[R-sig-ME] Why lme4 doesn't throw an error for illegitimate an model

[Simon Harmel]

Dear All,

As far as I understand, in a 2-level model, the use of a level-2 predictor
(here "sector") is illegitimate.

But I wonder why in the following model `lmer` doesn't throw an error to
indicate that?

library(lme4)
hsb <- read.csv('
https://raw.githubusercontent.com/rnorouzian/e/master/hsb.csv')

mn <- lmer(math ~ ses + (sector | sch.id), data = hsb)

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