Study Designs
Systematic Reviews
A systematic review evaluates the existing literature by identifying, appraising, selecting, and synthesizing all high quality research evidence relevant to a given question. The systematic review process includes a number of predetermined steps designed to identify and synthesize the best evidence-based information available.
For more information for more information about systematic reviews:
Systematic Reviews: Synthesis of Best Evidence for Clinical Decisions
Meta-analysis
“Meta-analysis is a statistical procedure that integrates the results of several independent studies considered to be ‘combinable.’ Well conducted meta-analyses allow a more objective appraisal of the evidence than traditional narrative reviews, provide a more precise estimate of a treatment effect, and may explain heterogeneity between the results of individual studies.”
For more information about Meta-anaylsis:
Matthias, E; Smith, G.D., and Phillips, A. N. (1997). Meta-analysis: Principles and procedures BMJ, 315: 1533 - 1537.
Randomized Controlled Trials
It has been noted that “randomized controlled trials are the most rigorous way of determining whether a cause-effect relation exists between treatment and outcome and for assessing the cost effectiveness of a treatment;” however, some RCTs are better structured than others. Jadad and colleagues have created a well-developed guide to assess the quality of RCTs.
For more information about randomized control trials:
Sibbald, B. and Roland, M. (1998). Understanding controlled trials: Why are randomised controlled trials important? BMJ, 316: 201.Jadad, A.R., Moore, R.A., Carroll, D., Jenkinson, C., Reynolds, D.J., Gavaghan, D.J., McQuay, H.J., (1996). Assessing the quality of reports of randomized clinical trials: Is blinding necessary? Controlled Clinical Trials, 17(1):1-12.
Observational Studies
“A type of study in which individuals are observed or certain outcomes are measured. No attempt is made to affect the outcome Observational studies are often conducted when randomized control trials are not feasible or not ethical.”
There are two common observational study designs:
- Cohort studies: Observations are made in a group of subjects that is followed over time.
Cohort studies can further be divided into prospective studies that begin in the present and follow subjects into the future, retrospective studies that examine information and specimens that have been collected in the past, or case-control design in which the investigator compares a group of people who have a condition with another group who do not. - Cross-sectional studies: Observations are made on a single occasion
For more information on observational study designs:
Hulley, S., et. al.(2006) Designing Clinical Research. 3rd Ed. Lippencott, Williams, and Wilkens, Philadelphia: Pennsylvania.The STROBE Statement is a checklist of 22 items that we consider essential for good reporting of observational studies. For the table listing the 22 items needed for a good observational study:
Case Series or Case Report
Case series studies are a common study design for new medications or for off-label use of medications. Such studies may be appropriate when a medication is first used for an indication, but they generally cannot provide evidence of the efficacy of a medication. Case series studies may be particularly misleading when the condition under study has a waxing and waning course. This is the case for conditions such as asthma, as well as psychiatric conditions such as bipolar disorder. In these remitting illnesses, an active or placebo control is essential in determining the efficacy of a medication.
Case series and case reports studies are defined as follows:
- Case Series: A descriptive study of a small group in which the study of an association between an observed effect and a specific exposure is based on detailed clinical evaluations and histories of the individuals. No control group is involved.
- Case Report: A descriptive study of an individual in which the study of an association between an observed effect and a specific exposure is based on detailed clinical evaluations and histories of the individual.