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Re: st: A quick question


From   Phil Schumm <pschumm@uchicago.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: A quick question
Date   Thu, 11 May 2006 19:40:26 -0500

On May 11, 2006, at 5:34 PM, Albert Newton wrote:
I have two varibles: weight and height of respondents in 1989.

weight1998 float %9.0g
height1998 float %9.0g

I want to create a new variable:

gen BMI = weight1998/(height1998^2)

My little problem is that: BMI takes strange values if weight1998 is missing. Could anyone tell me why? Thanks a lot!

height1998 weight1998 BMI1989
1.5748 290 53.04115
1.4732 256 53.50351
1.5494 285 53.84973
1.5494 306 57.8176
1.6764 375 60.52597
1.905 . 124.4902
1.778 . 142.9097
1.778 . 142.9097
1.6764 . 160.757
1.6764 . 160.757
1.6764 . 160.757

It is clear that the variable you are listing (BMI1989) was not calculated using the expression above. For example, if we take the first observation:


. di 290/(1.5748^2)
116.93572


which is not equal to 53.04115. We can help you a lot better if you include part of the actual log. For example:


. des

Contains data
obs: 11
vars: 2
size: 110 (99.9% of memory free)
------------------------------------------------------------------------ -------
storage display value
variable name type format label variable label
------------------------------------------------------------------------ -------
weight1998 int %8.0g
height1998 float %9.0g
------------------------------------------------------------------------ -------
Sorted by:
Note: dataset has changed since last saved

. gen BMI = weight1998/(height1998^2)
(6 missing values generated)

. li

+--------------------------------+
| wei~1998 hei~1998 BMI |
|--------------------------------|
1. | 290 1.5748 116.9357 |
2. | 256 1.4732 117.9551 |
3. | 285 1.5494 118.7183 |
4. | 306 1.5494 127.466 |
5. | 375 1.6764 133.4369 |
|--------------------------------|
6. | . 1.905 . |
7. | . 1.778 . |
8. | . 1.778 . |
9. | . 1.6764 . |
10. | . 1.6764 . |
|--------------------------------|
11. | . 1.6764 . |
+--------------------------------+


BTW, BMI is typically calculated as:

BMI = weight (kg) / height (m^2)

Your heights look about right (for meters), but your weights are off by about a factor of 4.


-- Phil

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