When measuring accuracy, it is necessary to test for auto-correlation within the errors. Auto-correlation occurs when there is dependence between the successive error values, which is also called a serial correlation. Durbin-Watson is the most widely used statistic to determine random errors. The Durbin-Watson statistic always lies between 0 and 4. If it is closer to zero, it indicates a positive auto-correlation. A value close to 4 indicates negative auto-correlation. A value close to 2 tends to reinforce the conclusion that no correlation exists among the error. Therefore values below 2 represent positive serial correlation amongst the errors and values above 2 represent negative serial correlation amongst the errors.

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