Defines the grouping to calculate standard deviation of data in [do_preprocess.pk()].
Usage
stat_sd_group(...)
Value
A list of expressions. This is added to the `pk` object.
Arguments
...
A set of unquoted variables whose unique combinations define
a group with which to impute missing SD values (using the
minimum non-missing SD value in the group).
Default is `Chemical, Species, Reference, Media`.