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Survey Sample Size
MKTG 3342
Fall 2008
Professor Edward Fox
Sample Size Determination
 Convenience – Say … about 100.
 Rule of Thumb - At least 30 per each
subgroup (e.g., males/females) that will be
analyzed.
 Budget Constraint - Have a $300 budget
for sampling. On average it costs $2 per
returned questionnaire. Then go for
sample size of 150.
Comparable Studies or Industry Average
Typical Sample Sizes for Studies of
Human and Institutional Populations
Number of
Subgroup
Analyses
People or Households
Institutions
National
Regional
or Special
Regional
or Special
National
None or few
500-1500
200-500
200-500
50-200
Average
1500-2500
500-1000
500-1000
200-500
Many
2500+
1000+
1000+
500+
Determining Sample Size Using Statistical
Methods
There are statistical formulas for computing
sample sizes. These consider three aspects:
Precision:
Percent of sampling error deemed
acceptable by the researcher
Confidence:How confident is the researcher that
the true average value lies in the
interval [lower, upper] estimated
Variance:
Dispersion of the true value across the
population
Determining Sample Size
In general:
 If you desire greater precision in your
estimate, you need a larger sample size,
other things being equal
 If you want greater confidence in your
estimate, you need a larger sample size
 If the estimated variance in the population
is high, then you need a larger sample
size
Determining Sample Size from Web
There are many sample size calculators on the
web.
Two such websites are:
http://www.steinermarketing.com/calc_sample_size.htm
http://www.dssresearch.com/toolkit/sscalc/size.asp
The formula on the first website is for estimating
proportions. Use 95% and 99% confidence
level, and confidence interval (allowable error)
to be 2% to 10%. Use any population size such
as 100, 1000, 10,000. See what happens to
sample size as you vary the parameters.
What About Response Rates?
 The calculated sample size is the number of
desired actual responses, or completed
questionnaires
 In the real-world not all surveys sent out are
completed (response rates are less than 100%)
 You must incorporate the expected response
rate when deciding how many questionnaires to
send out or how many people to call
Response Rate Calculation
For Mail Surveys:
Response
# Usable Surveys Returned
=
# Surveys - # Surveys Returned
Rate
Mailed
" Not Deliverable"
Mail Surveys
Estimating the number of surveys required to
achieve given sample size:
n
Surveys Required
=
[(1-U) RR]
n = required sample size
U = estimated proportion “not deliverable”
RR = estimated response rate (proportion)
Mail Survey Example
 You have determined that sample size of 200 will
allow reasonable precision and confidence for
your estimates of important population
parameters. You will be conducting a mail
survey of households in Highland Park. You
expect that about 5% of mail will be
undeliverable and the expected response rate is
10%. How many mail questionnaires should you
send out?
Response Rate Calculation
For Telephone Surveys:
#Completed Interviews
RR =
#Completed + #Refusals + #No Answers
Interviews
Telephone Survey
Estimating the number of calls required to
achieve given sample size:
n
Total Calls =
[(1-NE) (1-R) (1-NA)]
where:
n
NE
R
NA
=
=
=
=
required sample size
estimated proportion of non-eligibles
estimated proportion of refusals
estimated proportion of no answers
Telephone Survey Example
 You have determined that a sample size of 200 will
allow reasonable precision and confidence for your
estimates of important population parameters. You
will be conducting a telephone survey of university
students ages 20 and older. After checking with
university registration officials you know that 50% of
all university students meet this criterion. Further,
you expect about 20% of the people you contact not
to participate in the survey and about 15% not to be
reachable even after trying at several different times
on different days of the week. How may total calls
should you expect to make for this project?