Visual assessment of autocorrelation from the time series plot is subjective and depends
considerably on experience. Statistical tests based on the observed number of runs above and
below the mean are available (e.g., Draper and Smith 1981), though none are covered in this
course. It is a good idea, however, to look at the time series plot as a first step in analysis of
persistence. If nothing else, this inspection might show that the persistence is much more
prevalent in some parts of the series than in others.