Lesson 1: Epidemiology

• Epidemiology is the study of how often diseases occur in different groups of people and why.
• Epidemiology information is used to plan and evaluate strategies to prevent illness and as a guide to the management of patients in whom disease has already developed.
• Epidemiology is the method used to find the causes of health outcomes and diseases in populations.
• In epidemiology, the patient is the community and individuals are viewed collectively.
• Epidemiology is the study of the distribution (frequency, pattern) and determinants (causes, risk factors) of health-related states and events in specified populations.
• Epidemiology is the application of this study to the control of health problems.
• All findings in epidemiology must relate to a defined population.
• A key feature of epidemiology is the measurement of disease outcomes in relation to a population at risk.
• The population at risk in epidemiology is the group of people, healthy or sick, who would be counted as cases if they had the disease being studied.
• Conclusions in epidemiology cannot be drawn from purely clinical data on the number of sick people seen.
• In any epidemiological investigation, the target population should always be the basis for conclusions.
• Occasionally measurements can be made on the full target population in epidemiology.
• More often observations can only be made on a study sample, which is selected in some way from the target population.
• The confidence that can be placed in conclusions drawn from samples in epidemiology depends in part on sample size.
• Small samples can be unrepresentative just by chance, and the scope for chance errors can be quantified statistically.
• More problematic are the errors that arise from the method by which the sample is chosen in epidemiology.
• Systematic sampling errors can be avoided by use of a random selection process in which each member of the target population has a known (non-zero) probability of being included in the study sample.
• Case-control studies investigate the causes of disease with low incidence by comparing patients who have developed a disease with controls or referents who do not have the disease.
• Longitudinal studies vary enormously in their size and complexity.
• The two groups in a cohort study are followed up prospectively, and the incidence of disease in each is measured.
• In the simplest design a sample or cohort of subjects exposed to a risk factor is identified along with a sample of unexposed controls.
• Experimental studies are less susceptible to confounding because the investigator determines who is exposed and who is unexposed.
• Outbreaks of disease such as influenza, gastroenteritis, and hepatitis are still important.
• From time to time new communicable diseases such as Lassa fever, legionnaires disease, and, most recently, AIDS appear in epidemic form.
• A problem when the cohort method is applied to the study of chronic diseases such as cancer, coronary heart disease, or diabetes is that large numbers of people must be followed up for long periods before sufficient cases accrue to give statistically meaningful results.
• Screening patients for preclinical disease is an established part of day to day medical practice.
• Experimental study of populations involves studying the health of a population over time.
• In a longitudinal study subjects are followed over time with continuous or repeated monitoring of risk factors or health outcomes, or both.
• Cross sectional studies measure the prevalence of health outcomes or determinants of health, or both, in a population at a point in time or over a short period.
• Case-control studies require the identification of cases and controls, ascertainment of exposure, and analysis.
• Randomised controlled trials and crossover studies are examples of experimental studies.
• Most longitudinal studies investigate associations between exposure to known or suspected causes of disease and subsequent morbidity or mortality.
• Cross sectional studies can be used to explore aetiology, but associations must be interpreted with caution due to potential selection bias.
• The definition of a study population in epidemiology begins with some characteristic which all its members have in common.
• Ecological studies in epidemiology compare disease rates and exposures in different populations.
• The unit of observation in ecological studies is the population or community.
• Disease rates and exposures are measured in each of a series of populations and their relation is examined in ecological studies.
• The populations compared in ecological studies may be defined in various ways.
• Geographical comparisons are a common way of defining populations in ecological studies.
• Time trends are another common way of defining populations in ecological studies.