What is the meaning of "group-by-region interaction effects" (PET scan study of brains affected by schizophrenia)?
I was reading a recent study on schizophrenia and came across this sentence:
We found significant group and group-by-region interaction effects on volume of distribution (VT).
What is the meaning of this phrase in simple words?
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The authors did an experiment with two independent variables: group and region. The groups are "schizophrenia" and "no schizophrenia" (controls). The a priori regions were the frontal cortex, anterior cingulate, and hippocampus (they also had exploratory analyses of other regions).
Their study was a PET scan, measuring [11C]UCB-J as a marker for synaptic vesicle glycoprotein 2A.
"Group effects" means that there was a significant difference between groups overall (independent of region), with lower values in the schizophrenia group. "Group-by-region interaction" refers to a statistical interaction between group and region: that is, the difference between groups differed significantly by region (rather than observing the same difference in every region). In particular, the differences were larger in the neocortical areas compared to hippocampus.
Thinking about "main effects" versus "interactions" is part of interpreting the two-way ANOVA.
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