where for distinction the UM parameters are shown as au and bu, respectively. These two equations are the basis for the conservation of the arithmetic mean and variances of global solar radiation and sunshine duration data. The basic AM remains unchanged whether the restrictive or unrestrictive model is used. Equation 4.43 implies that in both models the centroid, i. e., averages of the solar radiation and sunshine duration data are equally preserved. Furthermore, another implication from this statement is that the AM and the UM yield close estimations around the cen-troid. The deviations between the two model estimations appear at solar radiation and sunshine duration data values away from the arithmetic averages. The simultaneous solution of Eqs. 4.43 and 4.44 yields parameter estimates as bu =
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