• Evidence Based Practice: Study Designs

    by Alan Batt. Last modified: 04/04/14

    sample size

    Randomised Controlled Trials

    Randomized controlled studies

    An experimental comparison study in which participants are allocated to treatment/intervention or control/placebo groups using a random mechanism. Best for study the effect of an intervention.


    • unbiased distribution of confounders;
    • blinding more likely;
    • randomisation facilitates statistical analysis.


    • expensive: time and money;
    • volunteer bias;
    • ethically problematic at times.

    Read more on blinding in trials: http://apps.who.int/rhl/LANCET_696-700.pdf

    Crossover Design


    A controlled trial where each study participant has both therapies, e.g, is randomised to treatment A first, at the crossover point they then start treatment B. Only relevant if the outcome is reversible with time, e.g, symptoms.


    • all subjects serve as own controls and error variance is reduced thus reducing sample size needed;
    • all subjects receive treatment (at least some of the time);
    • statistical tests assuming randomisation can be used;
    • blinding can be maintained.


    • all subjects receive placebo or alternative treatment at some point;
    • washout period lengthy or unknown;
    • cannot be used for treatments with permanent effects

    Cohort Study


    Data are obtained from groups who have been exposed, or not exposed, to the new technology or factor of interest (eg from databases). No allocation of exposure is made by the researcher. Best for study the effect of predictive risk factors on an outcome.


    • ethically safe;
    • subjects can be matched;
    • can establish timing and directionality of events;
    • eligibility criteria and outcome assessments can be standardised;
    • administratively easier and cheaper than RCT.


    • controls may be difficult to identify;
    • exposure may be linked to a hidden confounder;
    • blinding is difficult;
    • randomisation not present;
    • for rare disease, large sample sizes or long follow-up necessary.

    Case-Control Studies


    Patients with a certain outcome or disease and an appropriate group of controls without the outcome or disease are selected (usually with careful consideration of appropriate choice of controls, matching, etc) and then information is obtained on whether the subjects have been exposed to the factor under investigation.


    • quick and cheap;
    • only feasible method for very rare disorders or those with long lag between exposure and outcome;
    • fewer subjects needed than cross-sectional studies.


    • reliance on recall or records to determine exposure status;
    • confounders;
    • selection of control groups is difficult;
    • potential bias: recall, selection.

    Cross-Sectional Survey


    A study that examines the relationship between diseases (or other health-related characteristics) and other variables of interest as they exist in a defined population at one particular time (ie exposure and outcomes are both measured at the same time). Best for quantifying the prevalence of a disease or risk factor, and for quantifying the accuracy of a diagnostic test.


    • cheap and simple;
    • ethically safe.


    • establishes association at most, not causality;
    • recall bias susceptibility;
    • confounders may be unequally distributed;
    • Neyman bias;
    • group sizes may be unequal.
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    Alan Batt

    Alan Batt

    Paramedic, educator, researcher
    Alan is a critical care paramedic, paramedic educator and prehospital researcher, currently working around the world as an educator and researcher. He has previously worked and studied across Europe, North America and the Middle East. He holds a Graduate Certificate in Intensive Care Paramedic Studies, and an MSc in Critical Care. His main interests are in care of the elderly, end-of-life care, patient safety, professionalism (including role and identity), and paramedic education.

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