'desc' produces descriptive statistics for a
sample.  There can be multiple measurements on the sample, and
the sample can be stratified by one or more factors.
The possible statistics to be reported are:
mean ('mean')
median ('median')
minimum ('min')
maximum ('max')
standard deviation ('sd')
standard error of the mean ('se.mean')
variance ('var')
listwise valid cases ('nvalid.lw')
listwise missing ('nmis.lw')
variablewise valid cases ('nvalid')
variablewise missing ('nmis')
The default 'stats' argument ('all') requests all of these
statistics.  Individual statistics can be requested with
an appropriate character vector, e.g., c('mean', 'median')
requests only the mean and median.  By default, the number of
listwise valid and missing cases are always reported.
ARGUMENTS:
'...' is one or more quantitative variables, all of the same length.
'by' is a factor or a list, matrix or dataframe of factors.  Statistics
are computed for each subsample defined by the combinations of these
factors.
'stats' names the statistics to be reported ('all' is the default).
If 'na.rm' is TRUE (the default), then any case that is missing
on any of the dependent variables is deleted from the dataset
before any statistics are calculated.  If 'na.rm' is FALSE, then
missing values are retained in the dataset.  Note that functions
like 'mean' will return 'NA' if any case of a variable is NA.
'ord.byvar' has no effect if 'by' is null.  If 'by' is non-null
'ord.byvar = TRUE' there are 2 or more variables to be described,
then the output has one row per stratum combination and all statistics
for a variable are grouped together as consecutive columns.  If
'by' is non-null, 'ord.byvar = FALSE' and there are 2 or more
variables to be described, all statistics of the same type, e.g.,
all means or all sd's, are grouped together as consequtive columns.
If 'by' is null, then each column represents a different variable,
and each row presents a different statistic, e.g, row 1 might be
the medians, row 2 might be the means, etc.
'transpose = TRUE' transposes the output (strata in columns, one
row per stat).  The default is 'transpose = FALSE (strata in rows,
one column per stat).