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Thread: Sample size calculation

  1. #1
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    Sample size calculation

    Hi everyone, I'm a bit of a newbie here.

    I am trying to determine the appropriate sample size. I have a survey with several dozen questions, most of which are of the 4 item-response type. My total population is about 400, but I need to subdivide it further into sub-groups, ranging from n=40 to n=120.

    My question is the following: if I want a confidence level of 95% and a margin of error of +-10% for my estimates, what formula do I use to calculate sample size? First of all, I CAN have a conf. level of .95 and a margin of error of 10%, right?

  2. #2
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    Multan Punjab Pakistan
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    Re: Sample size calculation

    Yes Exactly
    Statistics is the best

  3. #3
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    Re: Sample size calculation

    Hi Aferim,

    To calculate your sample size you can use the following formula:

    Ns = (Np) (p) (1-p) / (Np - 1) (B/C) (B/C) + (p) (1-p)

    Ns = required completed sample size
    Np = population size
    p = how varied the population is expected to be (50/50 split is tipically assumed, which correspondes to the value of 0.5)
    B = acceptabale sampling error (in your case 10%, that is 0.1)
    C = Z statistics in relation to confidence inteval, 1.96 corresponds to 95% level (see calculator here http://www.measuringusability.com/zcalcp.php)

    Source: Dillman (2000) Mail and Internet Surveys: The Tailored Design Method, NY: John Whiley & Sonc, Inc. (pp. 206-207).

    Hope this helps :-)


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