One fundamental technique for identifying the cause of a given effect is to look for a common factor that is present in all cases in which the effect occurs. Mill called this technique the method of agreement: we look for some respect in which the different cases agree.

Example:

When doctors are confronted with a new disease, they typically try to see whether the people who have the disease ate the same food, or have the same virus in their blood, or are similar in some other way.