Table 1.

Reconceptualizing the known-unknown matrix as types of meta-analytic outcome data

	Registered	Unregistered
Reported	Registered-reported (known-known) Found by usual systematic review processes; extracting reported summary statistics	Unregistered-reported (unknown-known) Found by reference harvesting; searching data repositories
Unreported	Registered-unreported (known-unknown) Found by requesting known missing data; estimating effects from other reported data; using alternative information sources	Unregistered-unreported (unknown-unknown) Found by “imputing” hypothetically missing data; contacting authors of potentially related studies

Terminology in parentheses was the original from known-unknown matrix; found by indicates methods to locate the four types of meta-analytic data