An analysis of outdoor collector performance data collected as part of the NBS Durability/Reliability Test Program is presented. The data is analyzed for consistent patterns of performance degradation resulting from prolonged outdoor exposure at low flow rate and stagnation conditions. The analysis includes examination of the effects of pyranometer sensitivity changes with time and sensitivity dependence on temperature. Performance results are compensated for differences in test environments by use of an analytic correction routine. A statistical examination of the implementation of the testing procedure is made to identify interlaboratory differences. / Master of Science