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From |
"Nick Cox" <[email protected]> |

To |
<[email protected]> |

Subject |
st: RE: Re: strange world |

Date |
Wed, 9 Jan 2008 14:12:21 -0000 |

Ron�n is right to remind us that logical (true-or-false) conditions can be combined, and I'll say more about this in another post, but this example would I think not bite users hard even with a three-way logic in which such conditions coould take on possible values of True, False, or Missing. Thus condition | condition | condition | ... would surely be treated as True if at least one of the conditions were True, and as Missing if and only if all the conditions were Missing. Nick [email protected] Ronan Conroy There's another consideration too. Logical operators are often found in complex expressions. While sometimes you have to guard against missing values, some expressions depend on all variable being nonmissing, while others do not. Consider . gen underweight = (bmi1 < 19 ) | (bmi2 < 19) | (bmi3 < 19) . lab var underweight "At least one body mass index below 19" The variable thus defined can be calculated even when one or two of the bmi variables are missing. If that's fine by you, then Stata should not stand in your way. The user might specify . egen bmi_missing = rowmiss(bmi1 bmi2 bmi3) . gen underweight = (bmi1 < 19 ) | (bmi2 < 19) | (bmi3 < 19) if bmi_missing < 2 which would allow the expression to be evaluated if there were at least two BMI measurements. But the choice of how many missing measurements to tolerate has to be a scientific one. For this reason, I think that the user is the only person who knows under what circumstances a logical expression should evaluate to missing. It's unfortunate that Stata, SAS and SPlus/R have different ways of handling missing data in logical expressions, but I don't think that switching to the S philosophy that x < NA evaluates to NA is going to be any easier. * * 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**:**Re: st: RE: Re: strange world***From:*"Austin Nichols" <[email protected]>

**References**:**st: Re: strange world***From:*Maarten buis <[email protected]>

**st: Re: strange world***From:*Ronan Conroy <[email protected]>

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