The simplest approach to quality assessment of solar radiation data is comparison with physical limits. Are the data within “reasonable” bounds? Another basis for data quality assessment is Eq. (1.1), the “closure” equation between the solar components. However, given the instrumentation issues addressed above, it is clear that the closure can occur even if large errors are present in any one or all of the three components. This problem is also traceable to the component-summation and even shade-unshade calibration approaches, where unknown bias (type B) errors are embedded in the test instrumentation. Another approach to identifying the type-B errors and characterizing them under clear-sky conditions has been comparison with modeled clear-sky data. Physical limits, closure, and model comparison approaches will briefly be described in the next subsections.