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Introduction to the Scientific Basis of Medicine > Epidemiology > Flashcards

Epidemiology Flashcards

1
Q

What is epidemiology?

A

The quantitative study of the distribution, determinants and control of disease in populations.

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2
Q

What is the purpose of a study?

A

To measure the effects of a particular exposure to an outcome.

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3
Q

What are the causes of correlation?

A
  • Causative
  • Confounder
  • Chance
  • Bias
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4
Q

What is a confounding factor?

A
  1. Risk factor or predictive of outcome
  2. Associated with but not direct consequence of exposure
  3. Irrelevant to the study objective
  4. Unequally distributed amongst the
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5
Q

What are the 2 main confounding factors?

A
  • Age

- Sex

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6
Q

What is bias?

A

Systematic deviation from the truth, producing a mistaken estimate of the relationship of an exposure with the risk of disease.

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7
Q

What are Koch’s postulates for causation?

A
  1. The agent appears in every case of the disease.
  2. The agent occurs in no other disease.
  3. The agent is capable of inducing the disease in healthy subject.
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8
Q

What are the problems with Koch’s postulates?

A
  • There are usually multiple factors influencing a disease. These include:
    1. Agent
    2. Host (e.g. species, age, genetics…)
    3. Environment (Weather, housing, air…)
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9
Q

What are the different natures of studies?

A
  1. Therapeutic: Benefits patients, in addition to improving scientific knowledge.
  2. Scientific: May not benefit patients, but improves scientific knowledge.
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10
Q

What are the types of studies?

A
  1. Observational: Observing outcomes in patients with certain exposure.
  2. Descriptive: Using pre-existing statistics from other studies to draw conclusion (e.g. meta-analysis).
  3. Experimental/intervention: Actively exposing patients to certain exposure and measuring outcomes.
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11
Q

What are the criteria for causation?

A
  1. Strength of association
  2. Coherence (biological plausibility)
  3. Time sequence
  4. Consistency of association
  5. Dose response
  6. Reversibility
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12
Q

What are the different types of observational studies?

A
  1. Cross-sectional study: Measure the exposure and prevalence of a disease in a population at one moment in time.
  2. Case control (retrospective) study: Group of people with disease compared to group of people without disease to determine differences in exposure. (Present –> Past)
  3. Prospective study: Select group of people with disease and monitor the incidence of disease in group. (Present –> Future).
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13
Q

What are the advantages of case control studies?

A
  • Cheap
  • Suitable for rare diseases
  • Small number of subjects needed
  • Results quickly obtained
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14
Q

What are the disadvantages of case control studies?

A
  • Selection problems for cases
  • Selection problems for controls
  • Recall/observer bias
  • Time sequence difficult to determine
  • Only produces estimate of relative risk (odds ratio)
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15
Q

What is an odds ratio (relative risk, RR)?

A 
For study (+ is exposure, - is non-exposure) :
Case (+) = a
Control (+) = b
Case (-) = c
Control (-) = d

RR = Incidence (+) / Incidence (-) = (a/(a+b))/(c/(c+d))

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16
Q

What are the interpretations of RRs?

A

RR > 1: Increased risk
RR = 1: No risk
RR

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17
Q

What is excess/absolute risk (E/AR)?

A 
For study (+ is exposure, - is non-exposure) :
Case (+) = a
Control (+) = b
Case (-) = c
Control (-) = d

ER = Incidence (+) - Incidence (-) = (a/(a+b))/time - (c/(c+d))/time

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18
Q

What is proportionate ER?

A

Proportionate ER = ER / Incidence (+)

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19
Q

What us population ER?

A

Population ER = ER x Proportion of population exposed

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20
Q

What are the advantages of prospective studies?

A
  • No recall/observer bias
  • Allows incidence rates and true relative risk to be calculated
  • Allows other associations to be deduced
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21
Q

What are the disadvantages of prospective studies?

A
  • Bias in selection
  • Large sample required
  • Long follow up (expensive)
  • High drop-out rate
  • Methodology/criteria change in disease identification over time
22
Q

What information can be estimated from a retrospective study?

A 
For study (+ is exposure, - is non-exposure) :
Case (+) = a
Control (+) = b
Case (-) = c
Control (-) = d

RR ≃ ad/bc
Incidence (+) = a/b
Incidence (-) = c/d

23
Q

How can bias be reduced in retrospective study?

A
  • Blinding subject/observer to hypothesis

- Use objective methods of obtaining exposure

24
Q

What are 3 key aspects of trials?

A
  1. Randomisation
  2. Control
  3. Blinding
25
Q

Should patients who drop out of a trial be counted in final analysis?

A

Yes, to ensure randomisation is valid.

26
Q

What are the types of blinding?

A
  1. Single blinding (patients):
    - Eliminates placebo
  2. Double blinding (patients + observers):
    - Eliminates placebo
    - Eliminates recording bias
  3. Triple blinding (patients + observers + statisticians):
    - Eliminates placebo
    - Eliminates recording bias
    - Eliminates analysis bias
27
Q

What are the advantages of trials?

A
  • Minimises bias
  • Minimises confounders
  • Quantifies risks/benefits
28
Q

What are the disadvantages of trials?

A
  • Limited feasibility

- Very specific

29
Q

What is prevalence?

A

Proportion of defined group with condition at specific point in time.

30
Q

What is the equation for prevalence?

A

Prevalence = Number of cases / Total population

31
Q

What is incidence?

A

Rate of occurrence of new cases of a disease over a period of time.

32
Q

What is the relationship between prevalence and incidence?

A

Prevalence = Incidence x Duration of disease

33
Q

What does an increase in prevalence and incidence indicate?

A

Better treatment (greater duration of disease or survival time)

34
Q

What does an increased incidence and unchanged prevalence indicate?

A

High mortality

35
Q

What is prevalence used to measure?

A
  • Chronic diseases
  • Slow onset
  • Slow recovery/mortality
36
Q

What is incidence used to measure?

A
  • Acute diseases
  • Quick onset
  • Quick recovery/mortality
37
Q

What is mortality?

A

Death

38
Q

What is morbidity?

A

Illness

39
Q

What is “Number Needed to Treat (NNT)”?

A
  • The number of people that need to be treated in order for one person to prevent the disease in one person.
  • Calculated using ER.
40
Q

What is population excess risk?

A

Population excess risk = ER x Prevalence

41
Q

What are the different types of results that can be obtained from a diagnostic test?

A 
True positive (TP): Test was +ve and person had disease.
False positive (FP): Test was +ve and person didn't have disease.
True negative (TN): Test was -ve and person didn't have disease.
False negative (FN): Test was -ve and person did have disease.
42
Q

What is sensitivity?

A

Proportion of true positives correctly identified by test.

Sensitivity = TP/(TP + FN)

43
Q

What is specificity?

A

Proportion of true negatives correctly identified by the test.
Specificity = TN/(TN +FP)

44
Q

What is the predictive value of a positive test?

A

The likelihood that a person with a positive test has the disease.
Predictive value = TP/(TP + FP)

45
Q

What is the predictive value of a negative test?

A

The likelihood that a person with a negative test does not have the disease.
Predictive value = TN/(TN + FN)

46
Q

How are the predictive values dependent on the prevalence?

A
  • If prevalence is high, positive predictive value also high, and vice versa.
  • If prevalence is low, negative predictive value also low, and vice versa.
47
Q

What is the purpose of an asymptomatic test?

A
  • Detects possible condition in asymptomatic individual before symptoms manifest.
  • Allows early interventions that may improve outcome.
48
Q

What are the criteria for the condition to be worthy of testing?

A
  • Important health problem
  • Natural history understood
  • Detectable at latent stage
  • Early treatment is better than later treatment
49
Q

What are the criteria of the test?

A
  • Sensitive
  • Specific
  • High predictive values
  • Safe
  • Low cost
  • Chance of benefit should be higher than chance of harm
50
Q

What is lead time bias?

A

Increased survival time because disease diagnosed earlier, not because test extended life of participant.

Introduction to the Scientific Basis of Medicine (2 decks)

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