It improperly infers from the fact that a substance causally contributes to a condition that a reduction in the consumption of the substance is likely to eliminate that condition. It draws a conclusion about a causal relationship between two phenomena from evidence that merely suggests that there is a correlation between those phenomena. It confuses a condition that is necessary for establishing the truth of the conclusion with a condition that is sufficient for establishing the truth of the conclusion. It draws a conclusion about the population as a whole on the basis of a sample that is unlikely to be representative of that population.
Researchers recently studied the relationship between diet and mood, using a diverse sample of 1, 000 adults. It was found that those who ate the most chocolate were the most likely to feel depressed. Ttherefore, by reducing excessive chocolate consumption, adults can almost certainly improve their mood.
The argument is most vulnerable to criticism on which one of the following grounds?