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The MSE is a positive number, making zero for a constant dataset only.
The upper value of MSE is not limited and defined by the variable and its predictor, which can be troublesome in computations.
There are many normalized measures of prediction quality which are more comfortable for computations, with Coefficient of determination (R2) being the most popular.
See also
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Formal science / Mathematics / Statistics / Statistical Metric
[ Root Mean Square Error (RMSE) ] [ Coefficient of determination (R2) ]
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