Descriptive Studies: pPerson, Place and Time

D i ti E id i lDescriptive Epidemiology• Includes activities related toIncludes activities related to

characterizing the distribution of diseases within a populationdiseases within a population

Analytical EpidemiologyAnalytical Epidemiology• Concerns activities related to

identifying possible causes for the occurrence of diseases

Descriptive Epidemiology

• Epidemiological equivalent of the game “20 Questions”g Q

animal, mineral or vegetable?• In Descriptive Epidemiology:In Descriptive Epidemiology:

Who? - personWhere? placeWhere? - placeWhen? - time

Descriptive Epidemiology

PLACE

TIME

PERSON Think of this as the standard dimensions used to track the occurrence of a disease.

Person

• WHO is getting the disease?• Many variables are involved andMany variables are involved and

studied, but factors such as sex, age & race often have a major effect& race often have a major effect.

Characteristics of PersonCharacteristics of Person• AgeAge• Sex

Ethnic group• Ethnic group• Socioeconomic status• Nativity• Religione g o• Marital status• Occupation• Occupation

Age

Age

Sex

TimeTime• WHEN does the disease occur?WHEN does the disease occur?

“Temporal”Range from hours to decadesRange from hours to decades

• Type of disease dictates “time” element to be usedelement to be used

• Graphic format often usedy-axis (vertical) - frequencyx-axis (horizontal) - time

Characteristics Relating to Time

• Secular change (long-term) • Point epidemics (short-term)Point epidemics (short term) • Cyclic trends

S l i ti• Seasonal variation

Secular Change

Secular Change• Secular changes (“temporal• Secular changes ( temporal

variation”) occur slowly over long periods of timeperiods of time

Longer than one yearIncidence Rates of Cancers in Women

Incidence Rates of Cancers in Men

Point EpidemicsPoint Epidemics• Short-term changes occur over

limited time frameslimited time framesHoursDDaysWeeksMonths

• Used for short-term exposures or diseases with short incubation and/or illness durations

Point EpidemicsPoint Epidemics

Point Epidemicsp

Cyclic TrendsCyclic Trends• Cyclic trends may be either long-y y g

term or short term events.• Some are “seasonal” while othersSome are seasonal while others

are cyclic due to other factors:ImmigrationImmigrationSchool year Military deploymentMilitary deployment

Cyclic TrendsCyclic Trends

Seasonal Variation

• Seasonal i tivariation can

be seen for disome diseases

or conditions f lli ithifalling within a calendar year

Seasonal Variation• Seasonal variation can be used to

suggest possible etiologysuggest possible etiology.

Migratory Birds?

Time Clustering• Time clustering data can sometimes

be used to trace the “beginning” tobe used to trace the “beginning” to the introduction of a specific causal agentagent

Thalidomide & birth defectsFi t k t d i E i 1950’First marketed in Europe in 1950’s as sleeping pill and to treat morning sickness in pregnant women

Toxic Shock SyndromeStaphylococcus aureus infection in women using newly introduced hyperabsorbent tampons

Time Clustering

Place

• WHERE are the rates higher? lower?• Geographic location of sourceGeographic location of source• Geographic location of reservoir

John Snow and Cholera

5 Criteria of Place• ↑ Rate observed in all ethnic groups

in the areain the area• ↑ Rate NOT observed in persons of

similar groups inhabiting other areassimilar groups inhabiting other areas• Healthy persons entering area get ill

at same frequency• People who leave do NOT show

similar levels• Similar levels of infestation in otherSimilar levels of infestation in other

species (if zoonotic disease)

Characteristics Relating to Place

• International • Variation within countriesVariation within countries

Urban-ruralLocalLocal

• Building Maps

PlaceDistribution of AIDS in the US 1990

Local

Building Maps

Interactions ofInteractions of Time and Place

• Time-place clusteringMi ti• Migration