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From |
Maarten Buis <maartenlbuis@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Count data interaction terms |

Date |
Tue, 23 Aug 2011 11:20:14 +0200 |

On Mon, Aug 22, 2011 at 5:50 PM, DA Gibson wrote: > Im running an investigation into how unhealthy lifestyles (obestiy, > smoking etc) effect health service usage and ideally id like to use a > count analysis to do this. However im not sure whether as with OLS > analyses you can include interaction terms e.g. y = B0 > +B1(X1)+B2(X2)+B3(X1*X2) where X1 and X2 are binary variables. Is this > possible using count data? Yes, the tricky part is interpretation, but that is not a big deal as long as you interpret the incidence rate ratios rather than marginal effects. Consider the example below. The baseline tells you that a visitor to the park without camper, who visited the park alone and has no kid with him/her is expected to catch .39 fish per visit(*). An extra person added to the group increases that rate by a factor 3.07, i.e. (3.07-1)*100%=207 % (the main effect of person). If this extra person happens to a child, this effect of number of persons decreases by a factor .78, i.e. -22%. *------------------ begin example ----------------- webuse fish, clear gen byte baseline = 1 poisson count i.camper c.persons##c.child baseline, /// irr nocons *----------------- end example -------------------- (For more on examples I sent to the Statalist see: http://www.maartenbuis.nl/example_faq ) Also see: M.L. Buis (2010) "Stata tip 87: Interpretation of interactions in non-linear models", The Stata Journal, 10(2), pp. 305-308. Hope this helps, Maarten (*) I still have Stata 11, so I need this trick with a "variable" baseline which is secretly a constant. I understand that in Stata 12 the display of the baseline rates, odds, hazards, etc. are no longer suppressed, so there this trick is no longer necessary. -------------------------- 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/

**Follow-Ups**:**Re: st: Count data interaction terms***From:*DA Gibson <DAGibson1@sheffield.ac.uk>

**References**:**st: Count data interaction terms***From:*DA Gibson <DAGibson1@sheffield.ac.uk>

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