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Statistics for Decision- Making in Business - Maricopa Community ...

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( ̅)<br />

Conditional Probability<br />

The conditional probability of event provided that already occurred is written as<br />

( )<br />

And implies that the likelihood of may be different, know<strong>in</strong>g that already took place.<br />

Example 1: Due to wars at sea, shipwrecks, and other such disasters, there are (roughly)<br />

around 3,000,000 sunken vessels <strong>in</strong> the all of the seas <strong>in</strong> the world! Suppose an area of the ocean<br />

is mapped out due to the historic ships that have wrecked <strong>in</strong> that area. There is speculation that,<br />

of the estimated 20 ships <strong>in</strong> that region, 11 are orig<strong>in</strong>al pirate ships. Given that a pirate ship is the<br />

first of the 20 recovered, what is the probability that the next one found will also be a pirate ship<br />

SOLUTION:<br />

We would like to f<strong>in</strong>d the probability that a pirate ship is found, given that one pirate ship has<br />

already been removed. If one ship is removed, there are 19 ships left. S<strong>in</strong>ce the ship removed<br />

was a pirate ship, there are only 10 rema<strong>in</strong><strong>in</strong>g. That is,<br />

( )<br />

Note that this is different than,<br />

Why<br />

( )<br />

This probability has no condition placed on it. It assumes the very basic <strong>in</strong><strong>for</strong>mation: 20 ships, 11<br />

pirate ships. So,<br />

( )<br />

The conditional probability, <strong>in</strong> this case, is different than the unconditional probability.<br />

Example 2: Determ<strong>in</strong>e whether or not the follow<strong>in</strong>g situations represent and as<br />

<strong>in</strong>dependent or dependent events.<br />

a) : It ra<strong>in</strong>s <strong>in</strong> Chandler today<br />

There is a car accident <strong>in</strong> Chandler<br />

<strong>Statistics</strong> <strong>for</strong> <strong>Decision</strong>-<strong>Mak<strong>in</strong>g</strong> <strong>in</strong> Bus<strong>in</strong>ess © Milos Podmanik Page 108

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