Descriptive statistics of covariates by quartiles of peer problems at age 16 years.

Note that for ordinal variables ≤5 levels, descriptive statistics are only displayed for collapsed response levels, as indicated in the variable column, whereas bivariate associations were estimated using the full range of each variable.ap value from Mantel-Haenszel test (dichotomous variables, or ordinal variables ≤5 levels with bp value from Spearman’s rho (continuous variables, or ordinal variable ≥6 levels, or ≤5 levels with >20% cells with expected frequencies