Comparative politics is a field in political science, characterized by an empirical approach based on the comparative method. In other words, comparative politics is the study of the domestic politics, political institutions, and conflicts of countries. It often involves comparisons among countries and through time within single countries, emphasizing key patterns of similarity and difference. Arend Lijphart argues that comparative politics does not have a substantive focus in itself, but rather a methodological one: it focuses on "the how but does not specify the what of the analysis." In other words, comparative politics is not defined by the object of its study, but rather by the method it applies to study political phenomena. Peter Mair and Richard Rose advance a slightly different definition, arguing that comparative politics is defined by a combination of a substantive focus on the study of countries' political systems and a method of identifying and explaining similarities and differences between these countries using common concepts. Rose states that, on his definition: "The focus is explicitly or implicitly upon more than one country, thus following familiar political science usage in excluding within-nation comparison. Methodologically, comparison is distinguished by its use of concepts that are applicable in more than one country."
When applied to specific fields of study, comparative politics may be referred to by other names, such as for example comparative government (the comparative study of forms of government) or comparative foreign policy (comparing the foreign policies of different States in order to establish general empirical connections between the characteristics of the State and the characteristics of its foreign policy).
Sometimes, especially in the United States, the term "comparative politics" is used to refer to "the politics of foreign countries." This usage of the term, however, is often considered incorrect.
"Comparative political science" as a general term for an area of study, as opposed to a methodology of study, can be seen as redundant. The political only shows as political when either an overt or tacit comparison is being made. A study of a single political entity, whether a society, subculture or period, would show the political as simple brute reality without comparison with another society, subculture, or period.
The comparative method is – together with the experimental method, the statistical method and the case study approach – one of the four fundamental scientific methods which can be used to test the validity of theoretical propositions, often with the use of empirical data i.e. to establish relationships among two or more empirical variables or concepts while all other variables are held constant. In particular, the comparative method is generally used when neither the experimental nor the statistical method can be employed: on the one hand, experiments can only rarely be conducted in political science; on the other hand the statistical method implies the mathematical manipulation of quantitative data about a large number of cases, while sometimes political research must be conducted by analyzing the behavior of qualitative variables in a small number of cases.
Two major strategies are used in comparative research.
Most Similar Systems Design/Mill's Method of Difference: it consists in comparing very similar cases which only differ in the dependent variable, on the assumption that this would make it easier to find those independent variables which explain the presence/absence of the dependent variable. Most Similar Systems Design, or MSSD, is very helpful since it compares similar objects, it keeps many otherwise confusing and irrelevant variables in the research constant. In a basic sense, MSSD starts out with similar variables between subjects and tries to figure out why the outcome is different between the subjects. The main shortcoming that is said about this method is that when comparing countries, since there are such a limited number of them, all potential factors of explanation can never be kept altogether constant. As such, despite many possibilities of variables, there are only a limited number of cases to apply them to. There are two methods of applying MSSD, the first being a stricter application and the second being a more loose application. The stricter application implies that a researcher would choose various countries that have a number of similar variables, also called control variables, and would only differ from each other by one single independent variable. The looser application uses the same general concept, but the researcher chooses countries that have similar characteristics but those characteristics are not strictly matched to a set of control variables. Because of the complications of so many variables but not enough cases, a second method was devolved to be used in conjunction with MSSD.
Most Different Systems Design/Mill's Method of Similarity: it consists in comparing very different cases, all of which however have in common the same dependent variable, so that any other circumstance which is present in all the cases can be regarded as the independent variable. Most Different Systems Design, or MDSD, differs from MSSD with focus and the fact that it does not take a strict variable application. MDSD uses differences between countries instead of similarities between countries as variables because social scientists have found that differences between countries do not explain their possible similarities if they have any. A more basic idea of MDSD is it takes subjects with different variables within them and tries to figure out why the outcomes between them are similar in the end. When using MDSD as a comparative research method, scientists look at changing interactions between systems in countries and then after all data is collected, the results are compared between the different systems. If the results obtained from this research differ between each other, the researcher must move up to the system level and switch to the MSSD method. When using MSSD as a comparative research approach, there is the independent and depend variable that get introduced, specifically the dependent variable being something that is common in all the research subjects and the independent variable which would be the differing characteristic between the research subjects. MSSD is more precise and strict at finding the differing point along with similarities, but MDSD does not have so many variables and only focuses on finding one similarity or difference between a wide selection of systems.