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Proportion of Rainy Days
1.2
Proportion of Rainy Days Under July 31,2011 Overnight Conditions
1
0.8
0.6
0.4
0.2
0
0 20 40 60 80 100 120 140
Day with Specific Conditions
Graph: Based on a random simulation involving the true probability of a 50% chance of precipitation
and what occurs in the long-run.
As an interesting note, NWS has sophisticated helium “balloons” that they send up into the air to
measure properties such as wind speed and direction, humidity, and barometric pressure. Then
physics is used based on theories of fluid mechanics to make the prediction.
Among many others that we could begin to state, there is one other major misconception about
probability: that if the probability that it rains is said to be very small and yet it rains, then the
probability must be wrong. This is incorrect. Probability is a measure of uncertainty. As in the
case of meteorology, the predictions are scientific and are based upon prior data. Just because it
has only rained, say, 10% of the time on days like today, this is not to say that it won‟t rain. In
fact, it very well might! The moral of the story is that probability talks about likelihood. Only in
the instance of 0% and 100% probabilities is anything guaranteed. If there are situations in which
something either never happens or always happens, then we‟re probably not concerned about
understanding probabilities.
Probability
Probability is a measure of uncertainty, typically expressed as a number between 0 (0%) and 1
(100%), that describes how likely it is that an event will or will not occur under a specified set of
Statistics for Decision-Making in Business © Milos Podmanik Page 83