EPIDEMIOLOGI DESKRIPTIF
Study of the occurrence and distribution of disease
Correlational Studies (Ecological Studies)
Uses measures that represent characteristics of entire populations
It describes outcomes in relation to age, time, utilization of services, or exposures
advantages
• We can generate hypotheses for case-control studies and environmental studies
• We can target high-risk populations, time-periods, or geographic regions for future studies
limitation
• Because data are for groups, we cannot link disease and exposure in individual
• We cannot control for potential confounders
• Data represent average exposures rather than individual exposures, so we cannot determine a dose-response relationship
• Caution must be taken to avoid drawing inappropriate conclusions, or ecological fallacy
pattern
disease (population) burden [ Y axis ]
vs. prevalence of “risk factor” [ X axis ]
Case Reports (case series)
Report of a single individual or a group of individuals with the same diagnosis
advantages
We can aggregate cases from disparate sources to generate
hypotheses and describe new syndromes
limitations
We cannot test for statistical association because there is no relevant comparison group
Based on individual exposure {may simply be coincidental}
Important interface between clinical medicine & epidemiology
Most common type of studies published in medical journals
Cross-Sectional Studies (prevalence studies)
Measures disease and exposure simultaneously in a well- defined population
advantages
• They cut across the general population, not simply those seeking medical care
• Good for identifying prevalence of common outcomes, such as arthritis, blood pressure or allergies
limitations
• Cannot determine whether exposure preceded disease
• It considers prevalent rather than incident cases, results will be influenced by survival factors
• Remember: P = I x D
Can be used as a type of analytic study for testing hypothesis, when;
Current values of exposure variables are unalterable over time
Represents value present at initiation of disease E.g. eye colour or blood group
If risk factor is subject to alterations by disease, only hypothesis formulation can be done
descriptive epidemiology
place
person
time
periodic
seasonal
secular
epidemic
The long-time trend of disease occurrence
Temporal interruption of the general trend of secular variation
A cyclic variation in disease frequency by time of year & season
seasonal fluctuation
Environmental factors
Occupational activities
Recreational activities
An increase in incidence above the expected in a defined geographic area within a defined time period
geographic area
example
action level
Age
Sex
Occupation
hobbies
pets
medication
socioeconomic factors
hypothesis formulation
4 methods {derived from 5 canons of
inductive reasoning by John Stuart Mill}
Method of difference
Method of agreement
Method of concomitant variation
Method of analogy
methods for measuring
Rate
measure
Morbidity: Refers to the presence of disease in a population
Mortality: Refers to the occurrence of death in a population
Compute Disease Rate
Number of persons at risk = 5,595,211
Number of persons with disease = 17,382
Rate = 17,382 persons with heart disease
5,595,211 persons
= .003107 heart disease / resident / year
Prevalence
Rates are usually expressed as integers and decimals for populations at risk during specified periods to make comparisons easier.
310.7 heart disease / 100,000 residents / year
is the number of existing cases of disease in the population during a defined period.
incidence
is the number of new cases of disease that develop in the population during a defined period.
Incidence rate is a measure of the probability of the event among persons at risk.
IR = # new cases during time period X K /
specified population at risk
mortalitiy rate
Mortality rates are used to estimate disease frequency when incidence data are not available, case-fatality rates are high, goal is to reduce mortality among screened or targeted populations
Crude mortality: death rate in an entire population
Rates can also be calculated for sub-groups within the population
Cause-specific mortality: rate at which deaths occur for a specific cause
Case-fatality: Rate at which deaths occur from a disease among those with the disease
Maternal mortality: Ratio of death from childbearing for a given time period per number of live births during same time period
Infant mortality: Rate of death for children less than 1 year per number of live births
Neonatal mortality: Rate of death for children less than 28 days of age per number of live births
PR = # existing cases during time period / population at same point in time
basic measure of association
relative risk
odds ratio
• Ratio of the incidence rates between two groups • Can only be calculated from prospective studies (cohort studies)
• Interpretation
• RR > 1: Increased risk of outcome among “exposed” group
• RR < 1: Decreased risk, or protective effects, among “exposed” group
• RR = 1: No association between exposure and outcome
RR = incidence rate among exposed /
incidence rate among non-exposed
Odds: ratio of the probability of an event occurring to the probability of an event not occurring
Odds = P/(1-P)
Probability: Chance or risk of an event occurring (a proportion)
Probability= no. of times an event occurs/
no. of times an event can occur
odds ratio = ad / bc
interpretations
OR > 1: Increased odds of exposure among those with outcome
OR < 1: Decreased odds, or protective effects, among those with outcome
OR = 1: No association between exposure and outcome