Basic Epidemiology

Basic Epidemiology

Types of study Experimental study

Randomized controlled study

Observational study

Non Randomized study

Cohort study

Analytical study

Case control study

Descriptive study

Longitudinal study

Cross-sectional study

Experimental designs • • • •

Two or more than 2 groups One group – Standard treatment/ Placebo Other group(s)- New treatment Decision of treatment type depend on the investigators

Randomized controlled trial • Controlled- since control groups and biases are controlled • Randomized- process to reduce selection bias and ensure comparability between group • Randomization by – Coin tossing, dice – Random tables – Random number generators

Randomized controlled trial

Cohort design • Cohort- group with same characters • One or more group followed over time • Incidence of outcome in the group(s) are noted and compared • Relative risk is calculated

Cohort design

Case control design • Start from effect to cause • Group of people with outcome and group of people without outcome are compared. • Exposure factor in each group is analyzed and compared. • Calculate the odds ratio

Case control design

Advantage and disadvantage of case control studies Advantage • Easy to carry out • Rapid and inexpensive • Less no of subjects • Rare disease • No risk to subjects • Study multiple risk factors • No attrition • Ethical problem less

Disadvantage • Bias more chance • Selection of control difficult • Cannot measure incidence, Relative risk • Doesn’t recognize association from causation • Representativeness of case and control to population

Descriptive epidemiology • Study of the occurrence and distribution of disease • In Terms of Time Place Person

Descriptive epidemiology • Study of the occurrence and distribution of disease • In Terms of Time Place Person

Time trends • • • •

Secular Periodic Seasonal Epidemic

Long term trends over 10-100 yrs

over 2-5 years Over every season

Tetanus – by year, United States, 1955-2000 During 2000, a total of 35 cases of tetanus were reported. The percentage of cases among persons aged 25-59 years Has increased in the last decade. Note: A tetanus vaccine was first available in 1933. 900 800

Reported Cases

700 600 500 400 300 200 100 0

1955

1960

1965 1970

1975

1980

Year

1985

1990

1995 2000

Malaria – by year, United States 1966-2001 2.0

Reported Cases per 100,000 Population

Returning Vietnam Veterans

1.5

Foreign immigration from Malaria- endemic countries

1.0

0.5

0.0 1966

1971 1976

1981 1986

1991 1996

2001

Year Imported malaria cases have increased over the last 15 years, most likely as a result of increasing international travel and immigration and growing antimalarial drug resistance.

Whooping Cough - Four-monthly admissions, 1954-1973

Pneumonia-Influenza Deaths – By year, 1934-1980

Meningococcal Infection – By year, United States, 1935-1992

Place distribution

Deaths from Cholera per 10,000 houses by source of water supply, London 1854 Number of Houses

Deaths from Cholera

Southwark & Vauxhall Company

40,046

1,263

315

Lamberth Company

26,107

98

37

256,423

1,422

59

Water Supply

Rest of London

Deaths in each 10,000 homes

Person Age Sex Occupation Immunization status Underlying disease Medication Nutritional status Socioeconomic factors Crowding

Hobbies Pets Travel Personal Habits Stress Family unit School Genetics Religion

Cross-Sectional • The measurements of cause and effect are made at the same time which allows direct measurement of the variables. • Limited to studies of causes that are reasonably permanent characteristics of the individual so that cause and effect are present at the same time. • Describes prevalence relationships – not incidence

Prevalence • the number of cases of a disease occurring in a specified population at a designated time. • Point prevalence – at a specific point in time • Period prevalence – over a specific period of time

Incidence • the number of of new cases of a disease occurring in a specified population in a specified time period

July 1

August 1

Uses of Epidemiology • 1-To study historically the rise and fall of disease in the population • 2-Community diagnosis • 3-Planning and evaluation • 4-Evaluation of individual risk and chances • 5-Syndrome identification • 6-Completing natural history of disease • 7-Searching for causes and risk factors

Association and causation • Direct association – One to one causal relationship • Necessary and sufficient cause

– Multifactorial causation

• Indirect association • Spurious association

Criteria's for causality • • • • • • •

Experimental evidence Temporal association Strength of association Specificity of association Consistency of association Biological plausibility Coherence of association