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17 January, 14:12

A 20-minute consumer survey mailed to 350 adults aged 25-34 included a $5 Starbucks gift certificate. The same survey was mailed to 350 adults aged 25-34 without the gift certificate. There were 50 responses from the first group and 29 from the second group. From 95% confidence interval for the difference of proportion. Does it include zero?

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  1. 17 January, 14:26
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    -0.0133 < p < 0.10667

    - It does include zero

    Step-by-step explanation:

    Given:

    - The number of responses with certificate p_1 = 50

    - The number of responses without certificate p_2 = 29

    - Total sample size n = 350 adults

    Find:

    From 95% confidence interval for the difference of proportion. Does it include zero?

    Solution:

    - Formulate the proportion of people with and without certificate:

    p_hat with certificate = p_1 / n = 50/350 = 1/7

    p_hat w/o certificate = p_2 / n = 29/350 = 0.082857

    - Next compute the critical value of 95% confidence interval:

    Z_0.05 = 1.96

    - The confidence interval is:

    E = 1.96*sqrt [ (1/7) * (1-1/7) / 350 + (0.082857) * (1 - 0.082857) / 350 ]

    E = 0.04667

    C. I : E - (P_hat difference) < p < E + (P_hat difference)

    0.04667 - (0.06) < p < 0.04667 + (0.06)

    -0.0133 < p < 0.10667

    - It does include zero
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