The solar radiation values presented in the manual were calculated using improved models and data. The estimated data uncertainties assigned to the calculated values show how they might compare with true values. Random and Bias Errors. The two types of errors that contribute to uncertainties are random errors and bias errors. Random errors usually follow statistical distributions and result in values both above and below the true values.
Random errors tend to cancel when individual values are used to determine an average.
Consequently, random error sources do not contribute significantly to the uncertainty of 30-year monthly averages. Bias errors, however, are not reduced by averaging. Bias errors, which are often referred to as fixed or systematic errors, cause values to be in error by about the same amount and direction. When detailed information is not known about the bias errors, reasonable estimates of the bias error magnitude can be made using procedures similar to those described here. Climate change could also bias monthly average solar radiation values but was not considered a major source of error for this work.