Figure 1.
Schematic illustrations of more (A) and less (B) problematic cases for the estimation of fixed-effect covariates in random-intercept models. (a) Regression lines for several individuals with high (A) and low (B) between-individual variation in slopes (σb). (b) Two individual regression slopes with low (A) and high (B) scatter around the regression line (σr). (c) Regression lines with (A) many and (B) few measurements per individual (independent of the number of levels of the covariate).