Off-list requests have suggested that more details might be useful.
The hurdle or two-step models are based on the following approach:
Step 1 (the hurdle): divide the response variable into two classes: 0 and
non-zero. Predict the probability of 0 using a glm or glmm.
Step 2: analyze only the non-zero data using whatever model is appropriate opr
necessary.
See e.g. Ferguson, D. E., A. R. Stage, and R. J. Boyd. 1986. Predicting
regeneration in the grand fir-cedar-hemlock ecosystem of the northern Rocky
Mountains. Forest Science Monograph 1986, No 26.
Other tools, such as ZIP (zero-inflated Poisson) etc. might also be
appropriate.
Andrew.
--
Andrew Robinson Ph: 208 885 7115
Department of Forest Resources Fa: 208 885 6226
University of Idaho E : andrewr@uidaho.edu
PO Box 441133 W : http://www.uidaho.edu/~andrewr
Moscow ID 83843 Or: http://www.biometrics.uidaho.edu
No statement above necessarily represents my employer's opinion.
|