In December, 2016, PVTEC has offered a guideline for outdoor I-V characteristic measurement of PV module strings. This guideline is based on following assumptions: (1) Pmax is a linear function of PV module temperature at constant irradiance, (2) its gradient does not depend on irradiance, and (3) Pmax is in proportion to irradiance at constant PV module temperature. In this paper, applicability of the guideline to PV module measurement was inspected with enormous outdoor measurement data on five PV modules. Data analysis of a relation between PV module temperature and Pmax at the same irradiance condition indicated the validity of assumption (1). On the other hand, the validity of both assumption (2) and (3) was rejected because this analysis suggested that the gradient depended on irradiance and Pmax of wear-out PV modules had no proportional relation to irradiance under the same PV module temperature condition. This paper also pointed out that techniques to judge drop in power output of PV modules or strings had to consider probabilities of false positive and false negative judgments.