numeric vector of weights. If NULL (default), uniform weights (i.e. all equal to 1) are used.
na.rm
logical, indicating whether NA values should be silently removed before the computation proceeds. If FALSE (default), an additional level is added to the variables (see na.value argument).
na.value
character. Name of the level for NA category. Default is "NA". Only used if na.rm = FALSE.
digits
integer. The number of digits (default is 3). If NULL, the results are not rounded.