Survey Sample Size

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# Survey Sample Size - PowerPoint PPT Presentation

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.

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## Survey Sample Size

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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.

Number of

Subgroup

Analyses

Regional

or Special

Regional

or Special

National

National

• 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+

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

Deliverabl

e"

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 = required sample size

NE= estimated proportion of non-eligibles

R = estimated proportion of refusals

NA = 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?