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From |
"Scott Merryman" <smerryman@kc.rr.com> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: Re: how to calculate a function (sum ) of observations without creating a new variable ? |

Date |
Sat, 16 Aug 2003 22:00:08 -0500 |

-tabstat- is probably the easiest way to display a sum . use "C:\Stata8\auto.dta", clear (1978 Automobile Data) . tabstat price if mpg>20, stat(sum) variable | sum -------------+---------- price | 192611 ------------------------ Also, take a look at the saved results for -summarize- Scott ----- Original Message ----- From: "mikhail bontch-osmolovski" <bontchos@email.unc.edu> To: <statalist@hsphsun2.harvard.edu> Sent: Saturday, August 16, 2003 8:33 PM Subject: st: how to calculate a function (sum ) of observations without creating a new variable ? > Dear users of statalist. > This is a long question about functions in Stata . > I would like to know what is the best way to calculate a function of > the data without creating a variable, i.e. without using gen or egen > function. > Situation is simple: I have a big dataset of 8 mlns of observations and > I need to calculate weighted sums of > observations under certain conditions, I need: sum(wt) if b > Obvious way would be to: > 1. egen c=sum(wt) if b > 2. su c, or di c > 3. drop c. > However, this way is clumsy since I have to create additional 8 mln of > observation of c, all equal to each other, > so it takes memory which is very limited in my case and extra time (it > takes very long time) . > As I understand egen sum works by first running gen sum and than > replacing all observations with the last one. > I could not find anything online, so I wrote a simple program which > calculates sum of a variable called wt: > > scalar a=0 > local n=1 > while `n'<=_N { > scalar a=a+wt[`n'] > local n=`n'+1 > } > display a > > I could not believe my eyes, this simle program ran 3 times LONGER then > egen a=sum(wt). > Later I used scalar n instead of local n to save interpretation time, > but it stilled run 3 times longer than egen. > So I was forced to go back to egen, but it is estetically unpleasant to > creat 8 mln observations when you need just a constant and often low > memory does even allow me to have an extra variable. I am using Windows > XP, Stata 8.1. > > So I wonder if you know a good way to calculate a sum of variables and, > in general, and function of variables under certain conditions, like you > can do in Excel, without creating an extra variable?, why egen is faster > than plain sum ? Is this the good case for having a plugin which is > faster ? It is hard to believe that Stata has no commands for such a > simple operation. > > Misha Bonch-Osmolovski > UNC-Chapel Hill, Econ grad > ps. display which is also called hand calcualtor does not allow for if > condition, so it does not work > > > * > * For searches and help try: > * http://www.stata.com/support/faqs/res/findit.html > * http://www.stata.com/support/statalist/faq > * http://www.ats.ucla.edu/stat/stata/ * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**st: RE: Re: how to calculate a function (sum ) of observations without creating a new variable ?***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

**References**:**st: how to calculate a function (sum ) of observations without creatinga new variable ?***From:*mikhail bontch-osmolovski <bontchos@email.unc.edu>

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