Achieve Mastery of Medical Concepts

Study for medical school and boards with Lecturio

USMLE Step 1     |     USMLE Step 2   |    COMLEX Level 1   |     COMLEX Level 2   |     ENARM   |    NEET

Create your free account
The Lecturio Medical Concept Library
Epidemiological Studies

Epidemiological Studies

Epidemiological studies are used to establish associations between risk factors and health-related outcomes. These studies can take the form of observational or interventional studies. Observational studies are nonexperimental investigations of the associations between known exposures and outcomes. Interventional studies are designed to evaluate the direct impacts of a risk factor on a disease by applying an intervention to the subjects in an experimental group and prospectively comparing the effects with a control group. Interventional studies are more expensive and time-consuming but tend to provide stronger evidence for an association than observational studies. Depending on the situation, investigators will typically choose a combination of these study designs.

Last updated: 10 Mar, 2022

Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Contents

Observational Studies

Observational studies are used to observe and measure outcomes in a cohort with no control over risk factors or variables. They are often retrospective. Types of observational studies include cross-sectional studies, case-control studies, and cohort studies.

Cross-sectional studies

  • Collect data about an entire population at a single point in time (cross-section of time)
    • Participants are not followed over time.
  • Typically used to measure prevalence Prevalence The total number of cases of a given disease in a specified population at a designated time. It is differentiated from incidence, which refers to the number of new cases in the population at a given time. Measures of Disease Frequency (i.e., how many people have the disease in a population)
    • Also called prevalence Prevalence The total number of cases of a given disease in a specified population at a designated time. It is differentiated from incidence, which refers to the number of new cases in the population at a given time. Measures of Disease Frequency studies
  • Advantages:
    • Inexpensive, if making use of routinely collected data
    • Easy to perform
  • Disadvantages:
    • Unable to determine causality Causality Causality is a relationship between 2 events in which 1 event causes the other. Simply because relationships are observed between 2 variables (i.e., associations or correlations) does not imply that one variable actually caused the outcome. Demonstrating causality between an exposure and an outcome is the main objective of most published medical research. Causality, Validity, and Reliability because exposures and outcomes are measured simultaneously
    • Unable to include data on confounding variables Confounding variables A confound is an additional variable other than the independent variable that has an effect on the dependent variable, causing an erroneous relationship to be inferred between them. Types of Biases

Case-control studies

  • Start from an outcome and work backward (retrospectively) in time to see who had an exposure Exposure ABCDE Assessment
  • 2 groups are compared based on an outcome or the presence of disease:
    1. Cases: those who have or have had the outcome or disease
    2. Control: those who lack the outcome or disease
  • The proportions of those with an exposure Exposure ABCDE Assessment in the case and control groups are compared to see if there is an association.
  • Matching: selecting controls so that they share similar characteristics with cases that are identified as possible confounding factors (e.g., sex Sex The totality of characteristics of reproductive structure, functions, phenotype, and genotype, differentiating the male from the female organism. Gender Dysphoria, age, smoking Smoking Willful or deliberate act of inhaling and exhaling smoke from burning substances or agents held by hand. Interstitial Lung Diseases status) in an effort to remove the confounding effect
    • Individual matching: matching an individual control to an individual case
    • Group or frequency matching: equal percentages of controls and cases with a characteristics group with characteristics (e.g., 50% of cases and controls are men)
  • Advantages:
    • Inexpensive
    • Can be carried out by small groups of investigators
    • Shorter in duration
  • Disadvantages:
    • Cannot measure the incidence Incidence The number of new cases of a given disease during a given period in a specified population. It also is used for the rate at which new events occur in a defined population. It is differentiated from prevalence, which refers to all cases in the population at a given time. Measures of Disease Frequency
    • Cannot reliably determine a subject’s exposure Exposure ABCDE Assessment status over time (subject to observation bias Bias Epidemiological studies are designed to evaluate a hypothesized relationship between an exposure and an outcome; however, the existence and/or magnitude of these relationships may be erroneously affected by the design and execution of the study itself or by conscious or unconscious errors perpetrated by the investigators or the subjects. These systematic errors are called biases. Types of Biases)
    • Identifying a sample of controls can be difficult and subject to selection Selection Lymphocyte activation by a specific antigen thus triggering clonal expansion of lymphocytes already capable of mounting an immune response to the antigen. B cells: Types and Functions bias Bias Epidemiological studies are designed to evaluate a hypothesized relationship between an exposure and an outcome; however, the existence and/or magnitude of these relationships may be erroneously affected by the design and execution of the study itself or by conscious or unconscious errors perpetrated by the investigators or the subjects. These systematic errors are called biases. Types of Biases.

Cohort studies

  • An observational study that chooses a cohort that shares a common exposure Exposure ABCDE Assessment and observes this group over time (prospectively) to see who develops the outcome of interest
  • A longitudinal study in which the temporal relationship Relationship A connection, association, or involvement between 2 or more parties. Clinician–Patient Relationship between exposure Exposure ABCDE Assessment and outcome is established
    • Cohorts can also be “retrospective” (i.e., looking into medical records and seeing if the outcome developed itself prospectively in the past), but this restricts the ability of the investigator to control for confounding and bias Bias Epidemiological studies are designed to evaluate a hypothesized relationship between an exposure and an outcome; however, the existence and/or magnitude of these relationships may be erroneously affected by the design and execution of the study itself or by conscious or unconscious errors perpetrated by the investigators or the subjects. These systematic errors are called biases. Types of Biases.
  • Typically used for establishing possible cause and effect before randomized control trials (RCTs) are undertaken
Table: Advantages and disadvantages of cohort studies
Advantages Disadvantages
Direct calculation of incidence Incidence The number of new cases of a given disease during a given period in a specified population. It also is used for the rate at which new events occur in a defined population. It is differentiated from prevalence, which refers to all cases in the population at a given time. Measures of Disease Frequency rates Time-consuming
May yield information on the incidence Incidence The number of new cases of a given disease during a given period in a specified population. It also is used for the rate at which new events occur in a defined population. It is differentiated from prevalence, which refers to all cases in the population at a given time. Measures of Disease Frequency of disease Often requires a large sample size Sample size The number of units (persons, animals, patients, specified circumstances, etc.) in a population to be studied. The sample size should be big enough to have a high likelihood of detecting a true difference between two groups. Statistical Power
Clear temporal relationship Relationship A connection, association, or involvement between 2 or more parties. Clinician–Patient Relationship between exposure Exposure ABCDE Assessment and disease Expensive
Particularly efficient for the study of rare exposures Not efficient for the study of rare diseases
Can yield information on multiple exposures Losses to follow-up may diminish their validity Validity Validity refers to how accurate a test or research finding is. Causality, Validity, and Reliability
Can yield information on multiple outcomes of a particular exposure Exposure ABCDE Assessment Changes over time in diagnostic methods may lead to biased results
Minimizes bias Bias Epidemiological studies are designed to evaluate a hypothesized relationship between an exposure and an outcome; however, the existence and/or magnitude of these relationships may be erroneously affected by the design and execution of the study itself or by conscious or unconscious errors perpetrated by the investigators or the subjects. These systematic errors are called biases. Types of Biases
Table: When to use which research Research Critical and exhaustive investigation or experimentation, having for its aim the discovery of new facts and their correct interpretation, the revision of accepted conclusions, theories, or laws in the light of newly discovered facts, or the practical application of such new or revised conclusions, theories, or laws. Conflict of Interest design
Exposure Exposure ABCDE Assessment Outcome Study type
Uncommon Common Cohort
Common Uncommon Case-control
Scenario Study type
Short-term study of current, uncommon exposures and common outcomes Prospective cohort
Short- or long-term study of historic, uncommon exposures and common outcome Retrospective cohort
Outbreak investigation Case-control
Instantaneous survey, common outcomes and no change over time Cross-sectional
Is there an association between climbing Mount Everest and getting diabetes Diabetes Diabetes mellitus (DM) is a metabolic disease characterized by hyperglycemia and dysfunction of the regulation of glucose metabolism by insulin. Type 1 DM is diagnosed mostly in children and young adults as the result of autoimmune destruction of β cells in the pancreas and the resulting lack of insulin. Type 2 DM has a significant association with obesity and is characterized by insulin resistance. Diabetes Mellitus? Cohort
Is there an association between left-handedness and getting mad cow disease? Case-control
Is there an association between left-handedness and gender Gender Gender Dysphoria? Cross-control
What factor was likely responsible for the salmonella Salmonella Salmonellae are gram-negative bacilli of the family Enterobacteriaceae. Salmonellae are flagellated, non-lactose-fermenting, and hydrogen sulfide-producing microbes. Salmonella enterica, the most common disease-causing species in humans, is further classified based on serotype as typhoidal (S. typhi and paratyphi) and nontyphoidal (S. enteritidis and typhimurium). Salmonella outbreak at the office holiday party? Case-control
Was there an association between working on the nuclear bomb project in World War II and developing cancer 5 years later? Retrospective cohort
General categories of population research design

A flowchart of the different types of research Research Critical and exhaustive investigation or experimentation, having for its aim the discovery of new facts and their correct interpretation, the revision of accepted conclusions, theories, or laws in the light of newly discovered facts, or the practical application of such new or revised conclusions, theories, or laws. Conflict of Interest (observational vs. experimental) and their representative studies. Observational studies passively look at a cohort with a disease or condition over a period of time (often retrospectively); experimental studies prospectively look at the effect of a particular intervention or exposure Exposure ABCDE Assessment in a group as compared to controls.

Image by Lecturio.

Related videos

3:03
Population Research Designs – Observational Studies (Study Designs)
Cross-sectional Study – Observational Studies (Study Designs)
3:11
Case-control Study – Observational Studies (Study Designs)
7:04
Cohort Study – Observational Studies (Study Designs)
3:21
Observational Studies (Study Designs): Summary
3:18
Matched Studies – Observational Studies (Study Designs)
0:31
Learning Outcomes – Observational Studies (Study Designs)

Interventional Studies

Interventional studies are prospective experimental trials in which investigators compare the effects of an intervention on subjects with a control group. The most compelling type of interventional study is the RCT.

Randomized controlled trial

  • Also called randomized clinical trials
  • Gold standard for evidence:
    • Provides the strongest evidence for establishing causation
  • Used to:
    • Compare the efficacy of new therapy regimens and drugs against current therapies
    • Evaluate new screening Screening Preoperative Care and prevention strategies
    • Come up with new ways to organize and deliver healthcare
  • Can be performed in both clinical and community settings
  • Study groups: also called study arms
    • Intervention/experimental group: Participants are exposed or receive the intervention.
    • Control group: Participants receive the current standard of care Standard of care The minimum acceptable patient care, based on statutes, court decisions, policies, or professional guidelines. Malpractice, a placebo, or nothing at all.
  • Groups are chosen so that the 2 groups have almost identical conditions, except for the exposure Exposure ABCDE Assessment of interest.
  • Groups must be balanced: equivalent in size
  • Principle of intention to treat:
    • If subjects change groups, the results should be analyzed according to the initial group allocation of subjects.
  • Randomization:
    • Random allocation of subjects to either intervention or control group
    • Participants are assigned to either group by chance.
    • Reduces the chances of selection Selection Lymphocyte activation by a specific antigen thus triggering clonal expansion of lymphocytes already capable of mounting an immune response to the antigen. B cells: Types and Functions bias Bias Epidemiological studies are designed to evaluate a hypothesized relationship between an exposure and an outcome; however, the existence and/or magnitude of these relationships may be erroneously affected by the design and execution of the study itself or by conscious or unconscious errors perpetrated by the investigators or the subjects. These systematic errors are called biases. Types of Biases because the chance of being in either group is 50/50
  • Blinding:
    • Subjects and/or investigators don’t know the groups to which subjects are assigned.
    • Single-binding: Subjects do not know if they are getting the intervention or a placebo.
    • Double-blinding: Both participants and investigators do not know to which groups subjects are assigned.
    • Triple-blinding: blinding of the meta-analyst
    • Blinding reduces the chances of the Hawthorne and Rosenthal effects
      • Hawthorne effect Hawthorne effect Refers to the tendency of subjects in a study to behave or act differently (i.e., work harder) when they know they are being watched. Types of Biases: the tendency of subjects to behave differently if they know they are being studied
      • Rosenthal effect Rosenthal effect Refers to the tendency of subjects or investigators to behave differently based on other’s expectations. Types of Biases: changes in the behavior of subjects based on the researcher’s expectations
  • Placebo:
    • A medically ineffective intervention that mimics a real intervention so that a subject will not know to which study group they are assigned
    • Placebo effect: Any treatment, even if ineffective, results in improvement because the recipient believes it will.
Table: Advantages and disadvantages of randomized controlled trials
Advantages Disadvantages
Confounding variables Confounding variables A confound is an additional variable other than the independent variable that has an effect on the dependent variable, causing an erroneous relationship to be inferred between them. Types of Biases are well-controlled Cost
Temporal relationship Relationship A connection, association, or involvement between 2 or more parties. Clinician–Patient Relationship is well-established Study groups do not necessarily represent the real world.
If blinded, provides strong evidence for causation Ethically problematic
Rct

Flowchart showing how a defined population is distributed into different study groups through randomization when setting up an RCT. Randomization is carried out by the investigators so participants don’t choose which group they’ll be a part of.

Image by Lecturio.
Table: Differences between randomized controlled trials and case-control studies
RCTs Case-control studies
Prospective Retrospective/prospective
Experimental study: An intervention ( exposure Exposure ABCDE Assessment) (e.g., drug, screening Screening Preoperative Care test) is applied to a group and effects are compared to a control group. Observational study: A group is chosen with a common exposure Exposure ABCDE Assessment and followed longitudinally to track to development of an outcome (e.g., disease, condition).
Subjects are randomized by investigators before exposure Exposure ABCDE Assessment occurs. The subjects, or medical records, report their exposure Exposure ABCDE Assessment.

Quasi-experiment

Quasi experiment

Conceptual map showing the distribution of the subjects in studies that employ a quasi-experimental design. Note the lack of randomization.

Image by Lecturio.

Natural experiments

  • Resembles an RCT but the allocation is decided by an external force (e.g., natural disaster)
Natural experiment

Conceptual map showing the distribution of the study population in natural experiments. Randomization is not carried out by the investigators but by an external force and is subject to bias Bias Epidemiological studies are designed to evaluate a hypothesized relationship between an exposure and an outcome; however, the existence and/or magnitude of these relationships may be erroneously affected by the design and execution of the study itself or by conscious or unconscious errors perpetrated by the investigators or the subjects. These systematic errors are called biases. Types of Biases.

Image by Lecturio.

Ecological studies

  • Populations are the unit of analysis instead of individuals.
  • Ecological fallacy: Associations for populations cannot be applied to individuals.

Related videos

8:02
Randomized Clinical Trial (RCT) – Interventional Studies (Study Designs)
6:09
Rct: Advantages and Disadvantages – Interventional Studies (Study Designs)
8:19
Groups – Interventional Studies (Study Designs)
7:15
Cross-sectional Study – Observational Studies (Study Designs)
0
3:56
Cross-sectional Study – Observational Studies (Study Designs)
1
3:33
Cross-sectional Study – Observational Studies (Study Designs)
2
0:52
Cross-sectional Study – Observational Studies (Study Designs)
3

References

  1. Hammill, B. G. (2013). Chapter 12: Observational study designs. In R. D. Lopes, & R. A. Harrington (Eds.), Understanding clinical research. New York, NY: The McGraw-Hill Companies. accessmedicine.mhmedical.com/content.aspx?aid=57836443
  2. Dawson, B., & Trapp, R. G. (2004). Chapter 2: Study designs in medical research. Basic & clinical biostatistics, 4th ed. New York, NY: The McGraw-Hill Companies. accessmedicine.mhmedical.com/content.aspx?aid=2046062
  3. Schnipper, J. L. (2017). Research in the hospital. In S. C. McKean, J. J. Ross, D. D. Dressler & D. B. Scheurer (Eds.), Principles and practice of hospital medicine, 2nd ed. New York, NY: McGraw-Hill Education. accessmedicine.mhmedical.com/content.aspx?aid=1137607526
© 2021 Lecturio GmbH. All rights reserved.

USMLE™ is a joint program of the Federation of State Medical Boards (FSMB®) and National Board of Medical Examiners (NBME®). MCAT is a registered trademark of the Association of American Medical Colleges (AAMC). NCLEX®, NCLEX-RN®, and NCLEX-PN® are registered trademarks of the National Council of State Boards of Nursing, Inc (NCSBN®). None of the trademark holders are endorsed by nor affiliated with Lecturio.

Study on the Go

Lecturio Medical complements your studies with evidence-based learning strategies, video lectures, quiz questions, and more – all combined in one easy-to-use resource.

Learn even more with Lecturio:

Complement your med school studies with Lecturio’s all-in-one study companion, delivered with evidence-based learning strategies.

START NOW FOR FREE

User Reviews

¡Hola!

Esta página está disponible en Español.

Versión en Español

🍪 Lecturio is using cookies to improve your user experience. By continuing use of our service you agree upon our Data Privacy Statement.

got it
Details