Download Determining Sample Size

Determining Sample Size
Variable
Population
Sample
Mean
μ
Proportion
π
p
Standard deviation
σ
s
Size
N
n
Variance
Standard error of the mean
Standard error of the proportion
Standardized variate (z)
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Chapter 13 -2
Confidence Interval
Approach
Means
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Education, Inc.
Proportions
Chapter 13 - 3
The Confidence Interval Approach
z
.
Chapter 13 - 4
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The Confidence Interval Approach
(Cont.)
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Education, Inc.
Chapter 13 - 5
The confidence interval is given by
We can now set a 95% confidence interval around the sample mean of $182.
The 95% confidence interval is given by
= 182.00 + 1.96(3.18) = 182.00 + 6.23
Thus the 95% confidence interval ranges from $175.77 to $188.23.
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Education, Inc.
Chapter 13 - 6
0.475
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Education, Inc.
0.475
Chapter 13 - 7
Steps
Means
Proportions
2. Specify the
confidence level (CL).
CL = 95%
CL = 95%
3. Determine the z
value associated with
the CL.
Z value is 1.96
Z value is 1.96
1. Specify the level of
precision.
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Chapter 13 - 8
Steps
4. Determine the
standard deviation of
the population
Means
Proportions
Estimate σ
σ = 55
Estimate π
π = 0.64
5. Determine the
sample size using the
formula for the
standard error
6. If necessary,
reestimate the
confidence interval by
employing s to
estimate σ
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Chapter 13 -9
Incidence rate refers to the rate of occurrence or the percentage of
persons eligible to participate in the study.
In general, if there are c qualifying factors with an incidence of Q1, Q2,
Q3, ...QC, each expressed as a proportion,
Incidence rate
= Q1 x Q2 x Q3....x QC
Initial sample size
= Final sample size
Incidence rate x Completion rate
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Education, Inc.
Chapter 13 - 10
International Marketing Research
• When conducting marketing research in
foreign countries, statistical estimation of
sample size can be difficult, because
estimates of the population variance might be
unavailable.
• When statistical estimation of sample size is
attempted, the differences in estimates of
population variance should be recognized
and factored in, if possible.
• When the goal is to examine differences
across countries or international markets, it is
desirable to have the same sample size
across countries.
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Education, Inc.
Chapter 13 - 11