Types of Interventional Study Design: Interventional study designs, also called experimental study designs, where the researcher or investigator intervenes at some point throughout the study.
Types of Interventional Study Design
- Randomized controlled trial (common and strongest interventional study design)
- Pre-post study design
- Non-randomized controlled trials
- Crossover randomized controlled trial study design
Randomized controlled trial study design
- Randomized controlled trials (RCTs) are the most common type of interventional study.
- These trials take a homogenous group of study participants and randomly divide them into two separate groups.
- If the randomization is successful then these two groups should be the same in all respects, both measured confounders and unmeasured factors.
- The intervention is then implemented in one group and not the other and comparisons of intervention efficacy between the two groups are analyzed.
- Theoretically, the only difference between the two groups through the entire study is the intervention.
- Example is the intervention of a new medication to treat a specific disease group of patients.
- This randomization process is arguably the largest strength of an RCT.
- Additional methodological elements utilized among RCTS further strengthen the causal implication of the intervention’s impact.
- These include allocation concealment, blinding, measuring compliance, controlling for co-interventions, measuring dropout, analyzing results by intention to treat, and assessing each treatment arm at the same time point in the same manner.
Pre-post study design
- A pre-post study measures the occurrence of an outcome before and again after a particular intervention is implemented.
- Example is comparing deaths from motor vehicle crashes before and after the enforcement of a seat-belt law.
- Pre-post studies may be a single arm, one group measured before the intervention and again after the intervention, or multiple arms, where there is a comparison between groups.
- Often there is an arm where there is no intervention.
- The no-intervention arm acts as the control group in a multi-arm pre-post study.
- These studies have the strength of temporality to be able to suggest that the outcome is impacted by the intervention, however, pre-post studies do not have control over other elements that are also changing at the same time as the intervention is implemented.
- Therefore, changes in disease that occurred during the study period cannot be fully attributed to the specific intervention.
- Outcomes measured for pre-post intervention studies may be binary health outcomes such as incidence or prevalence or mean values of a continuous outcome such as systolic blood pressure may also be used.
- The analytic methods of pre-post studies depend on the outcome being measured.
- If there are multiple treatment arms, it is also likely that the difference from begin ning to end within each treatment arm is analyzed.
Non-randomized trial study design
- Non-randomized trials are interventional study designs that compare a group where intervention was performed with a group where there was no intervention.
- These are convenient study designs that are most often performed prospectively and can suggest possible relationships between the intervention and the outcome. However, these study designs are often subject to many types of bias and error and are not considered a strong study design.
Crossover randomized controlled trial study design
- A crossover RCT is a type of interventional study design where study participants intentionally “crossover” to the other treatment arm.
- A crossover RCT begins the same as a traditional RCT, however, after the end of the first treatment phase, each participant is re-allocated to the other treatment arm.
- This design has many strengths, including demonstrating reversibility, compensating for unsuccessful randomization, and improving study efficiency by not using the time to recruit subjects.
- Allocation concealment theoretically guarantees that the implementation of the randomization is free from bias.
- A third party who is not involved in the treatment or assessment of the trial creates the randomization schema and study participants are randomized according to that schema.
- The traditional method of allocation concealment relies upon sequentially numbered opaque envelopes with the treatment allocation inside.
- Participants are then allocated to the specific intervention arm in the pre-determined order dictated by the schema.
- Blinding in an RCT if withholding the treatment arm from individuals involved in the study. This can be done through the use of placebo pills, deactivated treatment modalities, or sham therapy.
- Sham therapy is a comparison procedure or treatment which is identical to the investigational intervention except it omits a key therapeutic element, thus rendering the treatment ineffective.
- An example is a sham cortisone injection, where a saline solution of the same volume is injected instead of cortisone.
- This helps ensure that patients do not know if they are receiving active or control treatment.
- Blinding within an RCT includes patient blinding, provider blinding, or assessor blinding.
- Co-interventions, interventions that impact the outcome other than the primary intervention of the study, can also allow for erroneous conclusions in clinical trials. For example, if a placebo treatment arm utilizes more over-the-counter medication than the experimental treatment arm, both treatment arms may have the same therapeutic improvement and show no effect of the experimental treatment.
- The exclusion or tracking and statistical adjustment of co-interventions serve to strengthen an RCT by minimizing this potential effect.
- Participants drop out of a study for multiple reasons, but if there are differential dropout rates between intervention arms or high overall dropout rates, there may be biased data or erroneous study conclusions.
- Intention-to-treat (ITT) analysis is a method of analysis that quantitatively addresses deviations from random allocation.
- This method analyses individuals based on their allocated intervention, regardless of whether or not that intervention was received due to protocol deviations, compliance concerns, or subsequent withdrawal.
- Assessment timing can play an important role in the impact of interventions, particularly if intervention effects are acute and short-lived.
- The specific timing of assessments is unique to each intervention, however, studies that allow for meaningfully different timing of assessments are subject to erroneous results. For example, if assessments occur differentially after an injection of a particularly fast-acting, short-lived medication the difference observed between intervention arms may be due to a higher proportion of participants in one intervention arm being assessed hours after the intervention instead of minutes.
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