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Lets say I want to regress a variable on a covariate which has distinct value zero but the all other values are following some smooth function? It could be e.g. Years of beeing a mom (once you are a mom you are very different from 0 years, i.e. the effect of going from 0 to 0.0001 year should be a jump, but from 0.0001 to 0.0002 sould be smooth). Or maybe exposre is 'Time of exposure to HPV' when we study cervix cancer?

One simple thing would be to add two covariates 'Years of being mom' and 'Are you a mom (yes/no)', but then all values would be 'No' exactly when the other variable is '0'.

Any ideas? Hierarchical model? If so, how?

PeterStrom
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  • This question is answered in many threads here, covering many similar applications (regression, logistic regression, covariates, dependent variables, etc.). They are easier to uncover by including "log" in the search terms: [zero regression log](http://stats.stackexchange.com/search?tab=votes&q=Zero%20regression%20log). – whuber Apr 11 '14 at 14:29
  • What's your concern about using two co-variates? Surely you'd *want* values of 'Are you a mom' to be 'No' when & only when 'Years of being mom' is zero. – Scortchi - Reinstate Monica Apr 11 '14 at 14:39

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