Triangulation (social science)
In the social sciences, triangulation is often used to indicate that more than two methods are used in a study with a view to double (or triple) checking results. This is also called "cross examination".
The idea is that one can be more confident with a result if different methods lead to the same result. If an investigator uses only one method, the temptation is strong to believe in the findings. If an investigator uses two methods, the results may well clash. By using three methods to get at the answer to one question, the hope is that two of the three will produce similar answers, or if three clashing answers are produced, the investigator knows that the question needs to be reframed, methods reconsidered, or both.
Triangulation in research 
Triangulation is a powerful technique that facilitates validation of data through cross verification from more than two sources. In particular, it refers to the application and combination of several research methodologies in the study of the same phenomenon.
- It can be employed in both quantitative (validation) and qualitative (inquiry) studies.
- It is a method-appropriate strategy of founding the credibility of qualitative analyses.
- It becomes an alternative to traditional criteria like reliability and validity.
- It is the preferred line in the social sciences.
By combining multiple observers, theories, methods, and empirical materials, researchers can hope to overcome the weakness or intrinsic biases and the problems that come from single method, single-observer and single-theory studies.
The purpose of triangulation in qualitative research is to increase the credibility and validity of the results. Several scholars have aimed to define triangulation throughout the years.
- Cohen and Manion (2000) define triangulation as an "attempt to map out, or explain more fully, the richness and complexity of human behavior by studying it from more than one standpoint."
- Altrichter et al. (2008) contend that triangulation "gives a more detailed and balanced picture of the situation." 
- According to O’Donoghue and Punch (2003), triangulation is a “method of cross-checking data from multiple sources to search for regularities in the research data."
Denzin (1978) identified four basic types of triangulation:
- Data triangulation: involves time, space, and persons
- Investigator triangulation: involves multiple researchers in an investigation
- Theory triangulation: involves using more than one theoretical scheme in the interpretation of the phenomenon
- Methodological triangulation: involves using more than one method to gather data, such as interviews, observations, questionnaires, and documents.
- Cheng, Liying (2005). Changing language teaching through language testing: a washback study. Cambridge University Press. p. 72.
- Bogdan, R. C. & Biklen, S. K. (2006). Qualitative research in education: An introduction to theory and methods. Allyn & Bacon. ISBN 978-0-205-51225-6.
- Cohen, L., & Manion, L. (2000). Research methods in education. Routledge. p. 254. (5th edition).
- Altrichter, H., Feldman, A., Posch, P. & Somekh, B. (2008). Teachers investigate their work; An introduction to action research across the professions. Routledge. p. 147. (2nd edition).
- O'Donoghue, T., Punch K. (2003). Qualitative Educational Research in Action: Doing and Reflecting. Routledge. p.78.
- Denzin, N. (2006). Sociological Methods: A Sourcebook. Aldine Transaction. ISBN 978-0-202-30840-1. (5th edition).