Re: st: speed question: -collapse- vs -egen-

["Michael Blasnik" <michael.blasnik@verizon.net>]

...

You can gain some speed in regular Stata code by not generating a separate 
variable just to count the number of non-missings:

bysort rep78: gen mean=sum(price)/sum(price<.)
by rep78: keep if _n==_N

On my machine, this reduces the time required for the corrected Stas code from 
17.3 to 13.8 s.

Michael Blasnik

----- Original Message ----- 
From: "Sergiy Radyakin" <serjradyakin@gmail.com>
To: <statalist@hsphsun2.harvard.edu>
Sent: Friday, April 25, 2008 9:12 PM
Subject: Re: st: speed question: -collapse- vs -egen-



> Hello All!
>
> Jeph has asked about an efficient way of creating a dataset with means
> of one variable over the categories of another variable. He suggested
> two possible solutions and Stas added a third one.
>
> Below I report performance of each of these methods and compare it
> with the fourth: a plugin.
>
> I use an expanded version of auto.dta and tabulate mean {price} by
> different levels of {rep78}.
>
> 1. All methods resulted in the following table of results*
>
>    meanprice   rep78
>       4564.5       1
>     5967.625       2
>     6429.233       3
>       6071.5       4
>         5913       5
>
> 2. The timing is as follows (Stata SE, Windows Server 2003, 32-bit)
>
>   1:     33.80 /        1 =      33.7960
>   2:     31.22 /        1 =      31.2190
>   3:     21.33 /        1 =      21.3280
>   4:      5.58 /        1 =       5.5780
<snip>

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