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From |
"Lektzian, David" <[email protected]> |

To |
<[email protected]> |

Subject |
st: Mathematical precision question |

Date |
Tue, 29 Jan 2008 15:20:14 -0600 |

Dear Statalist, I have a simple problem that I am hoping somebody can help me with. I am trying to generate a variable that multiplies three other variables by quantities and then adds them. Specifically, here is what I am trying to do: Var1 = 1972 Var2 = 2 Var3 = 20 I want to generate the variable ddy as follows: .gen ddy = Var1*1000000 + Var2*1000 + Var3 The correct answer, both on my calculator and in excel is: 1972002020 So, what does Stata come up with? Formatting the answer into the 10 digits necessary to view the answer: .Format ddy %10.0f Reveals that Stata's answer is: 1972002048 I'm sure this must be something to do with precision, but I don't know how to fix the problem. When I run this repeatedly, Stata somewhat INconsistently gets this operation wrong every time. In the first example Stata was over by 28. In this next example Stata is under by 36. Var1 Var2 Var3 1973 002 020 = 1973001984 according to Stata when in reality it equals 1973002020 Here Stata is closer only going over by 7. Var1 Var2 Var3 2002 002 055 = 2002002048 according to Stata when in reality it equals 2002002055 I'm sure this problem must have something to do with precision, but I need these answers to be exact. Can somebody help explain to me how to get Stata to perform these mathematical operations correctly? *As a note, I will mention that I am using these variables as ID variables which is why I want them exact. There might be a better way to create ID variables, but I'm a bit of a novice with Stata syntax. Thank you * * 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: Mathematical precision question***From:*"Nick Cox" <[email protected]>

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