[R-sig-ME] question regarding predictor variable

Sally A. Roman saroman at vims.edu
Fri Mar 9 18:04:31 CET 2018

Hi -

I have a dataset I am working with to model the proportion of fish caught at length.  I have paired observations (n=96).  The paired observations consist of a 10 minute tow and a 15 minute tow.  Data collected are length measurements in 1 mm intervals, number of fish at length, and total catch (number of baskets) for each tow.

The traditional approach to model this type of data is to use a logistic regression to model the proportion caught at length in the 10 min to the total catch at length in both tows in a pair and then have the pair as the random effect.  Traditional fixed effects are length (L) and length^2 (L2).  I would like to include total catch in the model, but am struggling with how to include the variable because there is a total catch record for each tow.  I was hoping for some guidance on if it is appropriate to include total catch for both tows or for one of the tows, if total catch between both tows is correlated as a continuous variable.

Sally Roman
Fisheries Specialist
Virginia Institute of Marine Science
Marine Advisory Services

Phone: 804-684-7165
Fax: 804-684-7161

1375 Greate Road
Gloucester Point, VA 23062

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