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

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1. Are repeats/replacements allowed If yes, permutations/comb<strong>in</strong>ations are likely<br />

the <strong>in</strong>correct approach.<br />

2. Does order matter If yes, permutations should be used. If no, comb<strong>in</strong>ations should<br />

be used.<br />

You Might Be Wonder<strong>in</strong>g:<br />

You might be wonder<strong>in</strong>g why we must divide by to remove all repeats. This was<br />

probably somewhat obvious when work<strong>in</strong>g with two objects. Say there are 5 objects to<br />

select from. One is now gone, so <strong>for</strong> the second selection there are only 4. We proceed to<br />

cross out everyth<strong>in</strong>g along the and to the right of the diagonal s<strong>in</strong>ce they are either not<br />

possible or are s ’<br />

Object 1<br />

Object 2<br />

Object 3<br />

Object 4<br />

Object 5<br />

Object 1 Object 2 Object 3 Object 4<br />

We have essentially multiplied the first five possibilities by the next number of possibilities,<br />

which is only 4 (this is accounted <strong>for</strong> by cross<strong>in</strong>g out the diagonals, s<strong>in</strong>ce this subtracts out<br />

five possibilities to give ), and then divided that result by 2, s<strong>in</strong>ce half of the table is a<br />

repeat. That is,<br />

What happens when we select a third object We extend the above table as a multiple of 3,<br />

s<strong>in</strong>ce there are three objects left. Each table represents a pair<strong>in</strong>g with one of the three<br />

rema<strong>in</strong><strong>in</strong>g objects, as shown <strong>in</strong> the upper-left corner:<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 129

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