Which of the following refers to a sampling approach in structured observation whereby a sampled individual is observed for a set period of time?

The correct option is Focal sampling.

Step-by-Step Explanation:

1. Understanding the Question: The question asks for the term that describes a sampling technique used in structured observation where a specific individual is observed continuously for a predefined, set period of time.

2. Analyzing the Options:
• Focal sampling: In structured observation (particularly in behavioral studies and observational research), focal sampling involves selecting a single individual (the "focus") and recording all of their behaviors for a specified length of time. This matches the definition provided in the question perfectly.
• Snowball sampling: This is a non-probability sampling technique used in qualitative research and surveys where existing study subjects recruit future subjects from among their acquaintances. It is not an observational timing technique.
• Purposive sampling: This is a non-probability sampling method where researchers rely on their own judgment when choosing members of the population to participate in their study, typically based on specific characteristics.
• Cluster sampling: This is a probability sampling method where the researcher divides the population into separate groups (called clusters) and then selects a simple random sample of these clusters for analysis.

3. Conclusion: Since focal sampling is the only method listed that specifically refers to observing a selected individual for a set block of time, it is the correct answer.