Meta-analyses improve precision
Meta-analyses provide researchers with a single pooled result to answer whether treatment A is more beneficial than treatment B. This pooled result is usually more precise than the result from the individual studies. But why is this important?
Each of following studies has attempted to calculate the effectiveness of treatment A as compared to treatment B. However, it is likely that the results from these studies will vary either by differences in study protocols or purely by chance. Some studies may find that there is a large beneficial effect of treatment A as compared to treatment B. However, others may show that the treatment effect is a lot smaller or even that treatment A is harmful.
The precision with which each of these studies calculates the treatment effect depends on many factors, including the number of people in the study. Generally, as the number of people increases in a study the precision of the treatment effect will increase.
Therefore, by statistically combining all of the sample sizes together from the individual studies, the precision of our pooled result for the treatment effect can be improved.