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# st: creating a new variable

 From Amal Khanolkar To "statalist@hsphsun2.harvard.edu" Subject st: creating a new variable Date Wed, 18 Jul 2012 10:34:22 +0000

```Hi all,

I have a very simple problem that I'm unable to find a simple solution for:

Below is the data concerned:

Gestational age in weeks:

tab gestwk

gestwk |      Freq.     Percent        Cum.
------------+-----------------------------------
22 |        134        0.00        0.00
23 |        387        0.01        0.02
24 |        738        0.02        0.04
25 |      1,235        0.04        0.08
26 |      1,688        0.06        0.14
27 |      2,125        0.07        0.21
28 |      2,723        0.09        0.30
29 |      3,415        0.11        0.42
30 |      4,481        0.15        0.57
31 |      5,876        0.20        0.76
32 |      8,533        0.29        1.05
33 |     12,958        0.43        1.49
34 |     21,420        0.72        2.20
35 |     36,710        1.23        3.44
36 |     70,297        2.36        5.79
37 |    151,310        5.07       10.87
38 |    373,660       12.53       23.40
39 |    660,536       22.15       45.55
40 |    822,376       27.58       73.13
41 |    542,442       18.19       91.33
42 |    219,603        7.37       98.69
43 |     31,928        1.07       99.76
44 |      5,470        0.18       99.94
45 |      1,648        0.06      100.00
------------+-----------------------------------
Total |  2,981,693      100.00

Mean birth weight of my study sample:

. sum bw

Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
bw |   2980093    3502.431    575.7603        300       6780

sum bw if gestwk==26

Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
bw |      1610    902.7248    189.5523        350       1970

. sum bw if gestwk==26

Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
bw |      1610    902.7248    189.5523        350       1970

Below, if I would like to look at the mean birth weight for a particular gestational week:

. sum bw if gestwk==27

Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
bw |      2024    1014.961     201.809        380       1920

. sum bw if gestwk==28

Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
bw |      2613    1138.658     238.724        370       2000

. sum bw if gestwk==29

Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
bw |      3316    1295.815    278.1803        370       2480

What I would like to do is to create a single continuous variable that would give me the mean birth weight for each gestational week so that I don't have to look at it individually as above. I would like to ideally be able to use this variable in scatter plots.

If I plot as follows:

scatter twoway bw gestwk

I of course don't get a single estimate for each gestational week, but instaed the entire range of birth weight for a particular week is plotted.

Thanks!

/Amal.

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```