The Professor's Response

Hi Professor,
I'm having trouble with question 2 from the Now You Try it problems in section 7.2
I found my Z to be .04
When I calculated E I got 1.31 as my result so when finding my two intervals I got 319.7 and 317.08, but it is wrong.
Could you tell me what I did wrong?

  See the professor's answer below.

Professor McGukian

Hi Natasha,

Here is the problem:

For the 175 Facebook users, the mean number of friends linked to their profiles was 318.39. The standard deviation was 436. Construct a 97% confidence interval for the true mean number of friends on Facebook for college students.

You said you got 0.04 as your critical z-score, but that cannot be correct. In this problem the confidence level is 97%. Since 97% is not on the t-table, we will find Za/2 by dividing 0.97 in half and looking that up on the Z table. This means we will go to the z-table with 0.97/2 = 0.4850 which corresponds to 2.17 on the z table.  The last example in section 7.1 has a video on getting this critical Z value.

That is most likely where your error was occurring.

Professor McGuckian

Professor McGukian

Here is the link for the video that finds the critical Z value for 97% confidence:

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