A standard systematic review takes all of the information that’s been gathered in the extraction process, looks for any patterns or differences emerging across the studies, describes the landscape of that evidence, and when possible, uses statistical tools to conduct a quantitative synthesis (i.e., meta-analysis) of the data.
Although some rapid reviews include a quantitative synthesis, time and resources often do not allow for this. Usually, rapid reviews take a more cursory approach to describing the evidence and skip the quantitative synthesis.
This is a shortcut, but it’s also just a result of any compromises you might have made throughout the rapid review process. If you’ve taken steps that might result in missing studies and incomplete or inaccurate information (e.g., searched less than 2-3 databases at minimum, extracted minimal data), it will be difficult and perhaps inappropriate to combine and synthesize your limited data.
Consequences of this shortcut
By summarizing the evidence quickly, the results of the review may lack the depth and certainty of the findings of standard systematic review, and may be prone to human error if done quickly.