Error in xtile command-

[Karim Syed]

Hi all. Please help on the following error.

I run the following commands to generate variables x1_q to create 5 equal quintiles of x1.

sort x1
xtile x1_q=x1, nq(5)

It should create 5 quantiles based on the variable x1, however, when I browse the data I only firnd 1 2 3 and 5 quintiles. Quintile 4 is missing. Please help. Thanks.

[Nick Cox]

The answer will not be a program error, but a misconception about what xtile can do in the face of ties.

Consider the examples below. In one case, there are only 3 distinct values, so that is all xtile can do.

In another case, there are many more distinct values, but xtile can only return 2 quantile bins given the frequency distribution.

You don't show us your data, but I promise that

tab x1 x1_q

will underline the difficulty.

. webuse nlswork
(National Longitudinal Survey.  Young Women 14-26 years of age in 1968)

. xtile race5=race, nq(5)

. tab race race5

           |       5 quantiles of race
      race |         1          4          5 |     Total
-----------+---------------------------------+----------
     white |    20,180          0          0 |    20,180 
     black |         0      8,051          0 |     8,051 
     other |         0          0        303 |       303 
-----------+---------------------------------+----------
     Total |    20,180      8,051        303 |    28,534 


. xtile wks_ue5=wks_ue

. tab wks_ue wks_ue5

     weeks |
unemployed | 2 quantiles of wks_ue
 last year |         1          2 |     Total
-----------+----------------------+----------
         0 |    17,394          0 |    17,394 
         1 |         0        659 |       659 
         2 |         0        704 |       704 
         3 |         0        447 |       447 
         4 |         0        582 |       582 
         5 |         0        207 |       207 
         6 |         0        253 |       253 
         7 |         0        131 |       131 
         8 |         0        307 |       307 
         9 |         0        137 |       137 
        10 |         0        172 |       172 
        11 |         0         72 |        72 
        12 |         0        241 |       241 
        13 |         0        134 |       134 
        14 |         0         59 |        59 
        15 |         0         69 |        69 
        16 |         0         98 |        98 
        17 |         0         71 |        71 
        18 |         0         78 |        78 
        19 |         0         29 |        29 
        20 |         0         89 |        89 
        21 |         0         31 |        31 
        22 |         0         65 |        65 
        23 |         0         15 |        15 
        24 |         0         36 |        36 
        25 |         0         36 |        36 
        26 |         0        117 |       117 
        27 |         0         37 |        37 
        28 |         0         40 |        40 
        29 |         0         26 |        26 
        30 |         0         41 |        41 
        31 |         0         20 |        20 
        32 |         0         50 |        50 
        33 |         0         23 |        23 
        34 |         0         21 |        21 
        35 |         0         33 |        33 
        36 |         0         31 |        31 
        37 |         0         17 |        17 
        38 |         0         14 |        14 
        39 |         0         13 |        13 
        40 |         0         32 |        32 
        41 |         0          9 |         9 
        42 |         0         18 |        18 
        43 |         0         10 |        10 
        44 |         0         23 |        23 
        45 |         0          9 |         9 
        46 |         0         18 |        18 
        47 |         0         12 |        12 
        48 |         0         19 |        19 
        49 |         0          9 |         9 
        50 |         0         18 |        18 
        51 |         0          8 |         8 
        52 |         0         34 |        34 
        53 |         0          3 |         3 
        54 |         0          1 |         1 
        55 |         0          2 |         2 
        56 |         0          2 |         2 
        62 |         0          1 |         1 
        73 |         0          1 |         1 
        75 |         0          1 |         1 
        76 |         0          1 |         1 
-----------+----------------------+----------
     Total |    17,394      5,436 |    22,830

[Karim Syed]

Thank you, Nick Cox, for your detailed and very helpful response. Yes, I do encounter the second case you have mentioned. Thanks.