**Question:** Cluster testing is done in -

A.
B.
C.
D.

**Correct Answer:** C. Cluster testing is done in cluster randomization.

**Core Concept:**
Cluster randomization is a method of randomizing study units into treatment groups in a randomized controlled trial (RCT). In cluster trials, the units being studied are not individuals but groups of individuals, such as households, schools, or workplaces. Randomization ensures that the units within each cluster have an equal probability of being assigned to either treatment group.

**Why the Correct Answer is Right:**
Cluster randomization is the right choice for the question because it involves randomizing entire clusters of study units, rather than individual participants. This method is suitable when individual participant randomization is not feasible, as in cases where the unit of analysis (i.e., the level at which the intervention affects the outcome) is the cluster, not the individual. By randomizing clusters, it reduces the risk of contamination between treatment groups and maintains the integrity of the study design.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because cluster randomization involves randomizing the clusters, not individual participants.
B. Individual randomization is a different method where each participant is assigned to a treatment group based on random allocation. It is not the correct answer for this question.
C. Stratified randomization is a method used when there is a need to ensure balance between treatment groups based on certain characteristics (e.g., age, gender, or disease severity). It is not the correct answer for this question as it involves stratifying individuals, not clusters.
D. Percentage randomization is a method in which the proportion of clusters assigned to each treatment group is determined by a random number generator. It is not the correct answer for this question as it does not address randomizing clusters, but rather individual proportions within clusters.

**Clinical Pearl / High-Yield Fact:**
Cluster randomization is essential in situations where individual participant randomization is not feasible or appropriate. Examples include trials involving infectious diseases, where participants might be contagious and could affect others in the cluster, or trials in resource-limited settings where resources like medications or equipment are limited and cannot be allocated to individual participants. Cluster randomization ensures that the study design remains valid, and the risk of contamination between treatment groups is minimized.