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

Alday, Phillip Phillip.Alday at mpi.nl
Tue Aug 8 19:43:43 CEST 2017

Yes, it makes sense. This is what is often called an "item" in the discussion on crossed random effects and leaving it out can distort inferences - see Clark 1974 "Language as a fixed effect fallacy" and more recent work by  Westfall and Judd (I'm thinking of their 2012 paper on this, but I can't think of the title or author order and I'm not at my desk to look it up).

From: Chad Newbolt <newboch at auburn.edu>
Sent: Aug 8, 2017 7:25 PM
To: r-sig-mixed-models at r-project.org
Subject: [R-sig-ME] Question about inclusion of a random effect


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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