Oil analysts predict that if the price of oil falls by half, the consumer’s purchase price for gasoline made from this oil will also fall by half.
Prediction
If the price of oil falls by half, the consumer's purchase price for gasoline will fall by half.
Evaluate
Oil analysts are assuming a direct correlation between the price of oil and the price of gasoline. If the price of oil is reduced by half, they predict that gasoline prices will similarly be reduced by half.
To cast doubt on the analysts' prediction, we should look for evidence that breaks this assumed direct proportionality between oil prices and gasoline prices.
Since we're looking to refute this conditional, we want a circumstance where the trigger is true and the outcome is false.
Goal
We need to find an answer that allows us to say it's possible that oil prices fall by half (the cost per barrel of crude oil), but the price at the pump falls by less than half.