During noise storms narrow-band, short-lived bursts make up an important part of the activity. Frequently the bursts show different characteristic arrangements in the frequency-time plane (Elgaröy, 1977). In the present investigation the frequency of occurrence of single bursts and sequences of bursts has been determined. The work started out as a search for statistical evidence for the occurrence of echoes of type I bursts. As progress was made, more attention was devoted to the general question of what was the frequency distribution of events containing different numbers of bursts and the conclusions that could be drawn from the observed distribution.