Form deciles with no 0 observations

[Astrid Koss]

Hello,

Can you please help me with the following issue: I want to form deciles based on the credit rating for different bonds.
The credit rating variable (rating_num) can take values between 1 and 22.
I tried 2 different methods and they yield the same result:

bys monthly_dates: astile decile_cr_monthly = rating_num, nq(10)

egen decile_r_num = xtile(rating_num ) , by(monthly_dates ) nq(10)

The problem is that in deciles 5 and 7 I have 0 observations.
I understand that this is due to the cutoff point.
I was wondering if there is a way to define the cutoff points such that I have observations in each of the 10 deciles?

Thank you very much for your help!

[Nick Cox]

If you have 10 or more distinct values, then you can always group them into 10 classes, but very likely they won't be deciles (meaning, decile bins with each 1/10 of the values).

tab rating_num

quantile rating_num

will usually make the problem clear.

More discussion in

SJ-18-3 dm0095 . . . . . . . . . . . Speaking Stata: From rounding to binning
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N. J. Cox
Q3/18 SJ 18(3):741--754 (no commands)
basic review of how to bin variables in Stata, meaning how to
divide their range or support into disjoint intervals

https://www.stata-journal.com/articl...article=dm0095 [needs subscription access or one-off payment]


SJ-12-4 pr0054 . . . . . . . . . . Speaking Stata: Matrices as look-up tables
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N. J. Cox
Q4/12 SJ 12(4):748--758 (no commands)
illustrates the use of matrices as look-up tables

https://www.stata-journal.com/sjpdf....iclenum=pr0054 (see Section 4) [accessible to all]

With integer values, ties are often common and may well rule out equal frequencies in quantile bins. Here are some results making the point:

. sysuse auto, clear
(1978 Automobile Data)

* -distinct- is from the Stata Journal and must be installed before it can be used
. distinct

-------------------------------------
              |     total   distinct
--------------+----------------------
         make |        74         74
        price |        74         74
          mpg |        74         21
        rep78 |        69          5
     headroom |        74          8
        trunk |        74         18
       weight |        74         64
       length |        74         47
         turn |        74         18
 displacement |        74         31
   gear_ratio |        74         36
      foreign |        74          2
-------------------------------------

. tab trunk

Trunk space |
  (cu. ft.) |      Freq.     Percent        Cum.
------------+-----------------------------------
          5 |          1        1.35        1.35
          6 |          1        1.35        2.70
          7 |          3        4.05        6.76
          8 |          5        6.76       13.51
          9 |          4        5.41       18.92
         10 |          5        6.76       25.68
         11 |          8       10.81       36.49
         12 |          3        4.05       40.54
         13 |          4        5.41       45.95
         14 |          4        5.41       51.35
         15 |          5        6.76       58.11
         16 |         12       16.22       74.32
         17 |          8       10.81       85.14
         18 |          1        1.35       86.49
         20 |          6        8.11       94.59
         21 |          2        2.70       97.30
         22 |          1        1.35       98.65
         23 |          1        1.35      100.00
------------+-----------------------------------
      Total |         74      100.00



. xtile trunk10=trunk, nq(10)

. tab trunk10

         10 |
  quantiles |
   of trunk |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |         10       13.51       13.51
          2 |          9       12.16       25.68
          3 |          8       10.81       36.49
          4 |          3        4.05       40.54
          5 |          8       10.81       51.35
          6 |         17       22.97       74.32
          8 |          8       10.81       85.14
          9 |          7        9.46       94.59
         10 |          4        5.41      100.00
------------+-----------------------------------
      Total |         74      100.00

[Astrid Koss]

Hi Nick,

Thank you so much for your answer!