A group of children were identifi ed as slow learners. They were then randomly allocated to take either a cod liver oil capsule or a dummy placebo in a double blind trial. After 6 months the group allocated cod-liver oil appeared to have improved in terms of reading ability, relative to the dummy group and this difference was statistically signifi cant

(p-value < 0.01). Can one say this was a causal effect? Should one recommend all slow learners take cod liver oil?