Problem description


Bird (Dabbling Ducks) count data on 30 wetlands for 2 years for about 10 weeks.


Starting with a mixed effect Poisson regression model. Then moved to a mixed effect linear model using square root of the count as the response and week as a weight (due to heteroskedasticity by week).

Fixed effects

  • Week (numeric or categorical)
  • Emergent
  • Year
  • Area
  • Age of wetland
  • WC ?

Random effects

  • site
  • site x year


Heteroskedasticity appears to exist with more variability in early weeks compared to later.


Think about

  • At least for model building, use linear model with square root of count
  • Treat week as categorical
  • Consider repeated measures structure on random effects
  • Try the analysis in SAS so that you have easier control of random effects and error variances

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22 February 2018