Learning Objectives

By the end of this lesson, learners should be able to:

  • Explain factors affecting sample size.
  • Understand confidence level and margin of error.
  • Estimate practical sample sizes for business surveys.

Why Sample Size Matters

A sample that is too small may produce unreliable results, while a sample that is unnecessarily large wastes resources.

Example

Surveying only 20 customers may not represent millions of customers, whereas surveying 200,000 customers may provide little additional value relative to cost.

Key Factors Affecting Sample Size

Population Size

Larger populations generally require larger samples, but the increase is not proportional.

Desired Confidence Level

Common levels:

  • 90%,
  • 95%,
  • 99%.

Higher confidence requires a larger sample.

Margin of Error

The maximum acceptable difference between the sample estimate and the true population value.

Smaller margins of error require larger samples.

Population Variability

More diverse populations require larger samples.

Illustrative Sample Sizes

For large populations at 95% confidence:

Margin of Error

Approximate Sample Size

±10%

96

±5%

385

±3%

1,067

These values are widely used in business research planning.

Business Example

A global streaming service wants customer satisfaction estimates within ±5% at 95% confidence. A sample of about 385 respondents per major market is often sufficient.

Finite Population Adjustment

When the population is relatively small, required sample size can be reduced.

Example

Surveying employees in a company with 800 staff requires fewer respondents than surveying a national customer base.

Nonresponse Adjustment

If the expected response rate is 50% and 400 completed surveys are needed:

Required invitations = 400 ÷ 0.50 = 800

Researchers must plan for nonresponse.

Common Mistakes

  • Ignoring response rates,
  • Using arbitrary sample sizes,
  • Confusing sample size with representativeness,
  • Failing to stratify important subgroups.

Practical Exercise

Determine the number of invitations needed to obtain 500 completed surveys when the expected response rate is 40%.

Learning Materials / Reference Materials

  • Cochran, W. G. Sampling Techniques.
  • AAPOR Response Rate Guidance.

Lesson Summary

Sample size should be determined systematically using confidence level, margin of error, population variability, and expected response rate rather than guesswork.

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