Study Designs
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Why is study design important?
Study design defines the way in which the data on the health status of interest is measured, collected and analysed
Name 4 important factors to consider when designing a study
• Question you wish to answer • Availability of data • Sampling methods / Data collection • Cost of design
What are 2 main types of study design?
• Observational 2. Experimental
What are observational studies used for?
Monitor or describe the health status of a population
What are experimental studies used for?
Examine the effect of an intervention on the outcome of interest
What is the hierarchy of evidence?
Gives an indication of the quality of evidence, reflecting reliability and validity of various forms of evidence
Name 3 examples of weak studies in the hierarchy of evidence
• Ideas / Opinions • Case series • Ecological studies
Name 3 examples of strong studies in the hierarchy of evidence
• Systematic reviews / Meta analysis • Randomised controlled trials • Cohort studies
What are the 2 most useful designs in observational studies?
• Cohort study 22. Case-control study
Name 3 weak designs of observational studies
• Case reports • Cross-sectional studies • Ecological studies
Describe the relationship between causation and observational study
Causation cannot be inferred solely from the results of observational study
Describe an example of possible confounding by other variables during an observational study
• A study of whether smoking of cigarettes causes oral cancer • Must take into account of confounding by drinking alcohol
Describe a cohort study
• Starts by defining a subgroup of the population to follow-up over a period of time • Exposures in the participants are measured at outset of study • Disease experience during follow-up is recorded • In the form of epidemiological studies, it is usual to form categories
Describe a case-control study
• Starts by identifying a group of subjects with disease and second group without • Frequency of exposure is determined in both groups • If more cases are exposed than controls this is an indication of exposure being risk factor
Describe the preferences between cohort studies and case-control studies
• Cohort study thought to be better approach • More time consuming and costly • Unsuitable for rare diseases
What is the usual relationship between cohort and case-control studies?
Cohort studies are often performed to confirm interesting findings from case-control studies
What study is used to establish the efficacy of new treatments?
Randomised controlled trials
What is a trial?
Any study conducted on patients to compared two or more treatments
Describe a controlled trial
Contains a treatment which is included as a basis for comparison which can either be active or placebo
What is randomisation?
Allocation of treatments to patients using some random process such as the toss of a coin or a computer generated number
What is the main advantage of randomisation?
Produce study groups which are comparable with respect to both known and unknown factors which could influence the outcome
What is block randomisation?
• Commonly used in the two treatment situations where sample sizes for both treatments must be equal • Recruits patients in small blocks ensuring half patients within each block are allocated each treatment • Within each block order treatment allocation is random
What is straitification?
• Stratify patients into subgroups defined by important prognostic variables • Carry out restricted randomisation within each subgroup
Why is allocation concealment necessary?
The decision to enrol a patient in a study could be influences by the knowledge of which treatment the patient may receive