BIOSTATISTICS FOR MEDICAL STUDENTS : TYPES OF DATA

Don't run away; it's simple : PARALLEL Vs CROSS OVER DESIGNS IN RANDOMIZED CONTROLLED TRIALS

 CONTROLLED TRIALS 

⏸Parallel groups 

➖➖➖➖➖➖➖

(👳🏻🔷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 

➖➖➖➖➖➖➖

(💂🔷  ➡ ️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

➖➖➖➖➖➖➖➖➖➖➖➖➖➖

✔️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 

➖➖➖➖➖➖➖➖➖➖➖➖➖➖➖

✖️The comparison is between patients and so usually needs a bigger sample size than the equivalent cross-over trial 

😃Advantages of crossover designs 

➖➖➖➖➖➖➖➖➖➖➖➖➖

✔️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 

➖➖➖➖➖➖➖➖➖➖➖➖➖➖

✖️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➕ 

〰〰〰〰〰〰〰

🏃🏾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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EXAMPLES OF RESEARCH QUESTIONS🔢

(From : Oxford Handbook of Medical Statistics, Janet L. Peacock, Philip J. Peacock, P:5)

❓What is the prevalence of diabetes mellitus in the population? 

🔴This is a simple descriptive study 

❓How effective is influenza vaccination in the community-based elderly? 

🔴This is a comparative study, comparing individuals who had vaccines with those who did not 

❓Does lowering blood pressure reduce the risk of coronary heart disease? 

🔴This is an evaluative study, investigating the effi cacy of lowering blood pressure 

❓Is prognosis following stroke dependent on age at the time of the event? 

🔴This is an observational study 

❓Why does smoking increase the risk of heart disease? 

🔴This is an explanatory study investigating the mechanism behind an observed relationship 

❓What evidence is there for the effectiveness of antidepressants in treating depression? 

🔴This study is a meta-analysis of existing interventional studies