Geiger counters are provided to detect particles emitted from radioactively decaying nuclei. A given nucleus decays at a random time which cannot be predicted in advance. Thus, the distribution of decays from a radioactive source will show statistical fluctuations in time.
Measure the number of decays D registered by the Geiger counter
in a 15-second time interval. Make this measurement a total of
twenty times. Calculate 
, 
and 
from your twenty values for D.
Statistical theory predicts that 
for this sort of random event (characterized here
by the fact that the probability that any one nucleus
decays in a given time interval is very
small -- this sort of probability distribution is called
a Poisson distribution). Do your
results approximately agree with this?