Operational Transfer Path Analysis (OTPA) is very attractive from a practical point of view, as it not requires an expensive measurement campaign in which transfer functions are measured, as is the case in the traditional Transfer Path Analysis (TPA). Instead, transmissibilities are measured from operational data, making the method relatively cheap in terms of measurement efforts. In practice, however, a lot of difficulties have to be overcome. In the paper measurements on a small gearbox are discussed. Both OTPA and the traditional TPA measurements were performed, allowing an objective comparison of OTPA. The difficulties for practical application of OTPA are discussed and conclusion are drawn from these measurements.