Study Designs Overview and Concepts

Study Designs, Overview

• Study design refers to the approach used in a study.
• It considers whether the study is well-designed, and if so, the presence of potential errors like random error, bias, and confounding.
• It also specifies the methodological approach used in the study.

Study Design Concepts

• Study Purpose: Different types of studies, e.g., descriptive and analytical.
• Time Orientation: Prospective, starting in the present and going forward, or retrospective, starting and ending in the present but looking back in time.
• Investigator Orientation:
  • Experimental trials: Researchers impose an intervention.
  • Quasi-experimental trials: Researchers compare groups but do not fully control the intervention.
  • Observational trials: Researchers observe and record data without manipulating any variables.

Case Reports

• Case reports are summaries of single patient cases.
• They detail exposures and outcomes, often adverse.
• Case reports are useful for hypothesis generation but not causation.
• Exceptions occur when the outcome is rare and characteristic of the exposure.

Case Series

• A case series is a collection of cases with a similar exposure.
• Clinicians often use case series for quantifying the incidence of a reaction after a drug launch.
• Case series are for describing a disease or patient characteristics and providing possible causes, but is not useful for causation.

Analyses of Secular Trends

• These studies examine trends in exposures (presumed causes) and outcomes (presumed effects) over time.
• They can be regional or global.
• Correlation analysis suggests a relationship but does not prove causation.

Case-Control Studies

• Case-control studies compare groups with and without a disease.
• They look back to find differences in exposures.
• Useful for studying rare diseases.
• They are often retrospective.
• Example: Women with venous thromboembolism versus those without it, looking back at different contraceptiv.
• Critical assumptions: Representative cases and controls, similar information gathering.

Cohort Studies

• Cohort studies follow groups of people sharing a common characteristic.
• They track outcomes/exposures over time.
• Prospective cohort studies recruit groups according to exposure and follow them over time, while retrospective cohort studies start with outcomes and work backward.
• Useful for studying the natural history of disease and exposures.

Prospective Cohort Studies

• Researchers assemble a cohort on the basis of exposure (e.g., exposure to a certain drug)
• They follow the cohort over time to observe the effect of the exposure on the outcome (e.g., the rate of side-effects).

Retrospective Cohort Studies

• Researchers start with a group of people who already have an outcome (e.g., a specific disease)
• They look back to see if they were exposed to a particular factor (e.g., a medication) at some point in time.

Cross-Sectional Studies

• Examine relationships between different factors (exposures and outcomes) at one point in time,
• Measures all variables for each study subject at a defined moment in time.
• It's descriptive rather than analytical.
• Useful for prevalence studies and exploring relationships in relatively short periods.

Randomized Clinical Trials (RCTs)

• RCTs are experimental studies in which researchers manipulate the exposure (intervention).
• Participants are randomly allocated to either an intervention or a control group.
• They are considered the gold standard for testing causality.
• Vital to determining whether an intervention causes an outcome, particularly with new treatments.
• Strengths: Random assignment, minimizing confounding influences.
• Critical assumptions to minimise bias:
  • Representitive cases
  • Comparable controls without the disease
  • Same information collection method.
• Blinding techniques:
  • Single-blind: Either the participants or the researchers are unaware of who is in which group.
  • Double-blind: Neither the participants nor the researchers are aware of who is in which group.
  • Triple-blind: The participants, researchers, and analysts are unaware of who is in which group.

Power Analysis of RCTs

• Sample size determination for RCTs requires planning.
• Essential for calculating the minimum number of participants needed to achieve sufficient power.
• Power is the ability of a study to correctly detect a true effect if it exists.