What is the basic difference between primary and secondary data, and what are the advantages and disadvantages of each?

Secondary data are facts and figures that have already been recorded before the project at hand. Primary data are facts and figures that are newly collected for the project. Secondary data should be used first, before the collection of primary data. Two main advantages of secondary data are the tremendous time savings because the data have already been collected and published and the relatively low cost. A greater level of detail is often available through secondary data, and industry data are usually more complete than data collected by private organizations. On the other hand, secondary data may be out of date, a good example being U.S. Census Bureau data that are collected at 5- or 10-year intervals. Furthermore, the definitions and categories might not be quite right for a particular project. And, finally, because secondary data are collected for another purpose, they may not be specific enough for particular needs. In the latter case, it might be necessary to collect primary data, which should be specific to the particular project or program for which the data collection is designed. Compared with secondary data, primary data have the advantage of being more specific to the problem being studied. The main disadvantages are that primary data are usually far more costly and time-consuming to collect than secondary data.