**Taking Credit for Chance Occurences**

According to the Cancer Information Center of the National Cancer
Institute "A disease cluster is the occurrence of a greater than
expected number of cases of a particular disease within a group of
people, a geographic area, or a period of time." (see
http://www.cancer.gov/cancertopics/factsheet/Risk/clusters) Disease
clusters can signify the occurrence of an epidemic, a biohazard, or some
other threat to public health. Therefore they must be taken very
seriously. But before panic sets in, one must always ask if the
"greater than expected number of cases" could merely have
happened by chance.

Consider some horrible non-contagious disease (SHND) that occurs in only
1% of the population. In a particular office building with 250 workers, 4
cases of SHND are reported. Should this raise an alarm concerning the
building, or could it have happened by chance?

Open the Building.xls
model and press the F9 key a few times to make sure
the model is working.

The building, with 25 floors and ten workers per floor, is modeled as
follows. For each of the 250 workers, we generate a random between 0 and
1 with RAND(). If the number is less than the population probability of
having SHND (in cell R13), the worker is represented by a 1. Otherwise
the worker is represented as a Blank. Note that by making the columns
narrow and adding decimal palces to the 1s, # signs appear on the screen
to represent the cases of the disease. Since R13 is set at 1%, we would
expect one worker in 100 to have the disease, or 2.5 cases on average for
a building of this size. Press the calculate key (F9) a few times to see
the range of outcomes. The indicator in cell R19 is equal to 1 if the
total cases in the building equals or exceeds the value in cell Q19 (4 in
this case).