An advantage is that primary data is information the researcher has collected and worked with personally. It relates directly to that person's research or study. A disadvantage of primary data is that it is expensive to collect the data, and it often takes a long time to process the information.
Depending on the research method, primary data may be numerical or qualitative. It may include a large set of subjects or one smaller in number. In either case, one of the issues with only using primary data is that the researcher has nothing to back it up against if the data is skewed or incorrect. If there is some degree of experimental error, it might be hard to tell if there is nothing to use for comparison.
One of the positive sides of using primary data is that the researcher controls when and how he collects the information. This also means that the researcher is responsible for the data and may be asked questions about its source and reliability later on in the process. Data can be very useful when researchers are doing a small experiment or running a small survey, as it takes less time to analyze the results.