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From |
Adriaan Hoogendoorn <aw.hoogendoorn@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: How to model a positive continuous dependent variable with many zeros? |

Date |
Tue, 31 May 2011 09:24:01 +0200 |

Dear Statalisters, I try to run regression models for two dependent variables that concern the seclusions of psychiatric patients: y1 = the number of seclusion incidents and y2 = the seclusion duration. Fortunately (at least from the patients perspective), there are many zeroes. I successfully applied a Poisson model (xtpoisson) to model the number of seclusion incidents for patients (level 1) in different clinics (level 2) taking exposure time t (the time that a psychiatric patient spent in the clinic) into account: xtpoisson y1 x1 x2 x3, re exposure(t) I am running into problems when I try modeling the duration of seclusions. Because of the many zeroes (85%) and the successful analysis of the number of seclusion incidents, I applied the xtpoisson model for the duration variable as well. Obviously duration it is not exactly a count variable, but I can count the number of hours within the duration, or the number of days, can’t I? So I estimated the duration counting the number of hours. The problem appears when I alternatively estimate the duration by counting the number of hours. I seem to get different model estimates for duration when I count the number of days than when I count the number of hours. In fact, not so much the parameter estimates change, but their significance levels are very sensitive to the scale (days or hours or even minutes) on which duration is measured. It appears that in xtpoisson y2 x1 x2 x3, re exposure(t) it does not matter if “t” is measured in days or hours, but it does matter if the duration “y2” is measured in days, hours or minutes. So my trick of counting days or hours seems to fail, and modeling seclusion duration by a poisson model seems not a good idea. Therefore my question to you is: do you know of a model that can deal with a positive continuous dependent variable (duration) with many zeros? Kind regards, Adriaan Hoogendoorn GGZ inGeest, Amsterdam * * 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: How to model a positive continuous dependent variable with many zeros?***From:*Hitesh Chandwani <hchandwani.stata@gmail.com>

**Re: st: How to model a positive continuous dependent variable with many zeros?***From:*Maarten Buis <maartenlbuis@gmail.com>

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