These data may or may not have been collected for research purposes. For example, the Bureau of the US Census is the largest publisher of statistical data in the world. Census data are collected to provide information about the population of the United States, but researchers can use these data to answer questions about population demographics, social behavior, health and well-being, and general conditions of communities. Just as with primary data collection, secondary data analysis has several advantages and disadvantages. Certain information is only available from secondary sources.
Sample sizes can be large enough to enable the researcher to draw conclusions.
Secondary data often are longitudinal, allowing the researcher to look for trends and changes over time. These advantages are balanced by some significant disadvantages, which researchers must keep in mind. Always remember that the data were collected for another purpose and that the researcher has no control over the data collection process. This may result in problems with data accuracy. The data may not be reported in the required form or may be outdated.