ObjectiveBecause the NIDCD/VA Hearing Aid Clinical Trial was conducted across eight clinical sites, rigorous control of the electroacoustic characteristics of the experimental devices was required.DesignThe parameters monitored included the gain and output of the approximately 720 hearing aids in the trial, measured both in the 2 cm3coupler and in situ. Each measurement was repeated six times on each hearing aid across the 9-mo duration of the study to insure both the stability and the accuracy of the circuits under investigation.ResultsThe gain data obtained in the coupler and in situ adequately demonstrated the stability of the instrument and the repeated measurements over time and across study sites. The output values produced by the experimental device also maintained acceptable constancy, both within and across treatment periods.ConclusionsThese measurements reflected satisfactory stability and sufficient accuracy within the circuits to achieve the intended goals of the study.