TABLE III.

Types of Bias

No.	Bias	Description
1	Regulatory	Resulting from the inclusion of clinical studies of nonapproved U.S. products (products not approved may provide a negative bias)
2	Historical	Resulting from the comparison of clinical trials over decades, which may have led to the exclusion of studies not meeting current practices and thereby may not include studies of the first approved products
3	Effectiveness	Resulting from the exclusion of studies utilizing validated outcome measures at the time of design but not using the currently accepted or adopted outcome measures or methodology (excludes earlier studies)
4	Control	Resulting from the failure to distinguish true placebo controls from active comparators (saline solution injections)