One view of what links a cause to an effect is that causes make a difference to whether or not the effect is produced. This assumption is behind comparative studies, such as the method of randomized controlled trials, aimed at showing whether a trial intervention makes a positive difference to outcomes. Comparative studies are regarded as the gold standard in some areas of research but they are also problematic. There can be causes that make no difference and some difference-makers that are not causes. This indicates that difference-making should be taken as a symptom of causation: a feature that accompanies it in some, though not all, cases. Symptoms can be useful in the discovery of causes but they cannot be definitive of causation.
