We construe the term “empirical educational research” broadly to include experimental and quasi-experimental quantitative studies, qualitative studies of various kinds, and design experiments. To be classified as “empirical,” a study must make use of data/evidence from observation or experiment. To be “research,” the data must be gathered systematically. Empirical educational research is reported in peer-reviewed journals after being thoroughly vetted by others engaged in similar inquiry.
You may include any peer-reviewed sources in which authors a) make claims that have been systematically investigated and are empirically grounded, or b) systematically review the empirically grounded claims of other researchers, or c) use empirical studies to frame concepts that can be used to understand and investigate further the content learning under consideration.