[R-sig-ME] Different p_values from between groups to within groups

[Luca Danieli]

Hello everybody,

may I ask you a suggestion on how to interpret a weird result I have?

I have 3 groups (ExpertiseType), and through the use of contrast hypotheses, the first model gives me this output:

model = lmer(Score~Closure*ExpertiseType+(1|Participant)+(1|Item), database, REML=TRUE)
                                                                  Pr(>|t|)
Closurecl_c1:ExpertiseTypeexp_c1    0.42203
Closurecl_c2:ExpertiseTypeexp_c1    0.00601 **
Closurecl_c3:ExpertiseTypeexp_c1   9.32e-08 ***

Another, more detailed model, gives me the following:

model = lmer(Score~Closure*ExpertiseType+ExpertiseType*LastPosition+Closure*LastPosition+(1|Participant)+(1|Item), database, REML=TRUE)

                                                                  Pr(>|t|)
Closurecl_c1:ExpertiseTypeexp_c1    0.50738
Closurecl_c2:ExpertiseTypeexp_c1    0.01059 *
Closurecl_c3:ExpertiseTypeexp_c1   4.05e-08 ***

As you can notice, I have an interaction in both analyses, but if I look for the same contrast hypotheses within the group for which I have the interaction, the Closurecl_c2 effect disappeares.

model = lmer(Score~Closure*LastPosition+(1|Participant)+(1|Item), subset(database, ExpertiseType==3), REML=TRUE)

                           Pr(>|t|)
Closurecl_c1      0.4411
Closurecl_c2      0.1419
Closurecl_c3   5.00e-07 ***

Which one should I consider the most reliable output?
Or, alternatively, what does this difference mean? I really don't know how to interpret this outcome. I was expecting that within groups, the analysis would get more defined.

Best
Luca



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