Introduction
SMS and SRM are terms that have been recently introduced to the airport industry—they
highlight the need to understand some basic principles of probability because risk has two components:
severity and likelihood. While the meaning of severity is straightforward, understanding
the term likelihood requires some basic understanding of probability.
This appendix will help airport workers understand basic principles and notations used to
describe the chance of an accident or incident occurring. Reviewing these principles will help
airport workers understand and participate in discussions about risk and in panels assembled
for safety assessments.
Catastrophic accidents in aviation are rare and estimating the probability of occurrence is
best estimated using historical data. However, many types of incidents are frequent, particularly
at larger airports, because of the associated volume of operations and activities. Birdstrikes and
FOD incidents may occur daily at some airports.
The initial sections of this appendix describe basic concepts that airport staff can use in dealing
with SMS at the airport. The last section presents mathematical operations with probabilities
to help staff understand more advanced concepts.
Likelihood vs. Probability
The words probability and likelihood are often used as synonyms, but in statistical use, there
is a clear technical distinction. To illustrate the statistical difference between these terms we
can use a die. If we roll a die 6 times, what is the probability that we will get a “4” in every
roll? If we roll the die 6 times and we get a “4” in every roll, what is the likelihood that the
die is fair?
Statistically, it would be incorrect to switch likelihood and probability in the two sentences.
Probability has to do with the chances of an observation, and likelihood refers to the chances of
the parameters being correct, given an observation. However, for this guidance, no distinction
will be made between likelihood and probability and the terms will be used interchangeably.
Probabilities fall on a scale between 0, or 0%, (impossible) and 1, or 100%, (certain). There is
no such thing as a negative probability (i.e., less than impossible) or a probability greater than 1
(i.e., more certain than certain). Some types of accidents are very rare and the probability may
be very small (e.g., 0.00000001%), however, probability is never negative.