A periodogram is a graphical plot with abscissa given by the number
of consecutive numbers constituting a single period and ordinate
given by the correlation ratio
. The equation of the periodogram is
where each of the terms of the sequence consists of a simple periodic part of period
, together with a part which does not involve this periodicity
, so
is the standard deviation of the
s,
is the standard deviation of the
s, and
is the number of periods covered by the observations.