '17 at 22:49
  • $\begingroup$ @JRF1111 ok, that opens a whole lot of different answers. Causality is definitely directionally constrained, that is, for causal models the equal sign is not a literal equality, it's more like an assignment operator, and the relationship is asymmetric $y \leftarrow x$ . $\endgroup$ – Carlos Cinelli Aug 21 '17 at 23:19
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You do have a claim to directionality, it is just implicit.

A more explicit statement of the logistic model is:

$$ y \mid x \sim \text{Bernoulli}(p = logit^{-1}(\beta_0 + \beta_1 x_1 + \beta x_2 ... + \beta x_k)) $$

Here, $\sim$ means "is distributed as".

You are making a distributional statement about $y$ conditional on $x$, and conditioning is not symmetric. Because of the conditional, the $x$'s are not considered random by the model, so $y$ cannot be thought of as explaining variance in the $x$'s (at least from the point of view of the model).

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  • $\begingroup$ Great response. Perhaps this is best left to a new question, but do you know of some methods that allow for binary, categorical, and non-normal variables where the conditioning is symmetric or where the xs are conditional on y? $\endgroup$ – JRF1111 Aug 20 '17 at 22:44

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