  The present paper reviews results derived from statistical studies on solar
activity indices. The prolonged minimum phase of cycle 23 raised the question
of peculiarities inherent in cycle 23. The most important solar activity index
is the relative sunspot number and though most of other indices are closely
related, shifts are obtained between their peak activity of the order of 1-2
years. These shifts reveal a 22-yr pattern which can be attributed to solar
interior or dynamo related processes. The minimum phase of cycle 23 is not
found to be exceptional. Investigating the relative sunspot numbers over the
past 150 years, solar cycles of more prolonged minima are observed. Since 1920
solar activity is quite high ('modern maximum') and cycle 23 might be the
herald of the end of this phase.
