The correlation between values in a time series at time *t* and time *t-k* for a fixed lag *k*. Frequently, autocorrelation refers to correlations among adjacent time periods (lag 1 autocorrelation). There may be an autocorrelation for a time lag of one period, another autocorrelation for a time lag of two, and so on. The residuals serve as surrogate values for the error terms. There are several tests for autocorrelated errors. The Box-Pierce test and the Ljung-Box test check whether a sequence of autocorrelations is significantly different from a sequence of zeros; the Durbin-Watson statistic checks for first-order autocorrelations.