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From |
wgould@stata.com (William Gould, StataCorp LP) |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Generating time-varying covariates in multiple spell data |

Date |
Wed, 09 Apr 2008 09:53:50 -0500 |

Marjo Pyy-Martikainen <Marjo.Pyy-Martikainen@stat.fi> writes, > thanks for your [...] solution -it works perfectly. > A small remark. Shouldn't the lines (3) and (4) be replaced by > the following lines: > > . stsplit bot, at(11 12 23 24 35 36 47 48) (3) > . gen dummy=(mod(_t,12)==0) (4) > > because isn't it the interval (11,12] that contains > December and not the interval (12,13]? That is, . stsplit bot, at(12 13 24 25 36 38 48 49) <- I wrote . stsplit bot, at(11 12 23 24 35 36 47 48) <- Marjo suggests The answer is that it depends on what 11, 12, and 13 mean, which is to say, how Marjo defined them, or how they were defined for him in the original data. For Marjo to be right, think of continuous time, and define 11, 12, and 13 as being the instant the month ends |December|January | | | | -----------|--------|--------|-----------------> time 11 12 13 For me to be right, think of continuous time, and define 11, 12, and 13 to be the instant the month begins: |November|December| | | | -----------|--------|--------|-----------------> time 11 12 13 When I wrote my answer, I just assumed that months in the data recorded start of month. The fact probably is that 11, 12, and 13 are neither beginnings nor ends of months, but are entire months. The original data was intended to be read, "The value of x was 5 in December", etc. That probably corresponds to months being recorded as starts of months. But Marjo may be correct. There is no right answer and Marjo needs to look back at how the months numbers were defined in the original data, and check for any subsequent processing that might have changed that. Marjo wants a dummy for December. Thus, the dummy must be set to 1 at the span that begins at the start of December and set back to 0 at the span that begins at the start of January. Well, I'm being sloppy about how I just said that because in Stata, time spans are (t0,t1], but you know what I mean. The pictures say it all; don't get hung up on the open/closed end points, think instead in terms of the span. So the answer is either |December|January | | | | -----------|--------|--------|-----------------> time 11 12 13 (--------]-------------------- | | set dummy=1 | on this record | | set dummy=0 on this record or |November|December|January | | | | | -----------|--------|--------|--------|--------> time 11 12 13 14 (--------]-------------------- | | set dummy=1 | on this record | | set dummy=0 on this record -- Bill wgould@stata.com * * 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/

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