A systematic review of tension-free urethropexy for stress urinary incontinence: intravaginal slingplasty and the tension-free vaginal tape procedures.

Assessment of study quality The studies were graded as poor, average or good for internal and external validity. The grading for internal validity was based on an assessment of bias, confounding and the role of chance; the grading for external validity was based on the population, inclusion or exclusion criteria and follow-up. Each study was also assigned a level of evidence from I (highest) to IV (lowest). The authors do not state how the papers were assessed for validity, or how many of the reviewers performed the validity assessment.