## Distinguishing Feature: Study Design Structure **Key Point:** The fundamental difference between parallel-group and crossover RCT designs lies in how participants are allocated to treatment arms and the temporal sequence of treatment exposure. ### Parallel-Group RCT - Participants are randomly assigned to **one group only** (either intervention or control) - Each participant receives **only one treatment** throughout the entire study period - Comparison is made **between groups** at the end of the study - Suitable for studying permanent effects or conditions where carryover is a concern ### Crossover RCT - Participants receive **both treatments sequentially** (intervention in period 1, control in period 2, or vice versa) - A **washout period** separates the two treatment phases to eliminate carryover effects - Comparison is made **within participants** (each person acts as their own control) - Requires smaller sample sizes due to within-subject comparison ### Key Discriminating Features Table | Feature | Parallel-Group | Crossover | |---------|---|---| | **Treatment allocation** | One group only | Both treatments sequentially | | **Comparison unit** | Between groups | Within individuals | | **Washout period** | Not applicable | Essential | | **Sample size** | Larger | Smaller | | **Carryover bias risk** | Low | High (mitigated by washout) | | **Suitable for** | Permanent effects, chronic conditions | Acute, reversible conditions | **High-Yield:** In NEET PG exams, the defining characteristic of a crossover design is that participants receive **both treatments in sequence**, while parallel-group participants receive **only one treatment**. This is the most testable discriminator. **Clinical Pearl:** Antihypertensive trials often use parallel-group designs because blood pressure control is maintained over time and carryover effects could confound results if a crossover design were used.
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