A.4 Explanation Related to the Last Two
Columns to the Right
For small frequencies (up to 1/10 yrs (units)), one can assume annual probability =
frequency, as shown in Fig. A.1.
However, at frequency = 1/5 yrs (f = 0.2) the error of the approximation rises to
20%. After that, calculations are required to evaluate the probability if the frequency is known or vice versa.
For those who want to know a bit more, frequencies and probabilities are indeed
linked by a mathematical function known as the Poisson distribution. It is a
common mistake to believe that a frequency f means that the event will occur “once
every 1/f year” as that event can indeed occur 0, 1, 2, … n times with decreasing
probability within a selected interval. As a matter of fact, the probability of an event
occurring once or more within its return period is always 0.63.
Table A.1 Estimation of large probabilities
Colloquial
vocabulary used to
describe the event
x occurrence
Event x
Frequency equivalent of x
NB. If these events occur
with a known average rate
and independently of the
time since the last event
P x to see at
least one
event next
year
p xmin –p xmax
Usually,
Almost always
Finding at least one container
of ice cream in a family
freezer
1
0.63 to
*1.0
At least one sunny week-end
in the next year
Common,
Must be
considered,
Not always
A member of the family gets
a cold next year
0.7–1
0.5–0.63
Getting stuck in a traffic jam
for at least 20 min next year
(exclude commuting)
Not uncommon
A person between the age of
18 and 29 does NOT read a
newspaper regularly
0.36–0.7
0.3–0.5
May be,
Possibly
Getting stuck for more than
one hour in traffic (exclude
commuting). A celebrity
marriage will last a lifetime
0.23–0.36
0.2–0.3
Not usually,
Occasionally
Odds of dying from heart
disease or cancer in the US
(1/7) chance of drawing 1
when drawing a fair dice (1/6
= 0.16)
0.11–0.23
0.1–0.2
Rarely
Almost never
Never
NB: a non expert should stop at this level of scrutiny.
Experts can develop more in depth estimates for lower
probabilities levels using the next Table A.2
0–0.1
Appendix A: Making Sense of Probabilities and Frequencies
275
Columns to the Right
For small frequencies (up to 1/10 yrs (units)), one can assume annual probability =
frequency, as shown in Fig. A.1.
However, at frequency = 1/5 yrs (f = 0.2) the error of the approximation rises to
20%. After that, calculations are required to evaluate the probability if the frequency is known or vice versa.
For those who want to know a bit more, frequencies and probabilities are indeed
linked by a mathematical function known as the Poisson distribution. It is a
common mistake to believe that a frequency f means that the event will occur “once
every 1/f year” as that event can indeed occur 0, 1, 2, … n times with decreasing
probability within a selected interval. As a matter of fact, the probability of an event
occurring once or more within its return period is always 0.63.
Table A.1 Estimation of large probabilities
Colloquial
vocabulary used to
describe the event
x occurrence
Event x
Frequency equivalent of x
NB. If these events occur
with a known average rate
and independently of the
time since the last event
P x to see at
least one
event next
year
p xmin –p xmax
Usually,
Almost always
Finding at least one container
of ice cream in a family
freezer
1
0.63 to
*1.0
At least one sunny week-end
in the next year
Common,
Must be
considered,
Not always
A member of the family gets
a cold next year
0.7–1
0.5–0.63
Getting stuck in a traffic jam
for at least 20 min next year
(exclude commuting)
Not uncommon
A person between the age of
18 and 29 does NOT read a
newspaper regularly
0.36–0.7
0.3–0.5
May be,
Possibly
Getting stuck for more than
one hour in traffic (exclude
commuting). A celebrity
marriage will last a lifetime
0.23–0.36
0.2–0.3
Not usually,
Occasionally
Odds of dying from heart
disease or cancer in the US
(1/7) chance of drawing 1
when drawing a fair dice (1/6
= 0.16)
0.11–0.23
0.1–0.2
Rarely
Almost never
Never
NB: a non expert should stop at this level of scrutiny.
Experts can develop more in depth estimates for lower
probabilities levels using the next Table A.2
0–0.1
Appendix A: Making Sense of Probabilities and Frequencies
275