syn.passive(data, func)formula specifying transformations on data.
It is specified as a string starting with ~.formula.+, -, * and ^ have different meanings in formula syntax. Use
the identity function I() if they should be interpreted as arithmetic operators,
e.g. "~I(age^2)".
mice: Multivariate Imputation by Chained Equations
in R. Journal of Statistical Software,
45(3), 1-67.
http://www.jstatsoft.org/v45/i03/
syn