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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