### 7.2 Predicting future cycle amplitudes based on cycle statistics

The mean amplitude of the last n cycles gives the benchmark for other prediction techniques. The mean
of the last 23 cycle amplitudes is 114.1 ± 40.4 where the error is the standard deviation about the mean.
This represents a prediction without any skill. If other methods cannot predict with significantly better
accuracy they have little use.
One class of prediction techniques is based on trends and periodicities in the cycle amplitudes.
In general there has been an upward trend in cycle amplitudes since the Maunder Minimum.
Projecting this trend to the next cycle gives a prediction slightly better than the mean. A number of
periodicities have been noted in the cycle amplitude record. Gleissberg (1939) noted a long-period
variation in cycle amplitudes with a period of seven or eight cycles (Section 5.3 and Figure 34).
Gnevyshev and Ohl (1948) noted a two-cycle periodicity with the odd numbered cycle having larger
amplitude than the preceding even numbered cycle (Section 5.4 and Figure 35). Ahluwalia (1998)
noted a three-cycle sawtooth shaped periodicity in the six-cycle record of the geomagnetic Ap
index.

Another class of prediction techniques uses the characteristics of the preceding cycle as indicators of the
size of the next cycle. Wilson et al. (1998) found that the length (period) of the preceding cycle is inversely
correlated to the amplitude of the following cycle. Another indicator of the size of the next cycle is the level
of activity at minimum – the amplitude of the following cycle is correlated with the smoothed sunspot
number at the preceding minimum (Brown, 1976). This type of technique has led to searches for activity
indicators that are correlated with future cycle amplitude. Javaraiah (2007), for example, has found
sunspot areas from intervals of time and latitude that correlate very well with future cycle
activity.