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Comparative politics is a field and a method used 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. The case study approach cannot be considered a scientific method according to the above definition, however it can be useful to gain knowledge about single cases, which can then be put to comparison according to the comparative method.
Comparative strategies 
Several different strategies can be 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 because 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 different from each other by one single independent variable. The looser application uses the same general concept, but the researched 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.
Some major works in comparative politics 
- In his work The Politics, Aristotle compares different "constitutions", by introducing a famous typology based on two criteria: the number of rulers (one, few, many) and the nature of the political regime (good or corrupt). Thus he distinguishes six different kinds of "constitutions": monarchy, aristocracy, and polity (good types), versus tyranny, oligarchy and democracy (corrupt types).
- The Spirit of the Laws
- Democracy in America and The Old Regime and the French Revolution
- The Art of Not Being Governed
- In Social Origins of Dictatorship and Democracy: Lord and Peasant in the Making of the Modern World (1966) Moore compares revolutions in countries like England, Russia and Japan (among others). His thesis is that mass-led revolutions dispossess the landed elite and result in Communism, and that revolutions by the elite result in Fascism. It is thus only revolutions by the bourgeoisie that result in democratic governance. For the outlier case of India, practices of the Mogul Empire, British Imperial rule and the Caste System are cited.
- In their work, The Civic Culture, Almond and Verba embark on the first major cross-national survey of attitudes to determine the role of political culture in maintaining the stability of democratic regimes.
- The Third Wave and Political Order in Changing Societies
- Patterns of Democracy (1999), a comprehensive study of democracies around the world.
- Political man (1960)
- Political Man: The Social Basis of Politics
- Parties and party systems
- Strong Societies and Weak States.
- In States and Social Revolutions: A Comparative Analysis of France, Russia, and China Theda Skocpol compares the major revolutions of France, Russia and China: three basically similar events which took place in three very different contexts. Skopcol's purpose is to find possible similarities which might help explain the phenomenon of political revolution. From this point of view, this work represents a good example of a research conducted according to the Most Different Systems Design.
- Making Democracy Work (1993), a major work assessing why some democratic governments work and other fail, based on the study of the Italian regional governments.
See also 
- Lijphart, Arend (1971). "Comparative politics and the comparative method". American Political Science Review 65 (3): 682–693. doi:10.2307/1955513. JSTOR 1955513.
- Mair, Peter (1996). "Comparative politics: An introduction to comparative.overview". In Goodin, Robert E.; Klingemann, Hans-Dieter. A New Handbook of Political Science. Oxford: Oxford University Press. pp. 309–335. ISBN 0-19-829471-9.
- Rose, Richard; MacKenzie, W. J. M. (1991). "Comparing forms of comparative analysis". Political Studies 39 (3): 446–462. doi:10.1111/j.1467-9248.1991.tb01622.x.
- Hopkin, J. [2002 (1995)] "Comparative Methods", in Marsh, D. and G. Stoker (ed.) Theory and Methods in Political Science, Palgrave Macmillan, pp. 249-250
- van Biezen, Ingrid; Caramani, Daniele (2006). "(Non)comparative politics in Britain". Politics 26 (1): 29–37. doi:10.1111/j.1467-9256.2006.00248.x.
- Lijphart, A. (1971), cit., p. 683
- Hopkin, J. [2002 (1995)], cit., p. 250
- It should be noted however that, as Lijphart points out in the article cited above, the experimental and statistical methods share the same logic as the comparative method: they all imply a comparison between cases which differ on the variable which is being studied, while remaining identical on all the other possible variables.
- Lijphart, A. (1971), cit., p. 691
- Anckar, Carsten. "On the Applicability of the Most Similar Systems Design and the Most Different Systems Design in Comparative Research." International Journal of Social Research Methodology 11.5 (2008): 389-401. Informaworld. Web. 20 June 2011.
- Comparative Methods in Political & Social Research: useful resources from Prof. David Levi-Faur's course at the University of Haifa.
- Comparative Politics in Argentina & Latin America: Site dedicated to the development of comparative politics in Latin America. Paper Works, Articles and links to specialized web sites.
- Comparative Politics and the Comparative Method (Arend Lijphart)
- Comparative Politics and the Comparative Method (Arend Lijphart)