Let
be the residuals from a linear regression of
on
independent variables, including the mean, where the
values
can be considered as a time series. The Durbin–Watson test (see Durbin and Watson (1950), Durbin and Watson (1951) and Durbin and Watson (1971)) can be used to test for serial correlation in the error term in the regression.
The Durbin–Watson test statistic is:
which can be written as
where the
by
matrix
is given by
with the non-zero eigenvalues of the matrix
being
, for
.
Durbin and Watson show that the exact distribution of
depends on the eigenvalues of a matrix
, where
is the hat matrix of independent variables, i.e., the matrix such that the vector of fitted values,
, can be written as
. However, bounds on the distribution can be obtained, the lower bound being
and the upper bound being
where
are independent standard Normal variables.
Two algorithms are used to compute the lower tail (significance level) probabilities,
and
, associated with
and
. If
the procedure due to Pan (1964) is used, see Farebrother (1980), otherwise Imhof's method (see Imhof (1961)) is used.
The bounds are for the usual test of positive correlation; if a test of negative correlation is required the value of
should be replaced by
.