The Early vs Late Infantile Strabismus Surgery Study: do sources for bias exist in this non-randomised trial? Early vs Late Infantile Strabismus Surgery Study Group.

Background: The Early vs Late Infantile Strabismus Surgery Study Group investigates whether early or late surgery is preferable in infantile convergent strabismus, in a non-randomised, prospective, multicentre clinical trial. The current state of the study after end of recruitment is reported here, focusing on the question of possible sources for bias in this non-randomised trial.
Method: The prognostic factors were analysed at baseline in order to check for imbalances between the two treatment groups. Reasons for possible differences are discussed.
Results: There is no evidence for clinically relevant inhomogeneities between the two groups concerning the distribution of the three prognostic factors spherical equivalent, degree of amblyopia, and limitation of abduction. The fourth prognostic factor, horizontal angle of squint, differs significantly between the two groups.
Conclusion: In the analysis of the final results we may have to account for differences in angle of squint at baseline by its inclusion as a covariate or by stratification.