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Re: st: creating a new variable


From   Steve Nakoneshny <scnakone@ucalgary.ca>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: creating a new variable
Date   Wed, 18 Jul 2012 05:40:45 -0600

Hi Amal,

See -help egen-. A potential solution for you could look something like:

bys gestwk: egen mbw = mean(bw)

Steve

Sent via carrier pigeon

On 2012-07-18, at 4:34 AM, "Amal Khanolkar" <Amal.Khanolkar@ki.se> wrote:

> 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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