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

=