For your reference, here’s a chart about what kinds of missing outcomes indicate a low risk of reporting bias and which indicate a high risk of reporting bias.
|Low risk of bias||High risk of bias|
|Study protocol is available and all pre-specified outcomes are reported according to the protocol definitions||Report omits some pre-specified outcomes without an appropriate explanation|
|Study protocol is unavailable but researchers reported all expected outcomes||The study report contains different outcome definitions or analytic methods than the protocol|
|Report provides incomplete results about outcomes|
|Report fails to report an expected outcome|
When you’re assessing a study for reporting bias, you should look at every outcome individually to check to see if the reported outcome matches the outcome described in the protocol. If there’s a match, it’s a“low risk of bias.” If an outcome is missing or not completely reported, and the researchers have not explained why, it’s likely a “high risk of bias.” If you’re unsure, you can label this domain as “unclear.”
If you can tell from a study’s protocol that an outcome was collected but not reported, or the study doesn’t include enough data about a particular outcome, you can try to contact the study authors directly and ask for that unpublished data. Then, you could incorporate the results into your systematic review.