A formula defining the grouping of the data frame.

frac

The part of data to be sampled.

replace

Is the sampling with replacement.

data

A data frame.

systematic

Should sampling be systematic.

Value

A dataframe.

Details

If systematic=FALSE (default) then frac gives the fraction
of data sampled. If systematic=TRUE and frac=.2 then every 1/.2
i.e. every 5th observation is taken out.