Basic probability, the topic of the previous screen, is also known as independent probability - the chance of selecting a marble was not dependent on any previous outcome. Only the relative numbers of marbles in the box at the moment of selection determined the probability of choosing a certain marble.
Dependent probability, on the other hand, refers to situations in which the chance of an outcome occurring is influenced by a previous outcome. In such instances, the probability of both outcomes occurring is:
Probability of Outcome 1 Probability of Outcome 2 = Probability of Outcomes 1 and 2
The following example will illustrate:
A box contains 3 blue marbles, 2 green marbles, and 4 red marbles. If one marble is selected at random, and a second marble is selected without replacing the first, what is the probability that both marbles chosen are blue?
Probability of blue on 1st draw:
Probability of blue on 2nd draw:
Our first probability, , represents the chances of choosing a blue marble on the first draw; our second probability, , represents the chances of choosing a blue marble on the second draw. To find the combined probability of these two draws, we must multiply the two separate probabilities:
.
Note that we did not reduce our first draw probability fraction until we obtained the combined probability of both draws. If we had reduced to , we would not have been able to calculate the correct probability for the second draw because the reduction would have changed our numerator and denominator:
Correct:
Incorrect:
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