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st: creating a new variable


From   Amal Khanolkar <Amal.Khanolkar@ki.se>
To   "statalist@hsphsun2.harvard.edu" <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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