Type of missing data

Some possible reasons for missing data

Missing studies.

Publication bias;

Search not sufficiently comprehensive.

Missing outcomes.

Outcome not measured;

Selective reporting bias.

Missing summary data.

Selective reporting bias;

Incomplete reporting.

Missing individuals.

Lack of intention-to-treat analysis;

Attrition from the study;

Selective reporting bias.

Missing study-level characteristics (for subgroup analysis or meta-regression).

Characteristic not measured;

Incomplete reporting.