Let’s say you have a handful of studies about your research question. Some studies include thousands of participants, while the other studies have only 30 or 40 participants each.
Do you treat all of the studies the same? Or do you give preference to the studies with more people because they are based on more information?
But, what if the data from the smaller studies were collected in a more rigorous manner than the bigger studies. Now would you give preference to the studies with more people?
It would depend on what you valued more- quantity of information or quality of information.
The concept of weighting is as simple as it sounds – when you’re combining your data together, you add extra weight, or importance, to the studies that are more trustworthy, informative and relevant to your research question.