Unlike the four previous inductive methods, the method of accompanying variation does not involve the elimination of any circumstances. The change in size of one factor causes another factor to change in size. Mills` rule of understanding states that if, in all cases where an effect occurs, there is a single prior C factor common to all of these cases, then C is the cause of the effect. According to the table in this example, the only thing you ate was oysters. Therefore, if we apply the rule of concordance, we conclude that the consumption of oysters is the cause of the disease. This situation is an example of Mills` common method of agreement and difference: the first four students are proof that all those who got sick had eaten coleslaw, and the four matching couples are proof that only those who fell ill had eaten coleslaw. This is a strong combination of the first two methods, as it tends to support our idea that real causes are necessary and that the conditions for their effects are sufficient. It is important to remember that the use of the scientific method attempts to confirm or disprove a hypothesis; However, this process must always be considered partial and temporary. The weight we give to a confirmation or rebuttal is never all or nothing. We need to gather evidence over a long period of time.
If we make mistakes, they are revealed by the results of repeated experiments. The common method deals with both the method of agreement and the method of difference as indicated by the diagram above. The application of the common method should therefore tell us that this time it is beef that is the cause. Philosopher John Stuart Mill has developed a series of five methods (or canons) that analyze and interpret our observations in order to draw conclusions about the cause-and-effect relationships they have. Mills methods should not come as a surprise, as these rules articulate some of the principles we use implicitly in causal reasoning in everyday life. But it is important to respect the limits of these rules. Even simply referred to as the “common method,” this principle represents only the application of methods of concordance and difference. This method is also generally known as the most similar system design in the context of comparative policy. For a property to be a necessary condition, it must always be present when the effect is present.
Since this is the case, we are interested in examining cases where the effect is present and to learning about the characteristics that exist and are absent under the “possible necessary conditions.” Obviously, the properties missing if the effect is present cannot be necessary conditions for effect.
