I have read one cannot/should not calculate the basic R-Squared used in linear regression for a Poisson generalized linear regression model.
It is logical to me that one cannot determine the basic R-Squared for a logistic regression, since the R-Squared gives the proportion of explained variance of the dependent variable and the dependent variable is not metric in a logistic regression.
However, the dependent variable in a Poisson regression is clearly metric. It might not be fully continuous, but that should not be a requirement to calculate the variance. So I do not understand why one should not calculate the basic R-Squared for a Poisson regression model.