Epidemiology I
What is epidemiology?
What common measures are used in the field of epidemiology?
What are the subject areas studied by epidemiologists?
How has epidemiology evolved over time? What is the current focus of epidemiology?
What are the health challenges of modern medicine (and focus of epidemiology)?
Definition of Epidemiology
Study of the distribution and determinants of diseases and injuries in human populations Concerned with frequencies and
types of injuries and illness in groups of people
Focus is not on the individual Concerned with factors that
influence the distribution of illness and injuries
Relationship Between Clinical Medicine and
Epidemiology Focus in medicine is the individual
patient Community replaces the individual
patient in epidemiology
Fundamental Assumptions in Epidemiology
Disease doesn’t occur at random Disease has causal and preventive
factors* Disease is not randomly
distributed throughout a population
Epidemiology uses systematic approach to study the differences in disease distribution in subgroups
Allows for study of causal and preventive factors
Components of Epidemiology
Measure of disease frequency Quantification of existence or occurrence
of disease Distribution of disease - three
questions Who is getting disease? Where is disease occurring? When is disease occurring? Formulation of hypotheses concerning
causal and preventive factors Determinants of disease
Hypothesis are tested using epidemiologic studies
Incidence
Measure of new cases of disease (or other events of interest) that develop in a population during a specified period of time E.g. Annual incidence, five-year incidence
Measure of the probability that unaffected persons will develop the disease
Used when examining an outbreak of a health problem
Prevalence Number of existing cases of
disease or other condition Proportion of individuals in a
population with disease or condition at a specific point of time
Diabetes prevalence, smoking prevalence
Provides estimate of the probability or risk that one will be affected at a point in time
Provides an idea of how severe a problem may be – measures overall extent
Useful for planning health services (facilities, staff)
Ratio
Used to compare two quantities1:1.1 ratio of female to male births
Used to show quantity of disease in a population
cases population
Proportion
A specific type of ratio in which the numerator is included in the denominator, usually presented as a percentage
352 males undergoing bypass surgery
539 total patients undergoing bypass surgery
=
Calculation of proportion:
Males undergoing bypass surgery at Hospital A
Total patients undergoing bypass surgery at Hospital A
65.3%
Rate Special form of proportion that includes a
specification of time Most commonly used in epidemiology
because it most clearly expresses probability or risk of disease or other events in a defined population over a specified period of time
3 major types Crude rates Specific rates (age-specific, infant mortality) Adjusted rates
Crude rates Unadjusted, simple ratios
cases in defined period of timex K
population in defined period of time
(k denotes units 100’s, 1,000, etc.)
Crude mortality rate:
Total deaths in 2003 x 1,000 = U.S. death
rateEstimated U.S. pop in 2003
Adjusted or Standardized Rates Allow for comparison of
populations with different characteristics Statistically constructed summary
rates allow for appropriate comparisons by taking into account differences in populations (age, gender, etc.)
Example of use: Population in Arizona is much older than population in Alaska, so it would be inappropriate to compare mortality rates. Standardization allows for meaningful comparisons.
Incidence Rate
Also known as incidence density Measure of incidence that is able to
handle varying observation periods Denominator is sum of person-time
at risk
Relationship Between Incidence and Prevalence
Prevalence varies directly with both incidence and duration. If incidence is low, but duration is long
(chronic), prevalence will be large in relation to incidence.
If prevalence is low because of short duration (due to recovery, migration or death), prevalence will be small in relation to incidence.
Measures of Association
Calculations used to measure disease frequency relative to other factors
Indications of how more or less likely one is to develop disease as compared to another
Two by Two TablesUsed to summarize frequencies of disease and exposure and used for calculation of association.
Disease
Exposure
Yes No
Yes
No
Total
Total
a
c d
b a + b
c + d
a + c b + d a + b + c + d
Relative Risk
Measure of association between incidence of disease and factor being investigated
Ratio of incidence rate for persons exposed to incidence rate for those not exposed Incidence rate among exposed
RR = Incidence rate among unexposed
Estimate of magnitude of association between exposure and disease
Incidence rate among exposedRR = Incidence rate among unexposed
Formula for relative risk:
a / (a + b)RR = c / (c+ d)
• Risk ratioIf RR calculated from cumulative incidence
• Rate ratioIf RR calculated from incidence rate (person units
of time)
RISK RATIO: Example Breast No Breast Cancer Cancer
TotalAlcohol 70 2,930
3,000No alcohol 50 2,950
3,000
RR using Cumulative Incidence (CI):
a/(a + b) 70 / 3,000 c/(c + d) 50 / 3,000
= = = 1.4
Interpretation of Relative Risk
1 = No association between exposure and disease Incidence rates are identical between
groups > 1 = Positive association < 1 = Negative association or
protective effect Example: .5 = half as likely to
experience disease
Odds Ratio Breast No Breast Cancer CancerAlcohol 70 100No alcohol 50 140 a x d (70) (140) b x c (50) (100)* Used for case control studies because persons
are selected based on disease status so you can’t calculate risk of getting disease
OR = = = 2.0
Difference Measures Attributable risk
# of cases among the exposed that could be eliminated if the exposure were removed
= Incidence in exposed - Incidence in unexposed
Population attributable risk percent Proportion of disease in the study population
that could be eliminated if exposure were removed
Incidence in total population - Incidence in unexposed
incidence in total population
=