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Re: st: creating a new variable


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: creating a new variable
Date   Wed, 18 Jul 2012 12:40:38 +0100

Here are five solutions for a similar problem.

. sysuse auto

. tab rep78, su(mpg)

     Repair |      Summary of Mileage (mpg)
Record 1978 |        Mean   Std. Dev.       Freq.
------------+------------------------------------
          1 |          21   4.2426407           2
          2 |      19.125   3.7583241           8
          3 |   19.433333   4.1413252          30
          4 |   21.666667   4.9348699          18
          5 |   27.363636   8.7323849          11
------------+------------------------------------
      Total |   21.289855   5.8664085          69

. tabstat mpg , by(rep78)

Summary for variables: mpg
     by categories of: rep78 (Repair Record 1978)

   rep78 |      mean
---------+----------
       1 |        21
       2 |    19.125
       3 |  19.43333
       4 |  21.66667
       5 |  27.36364
---------+----------
   Total |  21.28986
--------------------

. graph dot (mean) mpg, over(rep78) vertical

. egen mean_mpg = mean(mpg),  by(rep78)

. scatter mean_mpg rep78

. dotplot mpg, over(rep78) bar


On Wed, Jul 18, 2012 at 11:34 AM, Amal Khanolkar <Amal.Khanolkar@ki.se> wrote:

> 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.
>

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