Vol. 12, No. 4
April 2010
PQ Systems
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Quality Quiz from Professor Cleary

"B" is correct.

The plus and minus 2 percent was simply an off-the-cuff remark by Quinn. The proper way to calculate the correct confidence interval is:

Assume that Quinn has taken a sample and derived a sample proportion of .50. This is how he can take his sample proportion and build a 95 percent confidence limit:

There are two ways in which Quinn could have made an accurate statement about these confidence levels:

First, with a sample of 100 outlets and residents of Quincy, and finding that 50 of these were customers of Quince’s Quality Quiche, one could say with a confidence level of 95% that the true percentage is half (50%) of the people in town, plus or minus 10 percent.

A second way to state this is to say that we are 95 percent sure that the true proportion is somewhere between .40 and .60.

If he wanted to be 95 percent confident of the plus or minus 2 percent interval, what would Quinn need to do?

Read next month’s quiz to find out.

Click here for a more complete video explanation


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