A primary source is a direct source of information. Articles reporting on a study that was directly conducted by the authors may be considered primary sources, as the researchers are reporting more or less directly to you without others' interpretations getting in between.
A secondary source is a mediated or interpreted source of information. Meta-analyses, for instance, pool the results of many different individual studies and view the different conclusions in relation to each other to make broad conclusions and observations based on a very large number of results.
To determine what kind of source you have on your screen, look for a Methods section. Was the research done in a lab, in the field, or by direct communication with subjects? If so, you probably have a primary source. If the research was done in libraries or databases, it is a secondary source.
Notice that even primary sources usually summarize the existing literature that relates to the topic before describing the study that was conducted. This is typically called a Literature Review or Review of the Research. You will probably see comparisons with previous research in the Discussion sections of primary source articles, as well.
Evidence-based practice (EBP) principles are being adopted by many professions, including: medicine; nursing; social work; and education.
The EBP model is built on five steps.
Generally speaking, the reliability and value placed on different types of information according to an EBP paradigm will be ranked thus:
As you work your way from less- to more-reliable, you will notice that the size of data sets and the evaluations and comparisons within them increase. A meta-analysis will study hundreds or thousands of individual cases, which allows for overarching trends to emerge. Individual case studies may not always enable the researcher to determine which characteristics are typical of similar cases and which characteristics pertain only to that particular case.
Keep in mind, however, that meta-analyses are only as good as the studies upon which they rely, and recruitment into these studies can also be a concern, as explained by John Barrow in this opinion piece for the BMJ: