Thus, the aim of the research is identifying the rules. A common mistake Association between two variables is not evidence for causality. Some examples are described in the correlation notes. A more difficult example relates to the relationship between predators and their prey. There are many well known examples of linked cycles in population size.
Many ecologists argue that predator numbers are controlled by the number of prey.
Demonstrating the type of relationship shown above does not differentiate between these two hypotheses. It is only possible to demonstrate causality if experiments are carefully designed, using techniques such as the random assignement of cases to treatments and control over the experimental conditions. However, failure to reject Ho is not evidence for the absence of a biological effect. Null hypotheses are never accepted. The magnitude of an effect that can be demonstrated by a statistical test is related to the power of the test.