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| From | Maarten Buis <maartenlbuis@gmail.com> |
| To | statalist@hsphsun2.harvard.edu |
| Subject | Re: st: Count variables and growth curves |
| Date | Thu, 28 Jun 2012 12:43:39 +0200 |
On Thu, Jun 28, 2012 at 12:05 PM, Brendan Churchill wrote: > I am grouping year of birth into small five-year birth cohorts. I have year of birth from 1909 to 1986. When I recode this into 14 birth cohorts (for example 1981-1986 = 14). These have been coded from 0 to 14. I have also tried 1 to 15, but still the erroneous constant problem remains. I have two guesses: 1) You made an error when coding this variable and the new variable does not go from 0-14 or 15, but there are still some outliers present. This will particularly be important when you incorrectly added the new cohort variable as a continuous variable. Such coarsening only makes (some very small amount of) sense when you added them as a categorical variable (i.e. use the factor variable notation, see -help fvvarlist-) 2) You correctly added this as a categorical variable, but the reference category is just too small to obtain reliable estimates of the expected outcome. You will see that the confidence interval around the constant will be huge. I would typically prefer not to coarsen birthyear, but instead use splines to add a flexible effect of year of birth. See -help mkspline-. Hope this helps, Maarten -------------------------- Maarten L. Buis Institut fuer Soziologie Universitaet Tuebingen Wilhelmstrasse 36 72074 Tuebingen Germany http://www.maartenbuis.nl -------------------------- * * 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/