One of the major factors influencing the power degradation rate of PV modules is the environmental condition of the power plant site. In a previous investigation conducted by ASU-PRL on about 1900 modules (from six different manufactures) aged between 12 and 18 years, we reported a power degradation rate ranging between 0.6%/year and 2.5% per year for the hot-dry climatic condition of Tempe, Arizona. Statistically analyzing the performance parameters (current, voltage or fill factor) responsible for the power degradation and determining the degradation modes responsible for the degradation of those performance parameters is of great importance to the industry, especially to design appropriate accelerated tests for the new modules with similar/same construction as that of the field aged modules. The statistical analysis of the results presented in this paper was obtained using the null hypothesis technique. This analysis indicates that the major degradation modes for the modules having glass/polymer construction are encapsulant discoloration (causing Isc drop) and solder bond degradation (causing FF drop due to series resistance increase).