stata skills 3：用xfill填充序列，类似excel中用鼠标下拉来填充序列-经管之家官网！

So now our data looks like this:

http://www.sealedenvelope.com/img/stata/xfill2.gif

Why would we want to do this? The reason is that it makes the output from other longitudinal commands easier to interpret when performed in subgroups. For instance, look at the results of xcount within by groups before we filled in sex:

http://www.sealedenvelope.com/img/stata/xfill3.gif

This suggests that we have 14 males, 1 female and 14 patients whose gender is unknown. In fact we know the sex of all 15 patients in this dataset. The 14 phantom patients of unknown gender are being produced because patients are recorded as either male or female in the 1st record and as unknown sex in 2nd and subsequent records. One patient only has a 1st record and no subsequent records which is why the number of patients of unknown gender isn't 15.

The simple solution to this problem is to replace the missing values with their true values, in this case the patient's sex:

http://www.sealedenvelope.com/img/stata/xfill4.gif

Now the 14 patients of apparently unknown gender are eliminated from our results. Similar problems occur when summarising static variables using if andin clauses. xfill prevents such confusion arising.