Question about less than missing (<".")

[Liang Wang Jack]

Hi all,

I have a question about

<"."

and

!="."

So what is the difference between these two?

Best,
Jack LiangWang

[Nick Cox]

Your context here is string values, but as with numeric values but for different reasons their results can be quite different. The operator < (less than) means "would sort before" but sorting before is a matter of dictionary order.

Here is a trivial example:

. di ("a" < ".")
0

. di ("a" != ".")
1

It's true that the character (letter) a is not equal to the character . but false that it would sort earlier.

[Steve Samuels]

In addition to ".", Stata has extended missing values for numeric variables and sort them: . .a .b .c .....

 if x <.

excludes all missing values

 if x !=.

will include missing values larger than .
If there are no extended missing, then the two are equivalent.

sysuse auto, clear
replace rep78 = .a in 5   //extended missing
tab rep78, missing
   Repair |
Record 1978 |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |          2        2.70        2.70
          2 |          8       10.81       13.51
          3 |         30       40.54       54.05
          4 |         17       22.97       77.03
          5 |         11       14.86       91.89
          . |          5        6.76       98.65
         .a |          1        1.35      100.00
------------+-----------------------------------
      Total |         74      100.00

 tab rep78 if rep78<., missing

     Repair |
Record 1978 |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |          2        2.94        2.94
          2 |          8       11.76       14.71
          3 |         30       44.12       58.82
          4 |         17       25.00       83.82
          5 |         11       16.18      100.00
------------+-----------------------------------
      Total |         68      100.00

. tab rep78 if rep78!=., missing

     Repair |
Record 1978 |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |          2        2.90        2.90
          2 |          8       11.59       14.49
          3 |         30       43.48       57.97
          4 |         17       24.64       82.61
          5 |         11       15.94       98.55
         .a |          1        1.45      100.00
------------+-----------------------------------
      Total |         69      100.00

[Liang Wang Jack]

Great, I got it. Thank you all.

Best,
Jack LiangWang