The Professor's Response

For HW Section 7.2 Number 12 I am confused when finding the alpha. Before I had just divided the confidence level by 2 but then watching the video for 7.1 you say to subtract the confidence level from 1 and then divide by half then look it up on the t-table. But when I multiply that by the standard deviation divided by the square root of the sample mean I dont get the right answer.

12. The confidence level is 99%. The sample size is 37. The sample mean is 25, and the population
standard deviation is 4.

  See the professor's answer below.

Professor McGukian

Hi Natalie,

The values should be: E = 2.576 * 4 / root(37)

It seems like you might need to redo the homework problems for section 7.1.  You do not understand the method for finding critical values. 

Here is what you need to do, but please rewatch the videos for section 7.1 again.

To find a Za/2 critical value for a confidence interval:

If we are dealing with 99%, 98%, 95%, 90%, or 80% confidence, we use the t-table and look up alpha/2. Remember alpha is 1 - confidence level (as a decimal). The z-scores are in the last row of the t-table.

If your confidence level is any other value than the ones listed above, we divide the confidence level (as a decimal) in half and look that value up on the z-table to find the corresponding z-value.

In 7.1 the first four problems are done with the t-table, and the last four are done with the z-table.

Professor McGuckian

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