A VISUAL TEST FOR A UNIT-ROOT - GEARYS COUNT OF SIGN CHANGES REVISITED

Prompted by Geary's (1970) suggestion in a different context, the stat ionarity of a plotted time series can be assessed by simply counting t he number of times the plot crosses the trend line linking the first a nd last observations. The 95 per cent critical value is conveniently a pproximated by twice the square root of the sample size. Despite its s implicity, the test has surprisingly good power.