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Math Help - stratified sampling

  1. #1
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    stratified sampling

    hi this is my question:
    A glass blower has made 8 wine glasses, 32 ornaments and 7 bottles in one day. he is going to check the quality of the items by looking at a 20% sample.

    A) Explain why he should use a straified sample rather than a random sample?

    i know using stratified sampling would work but i don't know why i would use it instead of random sampling? so could someone explain this to me please?

    Thanks!
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by andyboy179 View Post
    hi this is my question:
    A glass blower has made 8 wine glasses, 32 ornaments and 7 bottles in one day. he is going to check the quality of the items by looking at a 20% sample.

    A) Explain why he should use a straified sample rather than a random sample?

    i know using stratified sampling would work but i don't know why i would use it instead of random sampling? so could someone explain this to me please?

    Thanks!
    There are a number of reasons for choosing stratified sampling, but in this case the main reason would be that the population breaks down into a number of distinct categories which may vary substantially in quality, so you will probably want equal sample sizes from each of the strata so you know something of the quality of each.

    CB
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  3. #3
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    Quote Originally Posted by CaptainBlack View Post
    There are a number of reasons for choosing stratified sampling, but in this case the main reason would be that the population breaks down into a number of distinct categories which may vary substantially in quality, so you will probably want equal sample sizes from each of the strata so you know something of the quality of each.

    CB
    thank you very much!
    the next part of the question is, how many items of each type must he choose?

    on this would i do: 20% of 57= 11.4
    18/57 X 11.4= 3.6 =4
    32/57 X 11.4= 6.4 =6
    7/57 X 11.4 = 1.4 = 1

    4+6+1= 11?
    Last edited by andyboy179; May 29th 2010 at 05:23 AM.
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