Survey Sample Size

1
Survey Sample Size

• MKTG 3342
• Fall 2008
• Professor Edward Fox

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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.

3

• Comparable Studies or Industry Average

Typical Sample Sizes for Studies of Human and
Institutional Populations
People or Households
Institutions
500-1500 1500-2500 2500
200-500 500-1000 1000
50-200 200-500 500
None or few Average Many
200-500 500-1000 1000
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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
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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

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Determining Sample Size from Web
There are many sample size calculators on the
web. Two such websites are http//www.steinermark
eting.com/calc_sample_size.htm http//www.dssresea
rch.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.
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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

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Response Rate Calculation
For Mail Surveys
Returned
Surveys
Usable
Response

Returned
Surveys
Surveys
Rate
-
Mailed
e"
Deliverabl
Not
"
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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)

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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?

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Response Rate Calculation
For Telephone Surveys
Completed Interviews
RR
Completed Refusals No Answers Interviews
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Telephone Survey
Estimating the number of calls required to
achieve given sample size
n Total Calls (1-NE)
(1-R) (1-NA) where n required sample
size NE estimated proportion of
non-eligibles R estimated proportion of
refusals NA estimated proportion of no answers
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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?