A quality control manager wants to see how many defective products come off the line on average per day. They select three random cases of units at the end of the day to test how many defective units are in one of the three cases. What type of sampling method is this? 

• A. Simple random sampling
• B. Stratified sampling
• C. Cluster sampling
• D. Systematic sampling

Answer: (C)

Verified step by step guidance

1. Understand the problem: The quality control manager is selecting three random cases of units at the end of the day to test for defective units. The goal is to identify the type of sampling method used.
2. Review the definitions of the sampling methods: Simple random sampling involves selecting individual items randomly from the entire population. Stratified sampling divides the population into subgroups (strata) and samples from each subgroup. Cluster sampling involves dividing the population into groups (clusters) and randomly selecting entire clusters. Systematic sampling selects every nth item from a list or sequence.
3. Analyze the scenario: The manager is selecting entire cases (clusters) of units rather than individual units or dividing the population into strata. This aligns with the definition of cluster sampling.
4. Compare with other methods: The selection is not systematic (e.g., every nth case), not stratified (no subgroups are defined), and not simple random sampling (individual units are not selected randomly).
5. Conclude: Based on the analysis, the sampling method used in this scenario is cluster sampling.