Updated: 7/26/2020

Observational Studies

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Overview
  • Observational studies are studies in which subjects are observed for the effect of exposures on outcomes without intervention by researchers
  • Often done when randomization is not possible for logistical or ethical reasons
Descriptive Studies 
  • Case report
    • detailed description of a patient's symptoms, signs, treatment, and disease course
    • conducted for hypothesis generation or to raise awareness of emerging diseases, rare pathologies or unusual presentations 
    • not useful for hypothesis testing
  • Case series
    • a group of case reports of patients with a similar exposure, treatment, or disease 
    • no comparison group
    • not useful for hypothesis testing
Prospective and Retrospective Studies
  • Prospective studies
    • patients enrolled before data collection
    • participants followed over time 
    • data collected on exposures and outcomes as they arise 
  • Retrospective studies 
    • involve collecting historical information from a sample of patients 
    • enrollment occurs after exposure and outcome have  already occurred
Case-Control Study
  • Retrospective (almost always) 
  • Compares patients with and without disease to asses their odds of a certain exposure   
  • Odds ratio (OR) is measure of disease association    
    • OR = odds of exposure among cases/odds of exposure in non-cases = ad/bc
      • odds of exposure among cases = # cases with exposure/# cases without exposure = a/c
      • odds of exposure among non-cases = # non-cases with exposure/# non-cases without exposure = b/d
  • "How much more likely is it that patients with cirrhosis have been exposed to heavy alcohol use compared to controls?"
Cohort Study
  • Can be either prospective or retrospective
  • Compares groups with and without an exposure to assess associations with subsequent disease  
  • Relative risk (RR) is measure of disease association 
    • RR = incidence rate in exposed group/incidence rate in unexposed group = [a/(a+b)]/[c/(c+d)]
      • incidence rate in exposed group = # of exposed cases/(# total exposed cases and non-cases) = a/(a+b)
      • incidence rate in unexposed group = # of unexposed cases/(# total unexposed cases and non-cases) = c/(c+d)
  • "How much more likely are patients to develop cirrhosis if they are exposed to heavy alcohol use?"
Cross-Sectional Study
  • Exposure and outcome are assessed simultaneously 
  • Ascertains association not causality
  • Prevalence is measure of disease association 
  • "How much higher is the proportion of cirrhosis patients with heavy alcohol use compared to those without cirrhosis?"
Ecological Study
  • Exposure and outcome assessed at the population level
    • population defined geographically or temporally
  • Prevalence and incidence are measures of disease association
  • Often used for rapid hypothesis generation 
  • "How much higher is the proportion of cirrhosis patients in a country where average alcohol use is high compared to a country in which alcohol is illegal and of low use?"
  • Disadvantage is the potential for "ecological fallacy"
    • making spurious risk associations between populations which may not represent true risk association at the individual level
    • e.g. assuming that people who live in a country with high alcohol use rates must be more likely to get cirrhosis by virtue of living in that country
Sibling Studies
  • Measures relative influence of environment vs genetics on trait variation
  • Twin concordance study
    • compares how often monozygotic and dizygotic twins both have the same trait or disease
  • Adoption study
    • 2 designs
      • examines differences and similarities between child and adoptive vs birth parents
      • examines differences and similarities between siblings raised by adoptive vs birth parents
 

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(M1.ST.15.18) A researcher interested in the relationship between vaccination and autism sends a survey to parents of children who are active patients at a large primary care practice. The survey asks several questions, including whether their children received their childhood vaccines on-time, and whether their children currently have a diagnosis of an autism spectrum disorder. Which of the following correctly identifies the study design used by the researcher? Tested Concept

QID: 104024
1

Cross-sectional

54%

(19/35)

2

Case-crossover

6%

(2/35)

3

Cohort

23%

(8/35)

4

Randomized controlled trial

0%

(0/35)

5

Case-control

11%

(4/35)

M 1 D

Select Answer to see Preferred Response

(M1.ST.15.20) A recent study attempted to analyze whether increased "patient satisfaction" driven healthcare resulted in increased hospitalization. In this hospital, several of the wards adopted new aspects of "patient satisfaction" driven healthcare, whereas the remainder of the hospital continued to use existing protocols. Baseline population characteristics and demographics were collected at the start of the study. At the end of the following year, hospital use was assessed and compared between the two groups. Which of the following best describes this type of study? Tested Concept

QID: 102599
1

Retrospective cohort

18%

(30/164)

2

Prospective cohort

41%

(67/164)

3

Retrospective case-control

15%

(25/164)

4

Prospective case-control

11%

(18/164)

5

Cross-sectional study

11%

(18/164)

M 1 E

Select Answer to see Preferred Response

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(M1.ST.14.10) You would like to conduct a study investigating potential risk factors that predispose patients to develop cirrhosis. Using a registry of admitted patients over the last 10 years at your local hospital, you isolate all patients who have been diagnosed with cirrhosis. Subsequently, you contact this group of patients, asking them to complete a survey assessing their prior exposure to alcohol use, intravenous drug abuse, blood transfusions, personal history of cancer, and other medical comorbidities. An identical survey is given to an equal number of patients in the registry who do not carry a prior diagnosis of cirrhosis. Which of the following best describes the type of study you are attempting to conduct? Tested Concept

QID: 104016
1

Cohort study

10%

(2/20)

2

Meta-analysis

0%

(0/20)

3

Case-control study

75%

(15/20)

4

Cross-sectional study

15%

(3/20)

5

Randomized controlled trial

0%

(0/20)

M 1 E

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(M1.ST.14.13) A new study shows a significant association between patients with a BMI >40 and a diagnosis of diabetes (odds ratio: 7.37; 95% CI 6.39-8.50) compared to non-diabetic patients. Which of the following hypothetical studies most likely yielded these results. Tested Concept

QID: 103975
1

A study consisting of 1000 non-diabetic subjects; 500 patients with a BMI > 40 and 500 patients with normal BMI, followed for diagnosis of diabetes over their life time

21%

(7/33)

2

A study consisting of 500 patients with diabetes and 500 patients without diabetes comparing BMI of subjects in both groups

45%

(15/33)

3

A study consisting of 1000 genetically similar mice; 500 randomized to diet to maintain normal weight and 500 randomized to high caloric intake with the outcome of diabetes rates in both groups after 1 year

3%

(1/33)

4

A study of 1000 patients with BMI > 40 with diabetes; 500 randomized to inpatient diet and exercise with goal BMI <25, and 500 randomized to no treatment with an outcome of glycemic control without medication after 1 year

3%

(1/33)

5

A study of 1000 patients comparing rates of diabetes diagnoses and BMIs of diabetic and non-diabetic patients

18%

(6/33)

M 1 E

Select Answer to see Preferred Response

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