Chapter 4: Business mathematics and computer spreadsheets
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1.2 Probabilities and expected values
Probabilities
When there is risk or uncertainty about what will happen, it might be possible to
estimate probabilities of the different possible outcomes. The total of the
probabilities of different possible outcomes is always 100% or 1.0. For example:
If it is estimated that there is an equal probability that the cost per unit of a
product will be $5 and $6, there is a 0.50 probability of $5 and a 0.50 probability
of $6.
If a company thinks it might make either a profit of $20,000 from a venture or a
loss of $15,000, and it is three times more likely that there will be a profit rather
than a loss, there is a 0.75 probability of a profit and a 0.25 probability of a loss.
Estimates of the probabilities of different outcomes might be an ‘educated guess’
based on experience and judgement. Sometimes, probabilities can be estimated
more reliably, using past experience as a guide to what is likely to happen in the
future. For instance, probabilities for different possible outcomes in the future might
be based on an analysis of historical records. An example might be an estimate of
the rate of defective output from a manufacturing process: historical records might
show that 0.5% of items produced in a manufacturing process are rejected as
defective items. This historical record might be used to asses the probability that
output will be defective in the future is 0.005 (= 0.5%) and the probability that units
of output will be free from defects is 0.995.
Expected value (EV)
An expected value (EV) is a weighted average value of different possible outcomes,
where the probability of each different possible outcome can be estimated. An
expected value can also be described as the ‘weighted average of a probability
distribution’.
An EV can be calculated using the following formula.
Expected value (EV) = ∑px
where:
∑ means ‘the sum of’
p = the probability of the outcome occurring. When there is a 50% probability
of an outcome, p = 0.50 and when the probability is 7%, p = 0.07, etc. The
total of the probabilities of all possible outcomes is 1.0 or 100%.
x = the outcome for which the probability has been estimated
When there is risk or uncertainty, and there different possible outcomes, each
possible outcome (x) has an associated probability (p). For each outcome and its
associated probability, multiply x by p. Then add up the total these values for ‘px’
that you have calculated. This gives you the expected value of the future outcome –
i.e. the weighted average expected outcome.