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IHST 2010 Study Guide - Final Guide: Face Validity, Evaluation Strategy, Official Statistics

Global Health
Course Code
IHST 2010
Lesley Beagrie
Study Guide

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- Study of the distribution and patterns of health-related states and factors that affect these.
- Survey random people to see: if they ever had disorder, if so when it began, when it ended.
- Then determine when to count the cases typically one-month, six-month, or one-year window
Nature of Epidemiology
- As a discipline, epidemiology knows few, if any boundaries.
- Epidemiology is the study of our collective health.
- It compares two facets:
- First, descriptive, involving the identifying of patterns, trends and disease and injury differentials
- Second, it moves beyond the descriptive approach to embrace causation or etiology. This is also called analytic
Measures of Frequency incidence and prevalence
- Prevalence rate: proportion of people at risk who have a specific condition at a given time
- Point prevalence: when the window is a specific date
Period prevalence: when the window is wider, one-month,
- six-months, or a year
- Lifetime prevalence: number of people who have ever had the disorder
- Natural history of a disorder
- Frequency of disease cases in a population at a time
- Expressed as proportion
- Number of people in sample with disorder divided by the total sample size
- Multiply by 100 to express as percentage
- E.g. 37 cases in sample of 250
- Prevalence = 14.8%
Problems in estimating prevalence
- Need large sample representative of the population
- May be biased by refusal to participate
- Especially if people with disorder of interest are less likely to participate
- Measurement inaccuracy
- More confidence in prevalence estimates if there is agreement across several studies
Measures of Frequency (cont.)
- Incidence rate: looks at the only new cases, those who came down w/ the disorder w/n the window
- What if someone previously had this disorder are they counted?
o Use judgement or make an arbitrary decision
o Exclude people with disorder at start of window
- All incident cases are also prevalent ones, but not all prevalent cases are incident ones
- Cross-sectional survey: sample people at one point in time
- Longitudinal survey: people are interviewed repeatedly over time. More useful but more expensive
- Incidence density (ID): number of new cases in each time divided by total person minus time.
o Reflects the burden of the disorder on society
- Duration: how long the disorder lasts
Mortality Rate
- MR reflects how many people die from a disorder in a given time, usually one year.
- MR= Number of deaths due to a disorder in a given time/ Number of people at risk
- The MR by itself doesn’t tell us much, difficult to interpret
Case Fatality Rate
- CFR is the proportion of people with the disorder who die within a given time, usually one year
- CFR = Number of deaths due to a disorder in a given time/ Number of people with the disorder
- The CFR still doesn’t tell us much by itself and we also don’t know for sure how many people have the disorder
Proportional Mortality Rate
- PMR is the number of deaths due to a disorder in a give time compared to all deaths
- PMR = Number of deaths due to a disorder in a given time/ Number of deaths from all disorders
- Can increase if more people contract and die from the disorder or if fewer people die from other causes.
Standardized Mortality Rate
- SMR is the ratio of the number of observed deaths from a given cause in a given time expected number of deaths
o SMR <1 means people with the disorder are dying at a lower than expected rate
o SMR > 1 indicates a higher than expected death rate
o Cause-specific SMRs: SMRs in a specific group due to specific cause
Age-standardized Mortality Rate
- Adjusts the age distributions so that they are the same
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- Often shown on an age pyramid
Measures of Impact
- Risk: probability that an event will occur
- Risk factor: an aspect of one’s life that is known to be associated with a health-related condition
- Risk determinant: something that is casually related to an outcome
Risk factors
- Many diseases associated with a range of factors that are neither necessary nor sufficient to cause the disease
but exposure increases onset probability
- Epidemiology identifies risk factors and contributes to understanding the casual mechanisms through which they
Cross-sectional studies
- Measure risk factors and disease at a single time point
- Test associations between risk factors and disease
- Identified associations may implicate casual risks
- But there are other potential explanations
Measures of Impact (cont.)
- Risk marker: a factor that is related to a higher prevalence of an outcome, may or may not be casually related
- Modifiable risk factor: one that is casually related to an outcome, and whose change can affect the outcome
Analytical Methodologies in Epidemiology
- 2 major analytic methods used in epidemiologic investigations: cohort and case control
- Purpose of each a is to test the hypothesized cause-and-effect relationship b/w a suspected risk factor and a
disease, injury, or even a social condition such as welfare status.
Methodologies in Epidemiology (cont'd)
- Cohort Studies
o Cohort studies are sometimes called prospective studies.
o They work from a postulated cause to an effect.
o General design is to begin with a group of people (a cohort) and observe them over a period of time.
o Selection for the group can be based upon the presence or absence of a characteristic or at random.
o Individuals within the group will vary in exposure to one or more of the factors being observed.
o They determine the differences in the rate at which the characteristic occurs (disease, injury, and
behavioral problem) in relation to exposure to the factor(s).
- Case-Control Investigations
o Case-control investigations are sometimes referred to as
o retrospective studies.
o They start with a group of people who already have the characteristic (health problem) and compare them
with people who do not have that health problem (characteristic).
o People who have the health problem are referred to as "cases," while those in the other group are termed
"controls. "
o In searching for the "cause," the case-control method determines if the two groups differ in the degree of
exposure to different factors.
o Case-control method moves backward in time (effect to cause).
Cohort Investigations
- A cohort is a group of people with a common experience
- over a defined period of time.
- A current cohort study means that the subjects are selected at the beginning of the study (present time) and
followed over a period of time (future time).
o The cohort model can be modified by making it a retrospective in nature rather than a perspective one.
Cohort studies
- Longitudinal studies
- Follow a sample (‘cohort’) across time
- Example large-scale UK cohort study: National Child Development Study, 17,000 individuals born 1958, Studied
at birth and ages 7, 11,16, 23, 33, 42 and 50
Cohort studies and causality
- Strongest inference when cohort study begins prior to
- disease onset
- Rules out possibility that hypothesized risk factor is really an outcome of the disease
- However, causal evidence provided by longitudinal relationship not immune to ‘third variable problems’
o Experimental studies (e.g. randomized controlled trials) required for strong causal evidence
o But experimentation on questions of interest often limited by ethical/practical constraints
Case-Control Studies
- A case-control study requires the investigator to work
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- backward in time.
- Part of the sample is drawn from a population of patients with the outcome (the cases).
- The other part is drawn from a population who does not have the outcome (the controls).
- The investigator then compares the predictor variables to determine which ones contributed to the outcome.
- Generally, it is much less expensive than cohort investigations.
- Most studies require fewer subjects.
- Sources for case groups can be records of physicians and employers, hospital records, death certificates, etc.
Case-control studies
- Cannot estimate disease prevalence
- Can identify risk factors
o Recruit
A sample with disease of interest n A matched sample of controls
- Compare on hypothesized risk factors
Case-Control Studies (cont'd)
- Information can be gathered from subjects themselves or even from proxies for the subjects or patients.
o Oftentimes, a combination of records and actual subjects is used.
- In many case-control studies, selection of cases is often simple in that the number of sources is very limited.
- In control groups, the obvious point is to procure a sample from a population at risk for the outcome.
- Controls should be representative of the general population in terms of probability of exposure to the risk factor(s).
They should have had the same opportunity to be exposed.
- It is important that sources and methods of data collection are similar for both cases and controls.
Relative Risk
- The ratio of the risks for the two groups within a cohort study
- Cohort study: has two cohorts of people, one exposed to the causal factor and the other has not been exposed
- Calculate the risk for the exposed group and compare it to the risk for the unexposed group
Relative Risk, cont’d
- Risk of death (MHE)
- Risk of death (comparison cohort)
- Relative risk
- Relative risk reduction (RRR): express improvement as a percentage relative to the comparison group,
ARR= Risk↓Comparison=0.70-0.60=0.10
Population Attributable Risk
- How much the incidence of the outcome can be reduced in the population if we were able to eliminate the risk
- To calculate, we need to know risk difference (risk of outcome b/w exposed and unexposed) and population
Problems w/ Relative Risk
- The RR is non-invertible if you focus opposite outcome, you get different results
- The RR does not take into account the prevalence of the disorder
Number Needed to Treat or Harm
- Number needed to treat (NNT): number of people needed to be treated for there to be one additional positive
- Number needed to harm (NNH): how many people must be treated to produce undesirable side-effects in one
more person
Risk ratio Dancey p. 170
- Compare disease risk in
- Those exposed to a hypothesized risk factor with the risk in those who were not exposed
- Expressed as a ratio
- E.g. Exposed risk: .024, Unexposed risk: .004
- Risk ratio: .024/.004 = 6
- Exposed people 6 times more likely to have disease than those unexposed
- Risk ratios always positive.
- Risk ratio of 1 indicates no link between factor and disease
- Risk ratio of >1 indicates exposed group more likely to have the disease n Risk ratio <1 indicates factor is
protective less likely
Odds ratio
- Odds are the probability an event will happen divided by the
- probability that it will not happen
- Odds ratio is odds of disease in the exposed group divided by the odds of disease in the unexposed group
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