A key theme that we need to consider. Often, academics and rubrics refer to contemporary and relevant literature. This is often implied to mean that the 'best research' is conducted within the last 5 years and can be stretched to some degree to the last 10 years. This fallacy fails to consider much of the excellent literature that has been developed that has revolutionised the way we conduct modern medicine beyond the last 10 years. Moreover, the term 'best evidence' is subjective. Different researchers and clinicians may consider the best to be different depending upon their objectives. Therefore, be mindful that there is excellent literature beyond 10 years, however, most assessments and assignments will want you to support your argument with recent literature.
Broadly, we can group research as either Quantitative or Qualitative. Quantitative data incorporates research that aims to quantify a problem, meaning to address 'what' or 'how many' aspects related to a research question. It is often displayed numerically including graphical representations of the data. Qualitative data describes the characteristics or qualities and is often collected through observation, interviews or questionnaires. This often includes textual examples relevant to the research question. Whilst there are a variety of ways and methodologies to conduct either quantitative or qualitative data, the key learning is that both are relevant. But, it ultimately depends on what you're trying to understand and what literature would best support your argument.
References
Straus, S., Glasziou, P., Richardson, S & Haynes, B. (2019). Evidence-based medicine: How to practice and teach EBM (5th Edition). ProQuest Ebook Central https://ebookcentral.proquest.com