Re: st: mean and SD by categories

[Nick Cox <njcoxstata@gmail.com>]

There are various problems with your approach.

1. The immediate problem is that you cycle over combinations of
categories creating local macros with names like mean111, sd111, and
so forth.

But a command like

display mean111

is understood by Stata to mean that you want to see a value of a
variable -mean111- or a scalar -mean111-, but you have no such
variable or scalar, hence the message you received.

To see the value of a local macro, you would need to do something like

display `mean111'

2. Adding +/- 1.96 SD to the mean is an attempt to get 95% confidence
intervals. But there are at least two problems here. 1.96 is an
optimistic multiplier and a more accurate multiplier would be based on
the t-distribution. However, that's secondary compared with using SDs
when you should be using standard errors.

3. A strategic difficulty here is that creating lots of local macros
first is not a good way to create graphs. In fact if you are a novice
in programming trying to program graphs from scratch is not advisable.
In your case 70 cross-combinations seems likely to produce only a very
complicated graph. Much depends on whether each cross-combination is
well enough represented to show stable patterns.

-stripplot- from SSC may be of help. It has a -ci- option that means
that confidence intervals are shown for you.

Nick
njcoxstata@gmail.com


On 16 July 2013 12:43, Dherani, Mukesh <M.K.Dherani@liverpool.ac.uk> wrote:
> Dear all,
> I am very novice in programming.
> I have a dataset with BMI by three categorical variables age (7 groups), sex (2 groups) and SES (5 groups).  I was to generate a line graph showing mean BMI with 95%CI by age , sex  and SES.  Below is my code to create local that I want to use in twoway line graph:
>
> forval a=1/2 {
> forval b=1/7 {
> forval c=1/5  {
> sum bmi if sex==`a' & agegp==`b' & ses==`c'
> local mean`a'`b'`c'=r(mean)
> local sd`a'`b'`c'=r(sd)
> local up`a'`b'`c'=r(mean)+(1.96*r(sd))
> local lo`a'`b'`c'=r(mean)-(1.96*r(sd))
> di mean`a'`b'`c' sd`a'`b'`c' up`a'`b'`c' lo`a'`b'`c'
> }
> }
> }
>
>
> At the end of this command stata says mean111 was not found!
> Secondly, I want graph to show me mean bmi (y axis) and age (x axis) for each ses for each sex separately. Should I carry this out in the loop or outside the loop?
>
> Any help is highly appreciated.
>
> BW,m
>
>
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