Introduction to Clinical Epidemiology

Spring 1999 Elective

UT-H HSCJan Risser, PhD and Will Risser, MD PhD

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Epidemiology

Basic science concerned with the patterns of disease frequency in human population

Distribution of disease by person, place, time Assumption:

– Disease does not occur randomly – Disease has identifiable causes

» which can be altered and therefore

» prevent disease from developing

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Definition of Epidemiology

The study of the distribution and determinants of health-related states or events in specified population, and the application of this study to control of health problems. [source: Last (ed.) Dictionary of Epidemiology, 1995]

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Definition of Epidemiology

Study: surveillance, observation, hypothesis testing, analytic research, and experiments.

Distribution: analysis by time, place, and person.

Determinants: physical, biological, social, cultural, and behavioral factors that influence health.

Health-related states or events: diseases, causes of death, behavior, reactions to preventive regimens, and provision and use of health services.

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Definition of Epidemiology

Specified populations: those groups of people with identifiable characteristics

Application to control: refers to the goal of epidemiology, that is to assess the public health importance of diseases, identify the population at risk, identify the causes of disease,describe the natural history of disease, and evaluate the prevention and control of disease

Aims of epidemiologic research

Describe the health status of a population Explain the etiology of disease Predict the disease occurrence Control the disease distribution

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Preventable Causes of Disease

BEINGS Biological factors and Behavioral Factors Environmental factors Immunologic factors Nutritional factors Genetic factors Services, Social factors, and Spiritual factors[JF Jekel, Epidemiology, Biostatistics, and Preventive Medicine, 1996]

Causal model of risk factors for CVDCausal model of risk factors for CVD

Morbidity and Mortality(Stroke, MI)

Biological Risk Factors

(Hypertension, Blood Lipids, Homocysteine)

Genetic Risk Factors(Family History)

Behavioral Risk Factors(Cigarette, Diet, Exercise)

Environmental Factors(Socioeconomic Status, Work Environment)

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Epidemiology & Clinical Medicine

Epidemiology is used in clinical medicine to: describe the natural history of diseases discuss disease causality

- proximate: biological mechanisms of disease

- distal: social and environmental causes of disease provide disease surveillance

- essential for evaluating community health problems - - and setting disease control priorities

[Greenberg RS (ed.) Medical Epidemiology, 1993]

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Epidemiology & Clinical Medicine

Epidemiology is used in clinical medicine to: evaluate diagnostic testing

- evaluate usefulness, sensitivity, specificity

- to set cutoff points, and develop screening strategies

evaluate prognosis

- by identifying prognostic factors

- through cohort and case control studies

[Greenberg RS (ed.) Medical Epidemiology, 1993]

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Epidemiologic Process

In the epidemiologic process, researchers: – Define the problem: ask a well focused

question » hypothesis statement

– Identify the cause: test the hypothesis » is there an association between exposure and outcome

– Interpret the results: » threats to validity (bias, confounding),

» role of chance

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Brief History of EpidemiologyClassical Infectious Diseases Epidemiology

– Edward Jenner (1749-1823)

» developed a vaccine against smallpox using cow pox 160 years before virus was identified

– John Snow (1813-1858)

» described the association between dirty water and cholera 44 years before vibrio was identified

– Ignaz Semmelweis (1818-1865) » described the association between childbed fever and

physician’s unclean hands 32 years before causal agent was discovered

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Brief History of EpidemiologyClassical Nutritional Epidemiology

– James Lind (1716-1794) » conducted an experiment which showed that scurvy could

be treated and prevented with limes, lemons, and oranges ascorbic acid was discovered 175 years later

– Joseph Goldberger (1874-1927)» identified that pellagra was not infectious but nutritional

in origin and could be prevented by increasing the amount of animal products in the diet and substituting oatmeal for corn grits

niacin was discovered 10 years later

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Brief History of EpidemiologyEpidemiology of Chronic Diseases

Observational Studies:R Doll & AB Hill. Early case-control study. Smoking and carcinoma of the lung: Preliminary report. [Br. Med. J. 2:739, 1950]

Cohort Studies:An approach to longitudinal studies in a community: the Framingham study. 10,000 residents gave baseline information. Follow-up is now 50 years. [Annals New York Academy of Sciences 107:539;1963]

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Brief History of EpidemiologyEpidemiology of Chronic Diseases

Experimental Studies:

Hypertension Detection and Follow-up Program Cooperative Group. 10,500 subjects randomly assigned to two groups: 1. stepped care - antihypertensive therapy increased stepwise to achieve

and maintain blood pressure reduction to goal.

2. Referred care - subjects were referred to their primary care physician and

treated as usual.

mortality stepped care 9.0/100 referred care 9.7/100

final blood pressure 84.1 in stepped care 89.1 in referred care

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Brief History of EpidemiologyEpidemiology of Chronic Diseases

Physician Health Study– randomized, placebo-controlled, double-blind clinical trial

– conducted entirely through the mail

– 22,071 male physicians enrolled to study the effects of aspirin on cardiovascular disease and the effects of beta-carotene on cancer

– randomly assigned to one of four groups

aspirin beta carotene

active active

active placebo

placebo active

placebo placebo

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Observational and Analytical Epidemiology

Descriptive epidemiology– the occurrence of disease (cross-sectional)

Etiologic epidemiology (quasi-experimental)– cause of disease (cohort, case-control)

Experimental epidemiology– assessment of therapy (cohort, randomized clinical

trial)

Research Design Used in EpidemiologyResearch Design Used in Epidemiology

Case Reports / Case Series Case control Cohort: prospective or retrospective Cross sectional Qualitative (Randomized Clinical Trials) Secondary data analysis

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Definitions in Epidemiology

Bias Confounding Frequency Measures

– Prevalence

– Incidence

Measures of Association Causal Inference

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

BIAS

systematic errors in collecting or interpreting data such that there is deviation of results or inferences from the truth.

» selection bias: noncomparable criteria used to enroll participants.

» information bias: noncomparable information is obtained due to interviewer bias or due to recall bias

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

BIAS

Bias results from systematic flaws in study design, data collection, or the analysis or interpretation of

results.

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Selection BIAS

Selection bias

Hospital Mean duration Labor

deliveries of labor complications

Season

Summer 180 8.0 hr 7.8% (14/180)

Winter 240 10.5 hr 11.7% (28/240)

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Selection BIAS

DELIVERIES Mean duration Labor

Hospt. Home All of Labor Complications

Season

Summer 180 20 200 8.0 hr 8.0% (16/200)

Winter 240 160 400 8.0 hr 8.0% (32/400)

Home deliveries were more common in winter. Labor complications among home deliveries were low. Women with prolonged or complicated labor attempt to reach the hospital no matter what season.

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Information BIAS

Response bias - a systematic error that results when subjects respond inaccurately to an investigator’s questions

Lushton County Medical Society commissioned a survey of community perceptions regarding physician impairment by alcohol or drugs. [Biomedical Bestiary. Michael, Boyce & Wilcox]

Persons were asked for anonymous impressions regarding their

personal physician.

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Information BIAS

1000 persons were randomly chosen from the community and sent a simple postcard1. Is your physician’s medical judgment

ever compromised by the use of alcohol or drugs? 0.4%

2. Does the physician drug or alcohol abuse threaten the

professional image of physicians in Lushton? 78%

3. Are physicians who use drugs or alcohol a threat

to their patients? 85%

98% response rate

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Information BIAS

1000 original respondents re-interviewed after 12% of the licensed physicians in the county were cited for drug- or alcohol-related malpractice. 1. Is your physician’s medical judgment ever compromised by the

use of alcohol or drugs? [0.4%] 8%

2. Does the physician drug or alcohol abuse threaten the

professional image of physicians in Lushton? [78%] 80%

3. Are physicians who use drugs or alcohol a threat

to their patients? [85%] 82%

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Confounding

a mixing of effects » between the exposure, the disease, and other factors

associated with both the exposure and the disease

» such that the effects the effects of the two processes are not separated.

Jan Risser,

PhD Will

Risser, MD PhD

Confounding

Confounding results when the effect of an exposure on the disease (or outcome) is distorted because of the association of exposure with other factor(s) that influence the outcome under study.

Spring, 1999

Jan Risser,

PhD Will

Risser, MD PhD

Confounding [Biomedical Bestiary: Michael, Boyce & Wilcox, Little Brown. 1984]

Gambling Cancer

Smoking, Alcohol, other Factors

Observed association, presumed causation

Unobserved associationTrue association