When reading in data from a foreign spreadsheet source (Excel/Quattro/Lotus)
or from the clipboard, having the optionSuggest filling in missing factors set will bring up this dialog,
if any of the factor columns have a number of missing values.
The columns marked with a leading F will have their missing
values replaced from the last non-missing value above the cell.
To specify which columns are to filled, double click the column names,
and F is placed at the start of the column's name.
Alternatively, columns can be selected for conversion to a factor
by clicking on them, and then clicking the Fill button. They can
be deselected by clicking the Leave button.
Select all the columns to be filled.
Select all the columns to be left in their initial state, i.e.
don't fill any columns.