Biomedical Engineering Reference
In-Depth Information
To their mind, the coin is due to come up tails. Yet, probability theory confirms
that the probability of a seventh flip coming up heads is still 50/50 as it was on
the first, second, and all flips. The coin has no memory. This misunderstanding
illustrates the need for an understanding of probability as related to risk and risk
management. Clearly, patient health and safety cannot be based on ignorance or,
worse, luck.
4.4 DEFINITIONS OF PROBABILITY
It is hard to improve on this three-hundred-year-old definition of probability:
For it should be presumed that a particular thing will occur or not occur in
the future as many times as it has been observed, in similar circumstances, to
have occurred or not occurred in the past [3].
In addition, there are three common definitions or uses of the term probability:
1. The least quantitative is a degree of subjective belief, gut feeling, or intu-
ition. “I believe there is a small probability that he will be promoted this
year.”
2. The most empirical is the ratio of the number of outcomes to the total
number of possible outcomes. It is written as a fraction, percentage, or
proportion. “A four has been observed sixteen times in the last one hundred
rolls of the die.” This is the approach we prefer if we have the data to do
the calculation. Probability estimates from past data projected to future
probability assumes that the future will continue to look like the past. Of
course, this adds to the uncertainty of the exercise.
3. Last, the most quantitative is a theoretical value determined by the limiting
frequency of an infinite random series for an assumed model. In theory, a
fair coin will come up heads 50% of the time. In practice, it only comes
close.
4.5 RULES OF PROBABILITY
Probability is expressed as a number between 0 and 1. It is also expressed as
a percentage, from 0-100%. Zero is assigned to an event that is absolutely
impossible to occur. One or hundred percent is assigned to an event that is
absolutely certain to occur. In some situations, they can be given in smaller
categories such as low, medium, high, 0, 1, 2, 3, 4, or 0, 1, 2, 3, 4, 5, 6. Note
an odd number of categories starting with zero permits a center category that is
half the maximum. Reporting scales are often specific for a given project.
A probability of 0.5 or 50% is assigned to an event that would occur half
the time on average in an infinitely long series. It is also called the indifference
point. In some literature, an event is said to be “possible” if the probability is
50% or less. It is “probable” is the probability is greater than 50%.
Search WWH ::
Custom Search