Symbolically, the common method of concordance and difference can be presented as: knowledge expands when we can verify or distort a hypothesis. This is because experimental tests are designed in such a way that the hypothesis is probably a general explanation of certain facts and not an isolated case. This type of experiment is controlled, which means that the experimental structures differ only from one variable (see the miles of difference method). The experimental group is the one that recovers the variable, while the control group does not. 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. Thus, we might find that you felt a little sick after eating an oyster, while your sister was rather comfortable eating a few, and your father became seriously ill after eating ten in a row. Since the variation in the number of oysters corresponds to a variation in the severity of the disease, it would be reasonable to infer that the diseases were caused by oysters. 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. Consider as an example of the two similar countries difference method. Country A has a centre-right government, a uniform system and was a former colony.
Country B has a centre-right government, a single system, but has never been a colony. The difference between countries is that Country A easily supports anti-colonial initiatives, while country B does not. The difference method would or would not identify the independent variable as the status of each country as a former colony, the dependent variable supporting anticolonial initiatives. This is because the two similar countries have compared, the difference between the two is whether they were previously a colony or not. This then explains the difference with the values of the dependent variables, the former colony supporting decolonization rather than the country without a history of being a colony. 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. 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.
This method is also generally described in comparative policy as the most diverse conception of the system. Symbolically, the method of agreement can be presented as follows: also simply called “common method”, this principle simply represents the application of methods of concordance and difference. Mills` methods are five methods of induction described by the philosopher John Stuart Mill in 1843 in his book A System of Logic.  They must shed light on issues of causation. Mills` methods can only reveal evidence of probable causes; they don`t really offer an explanation. The discovery of causalities is an important step towards understanding the world, but it is only part of what we need. We also need to understand how and why some cases of causation work the way they do.