# strange behaviour of SPSS mixed with random effects of dichotomous predictors

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## strange behaviour of SPSS mixed with random effects of dichotomous predictors

 Hi all When I have a dichotomous predictor in any form of regression analysis I almost always set the codes to 0 vs 1 and treat it as continuous (i.e. I do the dummy coding myself) However, when playing around with something in SPSS mixed I decided to let SPSS do this coding for me by treating my dichotomous predictor, X, as categorical, using the 'by' keyword, i.e. entering the code as MIXED Y by X... as opposed to MIXED Y with X... And in doing so I ran up against something very odd... If I just look at the fixed effect of X I get the same result if I do the dummy coding or if SPSS does it e.g. MIXED Y with X  /fixed = X /random = intercept | subject(TEAMID) /print = solution testcov. gives the same interpretation as MIXED Y by X  /fixed = X /random = intercept | subject(TEAMID) /print = solution testcov. This is exactly as it should be But... if I also include X as a random effect, and allow for covarying intercepts and slopes e.g. MIXED Y with X  /fixed = X /random = intercept X | subject(TEAMID) covtype(UN)  /print = solution testcov. DOES NOT give the same results as when I use SPSS to do the dummy coding, e.g. MIXED Y by X  /fixed = X /random = intercept X | subject(TEAMID) covtype(UN)  /print = solution testcov. The former code seems OK in terms of the results given - but the latter code, i.e the automated spss way of dummy coding X, gives a very odd result, in that, when calculating the random effects, it seems to treat the reference category of X as a variable too, and gives a 3x3 variance-covariance matrix, albeit with un1,3), un(2,3) and un(3,3) unable to be estimated, and the usual warning re: the Hessian matrix!? Has anyone else run into this issue?! Any idea what is going on? cheers Chris -- -- Dr Chris Stride, C. Stat, Statistician, Institute of Work Psychology, University of Sheffield Telephone: 0114 2223262 Fax: 0114 2727206 "Figure It Out" Statistical Consultancy and Training Service for Social Scientists Visit www.figureitout.org.uk for details of my consultancy services, and forthcoming training courses, which are also available on an in-house basis:   - Data management using SPSS syntax   - Advanced SPSS syntax and SPSS macros   - Testing for Mediation and Moderation using SPSS   - Multi-level Modelling using SPSS   - Introduction to Structural Equation Modelling using Mplus   - Testing for Mediation and Moderation using Mplus   - Multi-level Modelling using Mplus   - Latent Growth Curve Modelling using Mplus ===================== To manage your subscription to SPSSX-L, send a message to [hidden email] (not to SPSSX-L), with no body text except the command. To leave the list, send the command SIGNOFF SPSSX-L For a list of commands to manage subscriptions, send the command INFO REFCARD