CONTROLLED TRIALS
⏸Parallel groups
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(👳🏻🔷vs 👳▪️ ) Simple, head-to-head comparison of two or more treatments
1️⃣Subjects are allocated at random to a single treatment or a single treatment programme for the duration of the trial
🆓The groups are independent of each other
🔀Crossover trials
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(💂🔷 ➡ ️vs 💂▪️)This involves a single group study, where each patient receives two more treatments in turn; i.e. Each patient acts as their own control and comparisons of treatments are made within patients
2️⃣Two or more treatments are given to each patient in random order
✅useful for chronic conditions such as pain relief in long-term illness or the control of high blood pressure where the outcome can be assessed relatively quickly
✴️It may not be feasible for treatments for short-term illnesses that once treated are cured, for example antibiotics for infections
😃Advantages of parallel group designs
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✔️The comparison of the treatments takes place concurrently
✔️Can be used for any condition, especially an acute condition which is cured or self-limiting such as an infection
✔️No problem of carry-over effects
😬Disadvantages of parallel group designs
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✖️The comparison is between patients and so usually needs a bigger sample size than the equivalent cross-over trial
😃Advantages of crossover designs
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✔️Treatments are compared within patients and so differences between patients are accounted for explicitly
✔️Usually need fewer subjects than the equivalent parallel group trials
✔️Can be used to test treatments for chronic conditions
😬Disadvantages of crossover designs
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✖️Cannot be used for many acute illnesses
✖️Carry-over effects need to be controlled
✖️Likely to take longer than the equivalent parallel designs
✖️Statistical analysis is more complicated if subjects do not complete all periods
ѦԀԀIṬIȎṄѦʟ IṄҒȎ➕
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🏃🏾In cross over trials, it is important to avoid the 'carry-over effect' of one treatment into the period in which the next treatment is allocated.
↔️This is usually achieved by having a gap or 'washout period' between treatments
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