Two equations for calculating yellowness, the ASTM Yellowness Index and the IES-SPI-NEM Yellowing Factor, were compared for characterizing the yellowness of several transparent plastics subjected to outdoor weathering. The ASTM Yellowness Index was preferred over the IES-SPI-NEMA Yellowing Factor for describing the test results for three reasons: (1) it gave separate information about exposed and unexposed samples; (2) it distinguished properly among samples with and without ultraviolet light stabilizers; and (3) it ranked samples in accord with visual experience in every case. For the samples tested, typical transparent plastics widely used in the lighting industry, the IES-SPI-NEMA Yellowing Factor was misleading in each of the above respects.