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From |
"Carlo Lazzaro" <carlo.lazzaro@tin.it> |

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

Subject |
st: R: creating differences when time periods are misssing |

Date |
Thu, 1 Nov 2007 11:11:59 +0100 |

Dear Scott, there possibly another problem arising from your post, that is the reason why these data are missing. Are they missing completely at random (MCAR), at random (MRA)or otherwise their missing depends upon the unobserved answers (not missing at random (NMAR)? Should your missing be MCAR or MRA type, I suppose you can take a look at an article I am going to send you off the list. Sorry I cannot be more helpful as Stata codes are concerned. Kind Regards, Carlo -----Messaggio originale----- Da: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] Per conto di Scott Cunningham Inviato: mercoledì 31 ottobre 2007 19.34 A: statalist@hsphsun2.harvard.edu Oggetto: st: creating differences when time periods are misssing My data is a longitudinal dataset of individuals who were interviewed from 1997 to 2004. I have data on individual ages (measured as months from birth month). Because this interview did not always, consistently, ask individuals exactly 12 months after the last interview, I have been trying to control for differences in time since the last interview by differencing their ages as so: . gen dt=d.amo where "amo" is "age in months." I notice that this works so long as I have values of amo in both the current and previous year. But there are some people who disappear from the survey only to return a year later. They look like this: +----------------------------------+ | id rp age amo dt year | |----------------------------------| 56. | 27 0 15 189 12 1997 | 57. | 27 . . . . 1998 | 58. | 27 4 18 226 . 1999 | 59. | 27 3 19 237 11 2000 | 60. | 27 8 20 247 10 2001 | 61. | 27 6 21 259 12 2002 | 62. | 27 4 22 273 14 2003 | |----------------------------------| 63. | 27 1 23 283 10 2004 | The relevant variables are: id (indiciating this is the same person), amo (age in months on day of interview), dt (time since last interview), and year. Ignore the "rp" variable, but note that this variable measures something which depends on "dt" since it is a measure of something done since the date of the last interview. So, the problem is "dt" is missing twice. Once when all values are missing because the person was not interviewed. A second time when he comes back in. Ideally, I would like to know how to create differenced values for dt equal to (226-189), since the respondent is 226 months old on the day of the interview and was 189 the last time interviewed. What's the most efficient code to do this? * * 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/

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