Used in detecting unusually large or small numbers. An
astronomical data point is one that is obviously, even
blatantly, different from the rest of the points; all
studying the chart would agree the point is unusual.
Astronomical points should not be confused with the
highest or lowest data points, which every run chart will
have. While Rules 1, 2 and 3 are probability based, Rule 4
is subjective and recognises the importance of the visual
display of the data in a run chart.