Comparative research is a research methodology in the social sciences that aims to make comparisons across different countries or cultures. A major problem in comparative research is that the data sets in different countries may not use the same categories, or define categories differently (for example by using different definitions of poverty).
Development of the tradition 
When the practice of comparative research began is a matter of debate. Karl Deutsch has suggested we have been using this form of investigation for over 2,000 years. Comparing things is essential to basic scientific and philosophic inquiry, which has been done for a long time. Most authors are more conservative in their estimate of how long comparative research has been with us. It is largely an empty debate over the definition of the tradition with those questioning whether comparing things counts as comparative research.
Textbooks on this form of study were beginning to appear by the 1880s, but its rise to extreme popularity began after World War II. There are numerous reasons that comparative research has come to take a place of honour in the toolbox of the social scientist. Globalization has been a major factor, increasing the desire and possibility for educational exchanges and intellectual curiosity about other cultures. Information technology has enabled greater production of quantitative data for comparison, and international communications technology has facilitated this information to be easily spread.
Comparative research defined 
Comparative research, simply put, is the act of comparing two or more things with a view to discovering something about one or all of the things being compared. This technique often utilizes multiple disciplines in one study. When it comes to method, the majority agreement is that there is no methodology peculiar to comparative research. The multidisciplinary approach is good for the flexibility it offers, yet comparative programs do have a case to answer against the call that their research lacks a "seamless whole".
There are certainly methods far more common than others in comparative studies, however. Quantitative analysis is much more frequently pursued than qualitative, and this is seen in the majority of comparative studies which use quantitative data. The general method of comparing things is the same for comparative research as it is in our everyday practice of comparison. Like cases are treated alike, and different cases are treated differently; the extent of difference determines how differently cases are to be treated. If one is able to sufficiently distinguish two cases, comparative research conclusions will not be very helpful.
Secondary analysis of quantitative data is relatively widespread in comparative research, undoubtedly in part because of the cost of obtaining primary data for such large things as a country's policy environment. This study is generally aggregate data analysis. Comparing large quantities of data (especially government sourced) is prevalent. A typical method of comparing welfare states is to take balance of their levels of spending on social welfare.
In line with how a lot of theorizing has gone in the last century, comparative research does not tend to investigate 'grand theories', such as Marxism. It instead occupies itself with middle-range theories that do not purport to describe our social system in its entirety, but a subset of it. A good example of this is the common research program that looks for differences between two or more social systems, then looks at these differences in relation to some other variable coexisting in those societies to see if it is related. The classic case of this is Esping-Andersen's research on social welfare systems. He noticed there was a difference in types of social welfare systems, and compared them based on their level of decommodification of social welfare goods. He found that he was able to class welfare states into three types, based on their level of decommodification. He further theorized from this that decommodification was based on a combination of class coalitions and mobilization, and regime legacy. Here, Esping-Andersen is using comparative research: he takes many western countries and compares their level of decommodification, then develops a theory of the divergence based on his findings.
Comparative research can take many forms. Two key factors are space and time. Spatially, cross-national comparisons are by far the most common, although comparisons within countries, contrasting different areas, cultures or governments also subsist and are very constructive, especially in a country like New Zealand, where policy often changes depending on which race it pertains to. Recurrent interregional studies include comparing similar or different countries or sets of countries, comparing one's own country to others or to the whole world.
The historical comparative research involves comparing different time-frames. The two main choices within this model are comparing two stages in time (either snapshots or time-series), or just comparing the same thing over time, to see if a policy's effects differ over a stretch of time.
When it comes to subject matter of comparative inquiries, many contend there is none unique to it. This may indeed be true, but a brief perusal of comparative endeavours reveals there are some topics more recurrent than others. Determining whether socioeconomic or political factors are more important in explaining government action is a familiar theme. In general, however, the only thing that is certain in comparative research issues is the existence of differences to be analysed.
See also 
- Analytic frame
- Sociological imagination
- Social research
- Comparative cultural studies
- Comparative sociology
- Comparative history
- Comparative mythology
- Deutsch 1987
- Clasen 2004
- Antal, Dierkes & Weiler 1987: 13
- Øyen 2004: 276
- Heidenheimer, Heclo & Adams 1983: 505
- Jones 1985: 28
- Deacon 1983
- Esping-Andersen 1990
- Taylor 1990: 14
- Przeworski & Teune 1970: 31
- Clasen, Jochen (2004). "Defining comparative social policy". A Handbook of Comparative Social Policy. Edward Elgar Publishing. ISBN 978-1-84064-886-7.
- Deacon, Bob (1983). Social Policy and Socialism. Pluto Press. ISBN 978-0-86104-721-5.
- Deutsch, Karl (1987). "Prologue: Achievements and Challenges in 2000 Years of Comparative Research," in Comparative Policy Research". Comparative Policy Research.
- Dierkes, Meinolf; Hans Weiler and Ariane Antal (1987). Comparative Policy Research. Gower. ISBN 978-0-566-05196-8.
- Esping-Andersen, Gøsta (1990). The Three Worlds of Welfare Capitalism. Princeton University Press. ISBN 978-0-691-02857-6.
- Heidenheimer, Arnold J.; Hugh Heclo, Carolyn Teich Adams (1983). Comparative Public Policy. St. Martin's Press. ISBN 978-0-312-00493-4.
- Jones, Catherine (1985). Patterns of Social Policy. Taylor & Francis. ISBN 978-0-422-77260-0.
- Przeworski, Adam; Henry Teune (1970). The Logic of Comparative Social Inquiry. Wiley-Interscience.