Developing a Study Design for Evidence Based Medicine

With the need for valid and consistent clinical information within the podiatric medical community, developing a study design will help provide the best available research evidence in decision making: asking answerable questions; accessing the best information; appraising the information for validity and relevance; and applying the information to patient care. Foot and ankle surgeons must look at many factors when choosing an appropriate study design and be mindful that different types of studies are subject to different types of bias.

Case reports & Case series

Case reports and case series are descriptive and exploratory, consisting of collection of reports on the treatment of individual patients or a report on a single patient. Because they are reports of cases and use no control groups with which to compare outcomes, they have no statistical validity.  For example, a foot and ankle surgeon might describe the characteristics of an outcome for 100 consecutive patients with hammertoe who received a simple MPJ release. 

Case Control Study
Case control studies are retrospective, analytical, observational studies designed to determine the association between an exposure and outcome in which patients who already have a certain condition are compared with people who do not. Investigators then look to see what proportion in each group were exposed to the same risk factors.  They often rely on medical records and patient recall for data collection.  These studies are commonly used for initial, inexpensive evaluation of risk factors and are particularly useful for rare conditions, but are often less reliable than randomized controlled trials and cohort studies.  Showing a statistical relationship does not mean that one factor necessarily caused the other.  The study take a large population and follow patients who have a specific condition or receive a particular treatment over time and compare them with another group that has not been affected by the condition or treatment being studied. 

Advantages:

• Simple, quick and inexpensive
• Only feasible method for studying very rare disorders or those with long lag between exposure and outcome
• Fewer subjects needed than cross-sectional studies.
• Calculates odds ration

Disadvantages:

• Single outcome
• Reliance on recall or records to determine exposure status
• Confounders
• Selection of control groups is difficult
• Risk for bias
• Can not calculate the relative risk
• No prevalence or incidence

Cohort Study

Cohort studies are observational, prospective or retrospective studies, in which a large population and follow patients who have a specific condition or receive a particular treatment over time and compare them with another group that has not been affected by the condition or treatment being studied.  A cohort study may also follow two groups, one containing the agent of interest and the other acting as a control group.

Advantages:

• Ethically safe
• Can establish timing and directionality of events
• Eligibility criteria and outcome assessments can be standardized
• Administratively easier and inexpensive than Randomized Controlled Trial
• Examine multiple outcome variables.
•  Calculates relative risk

Disadvantages:

• The controls may be difficult to identify
• Exposure may be linked to a hidden confounder
• Risk of bias/confounders
• Randomization not present
• Expensive
• Time consuming

Randomized controlled trial

Randomized controlled trials are carefully planned projects that study the effect of a therapy on patients.  They include methodologies that reduce the potential for bias (randomization and blinding) and allow for comparison between intervention groups and control groups (no intervention).  When properly executed, this study is the strongest evidence of the clinical efficacy of preventive and therapeutic procedures in the clinical setting.

Advantages: 

• Control for bias/confounding
• Blinding more likely
• Provide strongest evidence

Disadvantages: 

• Expensive
• Ethically problematic at times

Cross sectional study

Cross sectional studies occurs where a population or sample of subjects is studied at a single point in time.  A sample survey is an example of a cross sectional study.

Advantages:

• Can study entire populations or a representative sample.
• Yields prevalence of factors measured.
• Lead in to cohort

Disadvantages:

• Susceptible to selection bias
• Information on factors is collected simultaneously, so it can be difficult to establish “cause and effect”.
• Not useful for rare exposures or occurrences.
• No incidence.