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Hearst has opened his empirical law while studying Nile. It turned out later, that this law well describes many other natural phenomena. It became clear that time sequences of measurements of such values as temperature, river flow, precipitation, thickness of tree rings or sea wave height can be researched using the method of normalized amplitude, or Hearst's method. Such sequences are characterized with H parameter, or Hearst's parameter.
Time sequences for which H is above 0.5 belong to persistent class – they sustain the current trend. If increments were positive during some time in the past, i.e. there was an increase, then henceforth there will also be an increase, on average. Thus, for a process with H>0.5 a tendency to increase in the past means a tendency to increase in the future. And on the contrary, the tendency to decrease in the past means, on average, continuation of decrease in the future. The greater is H, the stronger is the tendency.
With H=0.5 there is no expressed tendency in the process, and there are no grounds to consider that it will appear in the future.
A case of H<0.5 is characterized with antipersistence – growth in the past means reduction in the future, while a tendency to decrease in the past makes increase in the future more likely. The lower is H, the higher is this probability. In such processes, after increase of a variable there usually comes its decrease, and decrease is followed with increase.
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