Should I combine studies?

This will be covered in more detail in a future module. For now, you need to make a few decisions about whether it is likely to be appropriate for your review to combine studies in a meta-analysis or not. You've already thought through a lot of this when defining the question - were there groups of participants, or categories of intervention that were so different you think the effect of the treatment is likely to be very different? Or do you have reason to believe the effect of treatment will vary over time? If so, it may be inappropriate to combine different studies as they are measuring a different effect.

The other key factor in deciding whether to combine studies is called statistical heterogeneity. This means variation between studies in the measured effect of treatment. This is covered in the module on heterogeneity. For now, you need to know that it is usually inappropriate to calculate an average effect (that is, perform a meta-analysis) if there is a large amount of heterogeneity. You'll find out more about how the significance of this is investigated later on. Again your review group may have a policy on wording this part of the methods section.