Re: st: Going through each observation of a variable

[David Kantor <kantor.d@att.net>]

At 10:52 AM 6/7/2013, Derya wrote:
> Dear Statalist users,
>
> I am newbie in Stata programming and I am stuck on something that is 
> probably very simple - but I could not find the answer on the list server.
> I would like to compute the mean and standard deviation of an 
> expression (P1*os1+P2*os2) that is computed for prices under 
> different scenarios. I have 500 scenarios, so the variables price1 
> and price2 have 500 observations.
> I came up with the program below, which works fine, but it chooses 
> random values of price1 and price2 variables. I would like like the 
> program go through each observation of price1 price2 one by one. How 
> would I do that?
> Any help will be appreciated!
>
> Thanks a lot!
> Derya
>
>
> gen k=0
> gen wmean=0
> gen wsum=0
> gen wsqdev=0
> gen wsd=0
>
>
> forv k=1/500 {
>
> gen r = uniform()
> sort r
> gen select =_n==1
>      scalar P1=price1
>      scalar P2=price2
> drop r select
> gen Y_`k'=P1*os1+P2*os2
> replace k=`k'
> replace wsum=wsum+Y_`k'
> replace wmean=wsum/`k'
> replace wsqdev=wsqdev+((wmean-Y_`k')^2)
> replace wsd=sqrt(wsqdev/(k-1))
>
> }
It is not clear what you are trying to do and why you are sorting on 
a random order 500 times.
(And even without the random ordering, there is usually no need to 
step through the observations.)
Just a few minor points to start with:
You indicated that variables price1 and price2 have 500 observations. 
It is more accurate to say that your dataset has 500 observations.
The commands
        scalar P1=price1
        scalar P2=price2
take their values from the first observation.
The variable select is unused.
----

Putting those matters aside, is all you want just the mean and 
standard deviation of price1*os1+price2*os2 ?
Then generate a variable containing that value, and summarize it:
gen newvariable = price1*os1+price2*os2
summ newvariable

HTH
--David

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