Outlier study in a meta-analysis Im currently working on a meta-analysis of randomized controlled trials (RCTs).
These RCTs have reported their outcomes at different time points.
The maximum follow-up points were as follows: 1 months, 3 months, 6 months, and 48 months.
To pool the studies without double counting, I have decided to include the outcome data of the longest follow-up period of each study only.
My problem is: one of the studies maximum follow-up period was 48 months, which is considered an outlier in this case. This study also reports outcomes at 6, 12, and 24 months.
My first question:
Can I include the data of the shortest follow up-period data of this study instead, which is 6 months? My intention is to only make the data homogenous.
My second question:
I decided to do subgrouping for the studies based on intervention techniques to test for subgroup differences. Is it okay if one subgroup contained a single study? (no study was double-counted)
 A: Trials usually have different lengths of intervention and follow-up. The question on homogeneity in this case is a clinical one. For example, in a previous SR/ MA on hydroxyethyl starch, trials with shorter follow-ups showed sporadic results while trials with a longer follow-up showed almost identical results. This was theorized to be a result of the long-term harms associated with the intervention.
In your case, the outlier might be the most accurate trial since it presents the longest follow-up. So I wouldn't be quick to exclude the longest follow-up from the analysis just because it is the only trial reporting it.
As for having a single trial in the subgroup analysis, there is nothing wrong with that. In essence you are reporting the results of that trial since it is not being meta-analyzed with any other trials. Having said that, since it does provide data, it can be used to detect between subgroup differences. As such, again I would not exclude it for the noted reason (i.e. only one trial in the analysis).
