Practitioners are not a cluster.
Step-by-step explanation:
A practitioner is a person who actively takes part in research. The performance of the individual in professional field or specific area is monitored. This helps researcher to complete their research successfully.
In the above scenario, the researcher randomly selects some people to conduct research on religiosity. The life of the people who participate in the research in monitored. The people who follow worshiping are shortlisted and interviewed. The practitioners selected for the interview are not clusters, they are just selected randomly from the town.