Once you have a focused question, you should think about the type of evidence you need to answer it. Medical research can generally be divided into two categories:
Primary research: Original research carried out in a clinical or laboratory setting. Examples include: observational studies and randomized controlled trials.
Secondary research: Analysis of primary research, often to pool data or review the body of evidence on specific conditions. Examples include: systematic reviews and guidelines.
The evidence pyramid illustrates different evidence types according to their methodological strength. Each level of the pyramid represents a different type of study. As we go up the pyramid, the stronger (i.e. more reliable and higher quality) the evidence. We should aim to locate evidence higher in the pyramid whenever available. It's important that you consider the type of evidence you need when searching for information and this will influence where you look for information
Hierarchies of evidence help you to identify the most robust evidence to answer your research question by helping you decide what evidence to look for. Generally study designs towards the top of the hierarchy have measures in place to minimise bias, so they are good places to start.
Different research questions require different study designs, so there is no one hierarchy of evidence. The pyramid below details a hierarchy of evidence for effectiveness questions, i.e. are steroid tablets more effective than combination inhalers in treating adults with severe asthma?
There won't always be high level evidence such as systematic reviews or randomised controlled trials available in the area you wish to research, so you will need to consider evidence further down the pyramid.