A Randomized, Parallel-Group, Controlled Clinical Trial Evaluates the Effectiveness of an Anti-Inflammatory

A randomized, parallel-group, controlled clinical trial evaluates the effectiveness of an anti-inflammatory intervention as treatment for neuropathic pain following spinal cord injury.  Participants with varying levels and severities of spinal cord injury are randomly assigned to either a 24-week, anti-inflammatory diet treatment group or a control group.  Primary outcomes consist of changes in pain scores and markers of inflammation at 6 months.  Preliminary analyses reveal a significant reduction in pain scores in the treatment group from baseline to 6 months (p = 0.03) .  There was a significant reduction in pain scores among nonobese subjects (p = 0.01) and a nonsignificant increase in pain scores among obese subjects (p = 0.06) .  Which of the following is the best explanation for these differences in findings between obese and nonobese subjects?

A) Confounding bias
B) Effect modification
C) Hawthorne effect
D) Randomization failure
E) Selection bias