Some of the methods by which data can be collected for a statistical study are interviews, questionnaires, focus groups, field studies, direct observation, surveys, testing and experimenting. Depending on the nature of the study, the data-collection method can be based on either primary or secondary sources or a combination of both. Primary-source data collection refers to that which is obtained directly by the team conducting the study, while secondary-source data collection is obtained through a third-party source or data base such as a government census, research compilation or from organizational records.
Although many statistical studies are best served by primary-source data-collection methods, secondary-source methods can help to overcome budget constraints and better leverage the investigator's time and labor resources. Secondary resources also carry the advantage of an already established degree of reliability and validity that relieves the investigator of what can be a significant amount of fact-checking.
Data-collection methods can be focused on either quantitative or qualitative data or both. Quantitative data collection provides numbers and enables the investigator to measure and compare the results of the study against a baseline. Qualitative data collection adds the element of interpretation to the collection process. The study's participants might provide highly subjective and possibly biased responses. Qualitative data can, however, be a source of valuable insight and provide the investigator with context-related feedback that can expand the depth of the study. Atypical responses can also be better explained by the results of qualitative data collection.