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Sample Size. (Click icon for audio). Relationship Between Sample Size and Error. Determine Size for Probability Sample—Practical Issues. Financial Statistical Managerial (how confident needed). Ways to Determine Sample Size. Blind guess Available budget Bayesian considerations - PowerPoint PPT Presentation
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Determine Size for Probability Sample—Practical Issues
• Financial
• Statistical
• Managerial (how confident needed)
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Ways to Determine Sample Size
• Blind guess
• Available budget
• Bayesian considerations
• Rules of thumb– Main group n > 100– Subgroups 20 < n < 100
• Standards for comparable studies
• Statistical precision
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Statistical Precision
Must know:
• Variability of population and individual stratum
• Acceptable level of sampling error
• Needed level of confidence
• Type of distribution (if non-normal)
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Online Sampling Calculators
From DSS Research:
• Sample Sizehttp://www.dssresearch.com/toolkit/sscalc/size.asp
• Sample Errorhttp://www.dssresearch.com/toolkit/secalc/error.asp
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Sample Size Formula
2
Ezs
n
where:n = sample sizez = confidence interval in standard error unitss = standard error of the meanE = acceptable magnitude of error
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Sample Size Formula:Example #1
Suppose a survey researcher, studying expenditures on lipstick, wishes to have a 95% confident level (Z) and a range of error (E) of less than $2.00. The estimate of the standard deviation is $29.00.
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Suppose, in the same example as the one before, the range of error (E) is acceptable at $4.00. By how much is sample size is reduced?
Sample Size Formula:Example #2
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99% Confidence
Calculating Sample Size
1389
265.372
253.74
2
2)29)(57.2(n
2
347 6325.18 2
453.74
2
4)29)(57.2(n
2
15
2
2
Epqz
nWhere:
n = number of items in samples
Z2 = square of confidence interval in standard error units
p = estimated proportion of success
q = (1-p) or estimated the proportion of failures
E2 = square of maximum allowance for error between true proportion and sample proportion, or zsp squared.
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Calculating Sample Size at the 95% Confidence Level
753001225.
922.
001225
)24)(.8416.3(
)035( .)4)(.6(.)96 1. (
n4.q
6.p2
2