The epidemiological tool-box Associations – quantification using rate ratios, rate differences, odds ratios

Analytical epidemiology 1: Ecological Studies The association between quantity of salt sold and oesophageal cancer mortality in counties of Henan province, China

Prevalence of carriers of hepatitis B surface antigen

Risk for cancer of the liver (cases/100000 population in areas of high risk for this cancer - sub Saharan Africa and eastern Asia

The epidemiological tool-box Biases – information bias, selection bias Confounding – detection and control Determination of causation Effect Modification/ Interaction/Synergism

2*2 if only life were so simple… Outcome Status absent present exposed Exposure Status not exposed

Design of a case-control study TIME direction of inquiry Exposed Cases with disease Unexposed Population Controls without disease Exposed Unexposed

Odds ratio and corresponding 95% confidence (stratified by age) Analytical Studies 2: Case control study of Hepatitis C seropositivity and lymphoma Odds ratio and corresponding 95% confidence (stratified by age)

Design of a cohort study TIME direction of inquiry people exposed disease population without the no disease disease not exposed disease no disease “at risk”

Analytical studies 3: Cohort of British Doctors Death by lung cancer Current smoker Non smoker Yes a (4,180) b (224) No c (45,820) d (39,776) # at beginning 50,000 40,000 Total 90,000

Cohort of British Doctors cont’d Relative risk: risk in exposed/risk in unexposed RR = a  b = 0.084 .0056 = 14.9 a+c b+d Attributable risk = risk difference = excess risk in smokers = 0.084 - 0.0056 = .078

Analytical studies 3: Shanghai - cohort study 18,000 men-synergy (Qian G-S et al, Can Epid Biom. Prev 1194;33) Note: Aflatoxins measured by adducts, interview unreliable

Effect Modification (or Interaction) is said to occur when the association between exposurex and outcomey differs by variablez

Prevalence of Actinic keratosis in Israeli Dermatologic Patients with and without Psoriasis 51 (11%) 200 (27%) 538 (73%) 409 (89%) Psoriatics Controls

What is actinic keratosis? Rough scaly spots on sun-damaged skin Pre-cancerous (can give rise to Squamous cell caricnoma SCC) Found on nose, cheeks, upper lips, temples, forehead Common in fair-skinned people and those in outdoor occupations

Adjusted odds ratios (OR) for solar keratosis by hair color comparing psoriatics and controls Adjusted for ethnic origin, gender, age and degree of sun exposure

Adjusted odds ratios (OR) for solar keratosis by propensity to sunburn comparing psoriatics and controls Sometimes Adjusted for ethnic origin, gender, age and degree of sun exposure

The epidemiological tool-box Formulating study question Gathering data Hypothesis testing Impact measures

The epidemiological tool-box Judging the methodology Kappa statistic & Reliability Life Expectancy – survival analysis, life table analyses

The epidemiological tool-box Measurement accuracy N = sample size Outcome Measures Proportional mortality/morbidity

The epidemiological tool-box Quality of data (and/or life!) Randomization; Risk – incidence, types of incidence, incidence vs. prevalence, crude vs. disease-specific

The epidemiological tool-box Standardization Time Trends Utility assessment Validity Weight of evidence

The epidemiological tool-box X-axis Y-axis and plotting the data Z-score and statistical inference Ж - ?????

Steps in Designing and Conducting an Epidemiologic Investigation You have made an observation based on descriptive cancer statistics……. Steps in Designing and Conducting an Epidemiologic Investigation

Specify TOPIC and PURPOSE Determining what is KNOWN and UNKNOWN State the CASE FOR ACTION

- Enrico Fermi, Physicist, 1901-1954 Formulate STUDY OBJECTIVES Formulate HYPOTHESES “There are two possible outcomes: If the result confirms the hypothesis, then you've made a measurement. If the result is contrary to the hypothesis, then you've made a discovery” - Enrico Fermi, Physicist, 1901-1954

Select appropriate STUDY DESIGN Choose & define STUDY VARIABLES Define STUDY POPULATION & SELECTION TECHNIQUES Develop DATA COLLECTION TOOLS

PRETEST developed instruments TRAIN FIELD WORKERS CONDUCT/MONITOR FIELD WORK Prepare data for ANALYSIS ANALYSE the data

WRITE-UP findings COMMUNICATE findings INCORPORATE results into health planning EVALUATE modified health program

It is easier to investigate problems than to solve them. BUT… It’s not so easy to study them either!