I am using the linear mixed function in SPSS and want to determine whether a continuous variable (attendance) has an effect on the repeated measurement of my DV (GLS). My hypothesis is that attendance does have an effect on the DV. It was suggested that I add attendance as a covariate but also as a fixed effect. From what I've read it seems like fixed effects are always categorical so does this make sense?
MIXED GLS BY TIME WITH ATTENDANCE
/FIXED=TIME ATTENDANCE |SSTYPE(3)
/METHOD=REML
/REPEATED=TIME | SUBJECT(PARTICIPANT_ID) COVTYPE(UN).
OR, should attendance only be a covariate?
MIXED GLS BY TIME WITH ATTENDANCE
/FIXED=TIME ATTENDANCE |SSTYPE(3)
/METHOD=REML
/REPEATED=TIME | SUBJECT(PARTICIPANT_ID) COVTYPE(UN).
Best Answer
Standard regression model includes slope and intercept, regression is related to ANOVA with dummy-coded group memberships. Mixed effects model can include, besides the fixed effects as in regression, also random effects: random slopes and random intercepts (cf. Gelman & Hill, 2006). Grouping variable cannot be continuous, so such case is impossible, however you can have fixed effects for dummy-coded groups - the same as in linear regression.
Gelman, A. & Hill, J. (2006). Data Analysis Using Regression and Multilevel/Hierarchical Models. Cambridge.