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Re: st: Controling precision for multiple runs of same code


From   Phil Clayton <philclayton@internode.on.net>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Controling precision for multiple runs of same code
Date   Tue, 4 Jun 2013 10:05:28 +1000

The first thing that stands out is:
merge m:m ...

You should pretty much never do this. Don't take my word for it - the manual entry for -merge- says "m:m specifies a many-to-many merge and is a bad idea" and explains why, including that you might get non-reproducible results.

You probably want m:1. If for some reason you definitely need to join all records in a many-to-many fashion based on one or more ID variables, you should use -joinby-.

Phil

On 04/06/2013, at 9:55 AM, Melanie Leis <melanieleis@gmail.com> wrote:

> Hello,
> 
> I'm having trouble with a section of my code that yields different
> results each time I run it.
> 
> I start out with a dataset, baseline_4.dta, which has 47,267,047
> observations and 16 variables, and run this:
> 
> merge m:m statefips agecat_census using "ABCD.dta"
> assert _merge==3
> drop _merge
> egen tot_pop=sum(pop), by(statefips countyfips agecat_census sexcat
> racecat iprcat_mpact iprcat coverage groupsize)
> checkpop
> rename pop oldpop
> gen pop=tot_pop*prob_agecat_mpact
> checkpop
> collapse (sum) pop, by(statefips countyfips agecat_mpact sexcat
> racecat iprcat_mpact iprcat coverage groupsize)
> checkpop
> sum
> sort _all
> save "baseline_5.dta", replace
> 
> checkpop is a program that tells me what my total population is each
> time I run it. My total population is the same before and after the
> collapse function (see results below).
> 
> At the end, my total population and my number of observations in
> baseline_5.dta is different every time I run this. I suspect the
> difference is in rounding when it executes the gen pop line, but I've
> tried replacing it for
> 
> gen double pop=tot_pop*prob_agecat_mpact
> 
> and
> 
> gen float pop=tot_pop*prob_agecat_mpact
> 
> And I still get differences.
> 
> I tried using
> 
> gen long pop=tot_pop*prob_acegat_mpact
> 
> But I lost too much precision by doing this.
> 
> Could you please recommend a solution to obtain the exact same numbers
> in each run, without sacrificing precision?
> 
> Thanks!
> 
> Melanie
> 
> The log file for 2 of the runs I've done:
> 
> ************* RUN A ***********************
> 
> . merge m:m statefips agecat_census using "ABCD.dta"
> 
>    Result                           # of obs.
>    -----------------------------------------
>    not matched                             0
>    matched                        47,267,047  (_merge==3)
>    -----------------------------------------
> 
> . assert _merge==3
> 
> . drop _merge
> 
> . egen tot_pop=sum(pop), by(statefips countyfips agecat_census sexcat
> racecat iprcat_mpac
>> t iprcat coverage groupsize)
> 
> . checkpop
> 
> Total pop:       347,095,179
> Observations:     47,267,047
> Missing:                   0
> 
> . rename pop oldpop
> 
> . gen pop=tot_pop*prob_agecat_mpact
> 
> . checkpop
> 
> Total pop:       332,455,972
> Observations:     47,267,047
> Missing:                   0
> 
> . collapse (sum) pop, by(statefips countyfips agecat_mpact sexcat
> racecat iprcat_mpact ip
>> rcat coverage groupsize)
> 
> . checkpop
> 
> Total pop:       332,455,972
> Observations:     36,351,520
> Missing:                   0
> 
> 
> ************** RUN B *************
> . merge m:m statefips agecat_census using "ABCD.dta"
> 
>    Result                           # of obs.
>    -----------------------------------------
>    not matched                             0
>    matched                        47,267,047  (_merge==3)
>    -----------------------------------------
> 
> . assert _merge==3
> 
> . drop _merge
> 
> . egen tot_pop=sum(pop), by(statefips countyfips agecat_census sexcat
> racecat iprcat_mpac
>> t iprcat coverage groupsize)
> 
> . checkpop
> 
> Total pop:       347,095,179
> Observations:     47,267,047
> Missing:                   0
> 
> . rename pop oldpop
> 
> . gen pop=tot_pop*prob_agecat_mpact
> 
> . checkpop
> 
> Total pop:       332,455,928
> Observations:     47,267,047
> Missing:                   0
> 
> . collapse (sum) pop, by(statefips countyfips agecat_mpact sexcat
> racecat iprcat_mpact ip
>> rcat coverage groupsize)
> 
> . checkpop
> 
> Total pop:       332,455,928
> Observations:     36,351,515
> Missing:                   0
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