measurement and comparison of solar radiation estimation models ...

precision. The value of RMSE is always positive, representing zero in the ideal case.
The RMSE may be computed from the following equation
3.2.3. t-Statistic Method
RMSE = (y − x ) /n
(3.2)
After an estimation of a coefficient, the t-statistic for that coefficient is the ratio
of the coefficient to its standard error.
In the literature, Stone (1993) demonstrated that MBE and RMSE separately do
not represent a reliable assessment of the model’s performance and can lead to the false
selection of the best model from a set of candidates. To determine whether or not the
equation estimates are statistically significant, Stone (1993) proposed t-stat as:
3.2.4. Relative Percentage Error (e)
(n − 1)MBE
t − stat =
RMSE − MBE
The relative percentage error is given by;
e =
(3.3)
100 (3.4)
where; y is the kth calculated value, x is the kth measured value. The “e” value
provides the percentage deviation between the calculated and measured data. The ideal
value of “e” is zero.
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