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From |
ciccarec@uniroma2.it |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: RE: sum: collapse vs egen |

Date |
Sat, 05 Sep 2009 18:40:01 +0200 |

Hello Nick, thanks for quick answering. Quoting "Nick Cox" <n.j.cox@durham.ac.uk>: 3. Not your question, but -egen, sum()- is a poor way to do a sum. but in the numerical example I provided egen,sum() turns out to provide the exact answer (8.76), which I can't get using collapse (sum). My problem is that I never use "egen, sum" when working with real data while I often use "collapse (sum)", that seems to be not very appropriate. I think the problem is in the way the "collapse" command and the "round" function are related: I verified that if the line "gen double r2=round(a*100)/100" (see after the "collapse" command) is separated in 2 parts, like: collapse (sum) a gen double temp=a*100 gen double r2=round(temp)/100 The resulting r2 is correct (but I don't know why). Many thanks, Carlo Quoting "Nick Cox" <n.j.cox@durham.ac.uk>:

I have three comments here. 1. -egen- by default will generate -float- variables (unless you have -set type double-). So, you shouldn't be surprised to lose a little precision there. Functions like -round()- that may make knife-edge decisions are likely to show this up. The first FAQ cited below is a similar case. 2. This is all part of a larger issue: Stata works in binary. Stata does not do decimal arithmetic! See for example FAQ . . . . . . . . . . . . . . . . . . . Results of the mod(x,y) function . . . . . . . . . . . . . . . . . . . . . N. J. Cox and T. J. Steichen 2/03 Why does the mod(x,y) function sometimes give puzzling results? Why is mod(0.3,0.1) not equal to 0? http://www.stata.com/support/faqs/data/mod.html FAQ . . . . . . . . . . . . . . . . . The accuracy of the float data type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . W. Gould 5/01 How many significant digits are there in a float? http://www.stata.com/support/faqs/data/prec.html FAQ . . . . . . . . . Comparing floating-point values (the float function) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. Wernow 3/01 Why can't I compare two values that I know are equal? http://www.stata.com/support/faqs/data/float.html FAQ . . . . . . . . . Why am I losing precision with large whole numbers? . . . . . . . . . . . . . . . . . . UCLA Academic Technology Services 7/08 http://www.ats.ucla.edu/stat/stata/faq/longid.htm SJ-8-2 pr0038 Mata Matters: Overflow, underflow & IEEE floating-point format . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. M. Linhart Q2/08 SJ 8(2):255--268 (no commands) focuses on underflow and overflow and details of how floating-point numbers are stored in the IEEE 754 floating-point standard SJ-6-4 pr0025 . . . . . . . . . . . . . . . . . . . Mata matters: Precision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . W. Gould Q4/06 SJ 6(4):550--560 (no commands) looks at programming implications of the floating-point, base-2 encoding that modern computers use SJ-6-2 dm0022 . Tip 33: Sweet sixteen: Hexadec. formats & precision problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N. J. Cox Q2/06 SJ 6(2):282--283 (no commands) tip for using hexadecimal formats to understand precision problems in Stata 3. Not your question, but -egen, sum()- is a poor way to do a sum. A better way is -summarize, meanonly-. However, I guess that your real problem is understanding what -egen- does with some real data, but nevertheless note using -summarize- directly and picking up r(sum) is always better for a single sum. Nick n.j.cox@durham.ac.uk Carlo Here is my code: clear version 9.2 set obs 1 gen double a=3.86 save data1,replace clear set obs 1 gen double a=4.895 save data2,replace use data1,clear append using data2 egen sum=sum(a) gen double r1=round(sum*100)/100 list collapse (sum) a gen double r2=round(a*100)/100 list Why are r1 and r2 not equal ? * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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**Follow-Ups**:**RE: st: RE: sum: collapse vs egen***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

**References**:**st: sum: collapse vs egen***From:*ciccarec@uniroma2.it

**st: RE: sum: collapse vs egen***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

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