What are the important issues to consider when using secondary data?
Reusing data gathered in previous research works save a lot of time, energy and may be money. However, there are some subtle details you need to keep in mind when using such practices.


Make sure the author providing the data knows what he is talking about.
Make sure the data are valid and genuine.


As I heard from one of my professors, citing results of another author does not mean that you would not be held responsible in case of false/inaccurate results.

Update: Following the comment about raw data I think such data can be trusted, unless proved otherwise. Also, it is better to go for raw data (data sets) used widely thus reducing the chance there would be something terribly wrong with the data.