[R-sig-ME] Question about inclusion of a random effect

[Chad Newbolt]

All,



I'm working on analyzing a data set from a survey.  In the survey, I asked a group of respondents to view a series of 94 images, or test questions, and I'm in process of evaluating the influence of various factors on their ability to correctly identify an item in an image.  The test questions likely show a considerable amount of variation in difficulty, with some being harder to correctly answer than others.  I understand that I clearly should include a random effect for each respondent (ID), however, I'm not sure if it is appropriate to include a random effect for question (1|Question) to account for variation.  I may be overthinking this one, but, including and removing (1|Question) dramatically changes my results so I want to make sure to get this one right.



My basic model is shown below for reference:



  results=glmer(Y~X1+X2+X3+X4+X5+X6+(1|ID)+(1|Question),data=datum,na.action = na.omit,family=binomial)



Thanks in advance for the help

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