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st: mysterious inaccuracy when adding big numbers


From   Trang Nguyen <nqtrang.hanoi@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: mysterious inaccuracy when adding big numbers
Date   Fri, 8 Apr 2011 10:49:34 -0400

Hi.

I am working on a dataset with households as observations that are
nested in communes, districts and provinces. I have variables
- province: province number (3 digits)
- district: district number within each province (max 2 digits)
- commune: commune number within each district (max 2 digits)
- household: household number within each commune (max 2 digits)

I wanted to make a unique ID for each household that doesn't repeat
across communes, districts and provinces, that also shows me all the
province/district/commune information. So I did this:
gen ID = province*1000000 + district*10000 + commune*100 + household

I got a variable ID that is correct for the province, district and
commune components, but the last two digits do not match the value of
the household variable. Instead they are 04 or 12 or 20.

Could someone please help me figure out why this is so? My output is
below. Thanks much!

. gen ID = province*1000000 + district*10000 + commune*100 + household

. count if ID != province*1000000 + district*10000 + commune*100 + household
 7932

. format ID %15.0g

. list province district commune household ID in 1/20

    +------------------------------------------------------+
    | province   district   commune   househ~d          ID |
    |------------------------------------------------------|
 1. |      101          1         3          1   101010304 |
 2. |      101          1         3          2   101010304 |
 3. |      101          1         3          4   101010304 |
 4. |      101          1         3          5   101010304 |
 5. |      101          1        17          4   101011704 |
    |------------------------------------------------------|
 6. |      101          1        17          5   101011704 |
 7. |      101          1        17          6   101011704 |
 8. |      101          1        17          8   101011712 |
 9. |      101          1        17          9   101011712 |
 10. |      101          1        17         10   101011712 |
    |------------------------------------------------------|
 11. |      101          1        17         11   101011712 |
 12. |      101          3         3          3   101030304 |
 13. |      101          3         3          4   101030304 |
 14. |      101          3         3          5   101030304 |
 15. |      101          3         3          6   101030304 |
    |------------------------------------------------------|
 16. |      101          3         3          7   101030304 |
 17. |      101          5        11          3   101051104 |
 18. |      101          5        11          6   101051104 |
 19. |      101          5        11          9   101051112 |
 20. |      101          5        11         10   101051112 |
    +------------------------------------------------------+

. codebook ID

-----------------------------------------------------------------------------------
ID                                                             (unlabeled)
-----------------------------------------------------------------------------------

                 type:  numeric (float)

                range:  [1.010e+08,8.231e+08]        units:  1
        unique values:  1308                     missing .:  0/8341

                 mean:   4.6e+08
             std. dev:   2.6e+08

          percentiles:        10%       25%       50%       75%       90%
                          1.1e+08   2.1e+08   4.1e+08   7.1e+08   8.1e+08

Thanks much!

Trang

------------------------
Trang Nguyen
Doctoral student
Johns Hopkins School of Public Health

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