Reference table of predictor matrix codes

[hanneoberman]

          I made a previous inventory. Perhaps we could that into a table of code by method

• Univariate imputation methods for two-level data use other codes than 0 and 1
• 2l.bin, 2l.lmer, 2l.norm, 2l.pan, 2lonly.mean, 2lonly.norm and 2lonly.pmm use code -2 to indicate the class variable
• 2l.bin, 2l.lmer, 2l.norm and 2l.pan use code 2 to indicate the random effects
• 2l.pan uses codes 3 and 4 to add class means to codes 1 and 2 respectively

Originally posted by @stefvanbuuren in https://github.com/amices/ggmice/issues/125#issuecomment-1746490567