Operational Transfer Path Analysis (OTPA) is very attractive from a practical point of view, as it does not require an expensive measurement campaign in which transfer functions are measured, as is the case in the traditional Transfer Path Analysis (TPA). In practice, however, a number of requirements have to be met in order to achieve reliable results. One aspect is that one needs to considered sufficient number of transmission paths in order to get meaningful results. It is known, for instance, that an omission of a transmission path in a TPA and especially an OTPA analysis leads to incorrect source strength estimates. To assist in this matter, a method is presented to estimate the number of physical transfer paths based on a singular value decomposition of the matrices involved. In addition a rather unexpected phenomena is discussed in the paper, causing big errors in the estimation of the transmissibilities that are required for OTPA. From the experiments it was found that resonant behavior of metal springs drastically affects the effective transmissibilities of the structure, causing erroneous results. It is believed that the main cause of this is the introduction of torques in addition to the forces in normal direction. The above described aspects are illustrated by means of experimental results. Results obtained with classical TPA will be compared with results obtained with OTPA.