where "T" is the number of observations. If one has a lengthy sample, then this can be linearly mapped to the Pearson correlation of the time-series data with its lags. Since "d" is approximately equal to 2(1 − formula_5), where formula_5 is the sample autocorrelation of the residuals, "d" = 2 indicates no autocorrelation. The value of "d" always lies between 0 and 4. If the Durbin–Watson statistic is substantially less than 2, there is evidence of positive serial correlation. As a rough rule of thumb, if Durbin–Watson is less than 1.0, there may be cause for alarm. Small values of "d" indicate successive error terms are positively correlated. If "d" > 2, successive error terms are negatively correlated. In regressions, this can imply an underestimation of the level of statistical significance.