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| Title | Calculating confidence intervals | |
| Author | Nicholas Cox, Durham University, UK | |
| Date | March 1999 |
| I am trying to use Stata to calculate confidence intervals quickly for a large amount of data. I have been using the immediate command cii to calculate each confidence interval but I do not want to have to retype the results to make use of them. How do I accumulate the results of each calculation automatically into a new data set? |
cii 12 56 34
cii 21 65 43
and so on, where the three numbers are the number of observations, the mean and
the standard deviation in each case.
To accumulate the results, we exploit the fact that cii leaves in its wake not just the printed results, but also saved results that can be used either interactively or in a program. So we can pick those up and put them in variables as part of a data set that grows as we calculate. We will explain how to do this under Stata 6.0, and then the small modification needed for Stata 5.0.
First set up the scenery. If you have data in memory, clear them and
set obs 1
gen N = .
gen mean = .
gen se = .
gen lb = .
gen ub = .
then set up a do file mycii.do
-------------- mycii.do
noi cii `1' `2' `3'
qui replace obs = r(N) in l
qui replace mean = r(mean) in l
qui replace se = r(se) in l
qui replace lb = r(lb) in l
qui replace ub = r(ub) in l
local n = _N + 1
qui set obs `n'
-------------------
Make sure that the l in in l is the letter l (standing
for last), not the numeral 1 (which would mean first). In this
program the r( ) are the saved results documented under Stata
Reference Manual Set — Volume 1 [R] ci. The '1', '2' and '3'
refer to the three numbers supplied to cii, its arguments in
programming jargon.
Now type away
run mycii 12 56 34
run mycii 21 65 43
and each time you run this do file, the last observation (initially also the
first) will be replaced and the number of observations in the data set will be
bumped up by 1.
You can promote your do file to a program
-------------- mycii.ado
program def mycii
version 6.0
cii `1' `2' `3'
qui replace N = r(N) in l
qui replace mean = r(mean) in l
qui replace se = r(se) in l
qui replace lb = r(lb) in l
qui replace ub = r(ub) in l
local n = _N + 1
qui set obs `n'
end
-------------------
so that you can go
mycii 12 56 34After the last calculation you have a new dataset: junk the last observation which is all missing values.
The same approach will work with any immediate command. Just write your do file or program to pick up the saved results as documented in the manual entry on the immediate command.
(Users of version 5.0 will not be able to use the r( ) notation, which was introduced in Stata 6.0. You must refer to $S_1, $S_3, etc. See the documentation of saved results in your manual. And of course version 6.0 in the program above should be version 5.0.)
