Information
Sometimes, if you are confident that most studies will be confined to a few key sources, you might only search a limited set of journals. This was how almost all research syntheses were done before online databases. No one uses this approach anymore because of the explosion of data accessibility (at least for those with access to scientific journals). However, whether to delve into unpublished or “gray” sources remains important (Gurevitch et al., 2018; Koricheva et al., 2013).
Remember that during a scoping search, try to find only some studies or obtain a preliminary estimate of the mean effect. The main goals are to:
a. Consider estimating the amount of data available to help you make informed decisions about expanding or focusing your study questions. This can really improve your research.
b. Work out what factors vary among studies that you might encode as potential moderators.
c. Decide what criteria mark a study as irrelevant (e.g., if your search identifies 2000 papers to read in full, you will have to make some exclusion decisions based on the title, abstract, and place of publication);
d. Work out what criteria each potentially relevant study must fulfil before you try to extract an effect size.
e. Establish the format of your data extraction form/spreadsheet and
f. Decide upon the most suitable measures of outcome (effect sizes).
This last decision will often depend on whether data are reported as a relationship between two continuous variables, in which case the effect size r is the most popular choice in ecology and evolution. Alternatively, the decision may involve comparing two groups, in which case there is a range of options depending on whether the response variable is discrete or continuous. It is sometimes most straightforward to conduct separate meta-analyses, dividing studies based on the most appropriate effect size.
