A.1 Definitions
• Frequency is a measure of how often an event occurs on average during a unit
of time (for example, how many times an engine supposed to start every
morning fails to start per year). It ranges from 0 to infinite. Time can be replaced
by other “counters”. For people in charge of performing risk assessments a
common unit of time is “per year”. One can measure frequency by long term
observations (building a “statistic”).
• Probability is, by definition, a number between nil and one, measuring the
chances some event may or may not happen. Nil means it is impossible to occur,
one means it is certain to occur. If you want to stay out of jail never use nil or
one!
A.2 Step 1
Making Sense of Probabilities
and Frequencies
Table A.1 can be used for estimating a large probability, high frequency event x.
For low probabilities p x 0.1, trained analysts can use Table A.2 delivered in
Step 2, which zooms into the lower range of probabilities.
The last two columns to the right display the “Frequency equivalent of x” and
the corresponding probability of seeing event x occurring (at least once) “next
year”. The next section delivers an explanation related to these two columns.
A.3 Step 2
Making Sense of Probabilities
and Frequencies
Table A.2 starts where the prior Table A.1 finished (0.1) and reaches 10
−6 , as this
value is commonly considered to be the threshold value of human credibility. Going
below credibility would require solid data that are normally not readily available,
thus it is highly recommended to stay away from that range.
Here again, the last two columns to the right display the “Frequency equivalent”
(return time) and the corresponding probability of seeing the event (at least once)
“next year”. The next section delivers an explanation related to these two columns.
274
Appendix A: Making Sense of Probabilities and Frequencies
• Frequency is a measure of how often an event occurs on average during a unit
of time (for example, how many times an engine supposed to start every
morning fails to start per year). It ranges from 0 to infinite. Time can be replaced
by other “counters”. For people in charge of performing risk assessments a
common unit of time is “per year”. One can measure frequency by long term
observations (building a “statistic”).
• Probability is, by definition, a number between nil and one, measuring the
chances some event may or may not happen. Nil means it is impossible to occur,
one means it is certain to occur. If you want to stay out of jail never use nil or
one!
A.2 Step 1
Making Sense of Probabilities
and Frequencies
Table A.1 can be used for estimating a large probability, high frequency event x.
For low probabilities p x 0.1, trained analysts can use Table A.2 delivered in
Step 2, which zooms into the lower range of probabilities.
The last two columns to the right display the “Frequency equivalent of x” and
the corresponding probability of seeing event x occurring (at least once) “next
year”. The next section delivers an explanation related to these two columns.
A.3 Step 2
Making Sense of Probabilities
and Frequencies
Table A.2 starts where the prior Table A.1 finished (0.1) and reaches 10
−6 , as this
value is commonly considered to be the threshold value of human credibility. Going
below credibility would require solid data that are normally not readily available,
thus it is highly recommended to stay away from that range.
Here again, the last two columns to the right display the “Frequency equivalent”
(return time) and the corresponding probability of seeing the event (at least once)
“next year”. The next section delivers an explanation related to these two columns.
274
Appendix A: Making Sense of Probabilities and Frequencies