4 page glossary of Epidemiological and related terms

Association.
The relationship between two or more events, characteristics, or other variables, often quantified in statistical terms. Synonyms are correlation, statistical dependence and relationship. A (numerical) relationship between 2 or more events, characteristics, or other variables.

Bias.
A systematic deviation from the truth. Any trend in the collection, analysis, interpretation, publication, or review of data that can lead to conclusions that are systematically different from the truth. We understand bias as either selection or information bias. See confounding.
Selection bias. This occurs where there are systematic differences in characteristics between those in the population selected for study and those that are not. 
Information bias. This occurs where there are systematic errors in measurement, recording, management, analysis or reporting of data. Most often, information bias is an error due to different quality or accuracy of information between groups compared. It refers to measurement of exposure or of effect. 

Community.
The general public. The people in the catchment area for which a service is provided. A group of populations. 

Confounding.
This occurs where the effects of two concurrent or simultaneous processes in causing an outcome are not explicitly separated. (Risk factor 1 seemingly causes an outcome, but in reality another risk factor 2 is mixed-in with risk factor 1 and distorts the apparent contribution of risk factor 1, by having an impact on the outcome.) See bias.

Distribution.
The complete summary of the frequency and pattern of the values or categories of a measurement. In epidemiology, distribution is the frequency and pattern of health-related characteristics and events in a population. Classically described in terms of time place and person/ population. 

Epidemic.
The occurrence of more cases of a particular type of disease, chronic condition, or injury than expected in a given area, or among a specific group of people, over a particular period of time. (Think miles, many people, and months). An Outbreak is a confined epidemic, in time, place, and population. In the public's mind, "epidemic" has a far more serious connotation than "outbreak." A Pandemic is an epidemic occurring over a very wide area (several countries or continents) and usually affecting a large proportion of the population. An endemic health condition is a disease, chronic condition, or type of injury that is constantly present in a given geographic area or group. 

Epidemiology.
The study of the distribution and determinants of health conditions or events in populations, and the application of this study to control health problems. The study of disease occurrence and causes in human populations.
Occupational epidemiology is the study of the distribution and determinants of health in working humans.
 Analytic Epidemiology. The aspect of epidemiology concerned with why and how a health problem occurs. Analytic epidemiology uses comparison groups to provide baseline data so that associations between exposures outcomes can be quantified and hypotheses about the cause of the problem can be tested. 
 Descriptive epidemiology. The aspect of epidemiology concerned with gathering, organizing, and summarizing data on "person" (Who is ill?), "time" (When did they become ill?), and "place" (Where could they have been exposed to the illness?). This information is then used to conduct analytic epidemiology. 

Evaluation.
Systematic and objective examination of activities to determine how relevant and effective they are. 

Hazard.
A physical, chemical, biological, ergonomic or psychosocial agent that can cause disease or ill-health.
Related words are:
 Agent. A factor that is essential for a disease, chronic conditions, or injury to occur. In occupational health we divide classify these into the five hazard classes. In theory, agents can cause a health problem by being introduced, being present in excess, or being present at deficient levels.
 Determinant. Any factor that brings about change in a health condition or in other defined characteristics. 
 Environmental factor. An extrinsic factor, such as geology, climate, insects, sanitation, or health services, which affect an agent and the opportunity for exposure. 
 Exposure. Coming into contact with a cause of, or possessing a characteristic that is a determinant of, a particular health problem. 
 Exposed group. A group whose members have had contact with a cause of, or possess a characteristic that is a determinant of, a particular health problem. 
 Host factor. An intrinsic factor (e.g., age, race, sex, behaviours) that influences an individual's exposure, susceptibility, or response to an agent. 
 Risk factor. An aspect of personal behaviour or lifestyle, an environmental exposure, or a hereditary characteristic that is associated with an increase in the occurrence of a particular disease, chronic condition, or injury. A protective factor is one which decreases the occurrence of the outcome. 

Health.
A state of complete physical, mental, and social well-being and not merely the absence of disease or other infirmity.
 Health indicator. Any of a variety of measures (e.g., mortality rate) that indicates the state of health of people in a defined population. 

Hygiene.
The principles and practice of maintaining health. ‘Occupational Hygiene is the discipline of anticipating, recognising, evaluating and controlling health hazards in the working environment with the objective of protecting worker health and well-being and safeguarding the community at large’. (IOHA definition) These hazards may be physical, chemical, biological, ergonomic or psychosocial, and may cause injury, illness, impairment, or affect the well-being of workers and members of the community.

Natural history of disease.
The course of a disease from the time it begins until it is resolved. 

Notifiable disease.
A disease that, by law, must be reported to public health authorities upon diagnosis. Lung cancer is one such disease in occupational health.

Outcome, or disease, or disease outcome.
Injury, illness, impairment, or changes in the well-being of workers. Any or all of the possible results that may stem from exposure to a causal factor or from preventive or therapeutic interventions. All identified changes in health status that result from the handling of a health problem. 

Population.
The total number of inhabitants of a given area or country. In sampling, the population may refer to the units from which the sample is drawn, not necessarily the total population of people. A population can be a particular group at risk, such as everyone who is engaged in a certain occupation. A population need not be human, eg. farms, households, folders.
A group sharing certain common characteristics, about whom inferences can be made. 

Sample.
A selected subset of a population. A sample may be random or non-random and representative or non-representative. 

Study. 
An epidemiological study often compares two groups of people who are alike except for one factor such as exposure to a chemical or the presence of a health effect. The investigators try to determine if the factor is associated with the health effect.
 Analytic study. A study examining associations between groups, typically looking for causality. The common types of analytic study are cross-sectional, case-control and cohort studies.
 Descriptive study. A study describing the existing distribution of variables, without regard to causal or other hypotheses. Classically, there is no comparison group. Examples are case studies, case series, and descriptive cross-sectional studies.
 Interventional study. An analytical study where the investigators systematically and intentionally change the exposure status of the subjects, by e.g. giving therapy, or removing a risk factor. 
 Observational study. An analytic epidemiologic study in which there is no intervention. Changes or differences in one characteristic are studied in relation to changes or differences in others. 

 Case-control study. An analytic observational study where cases (with the disease or outcome) are compared with suitable controls (without the disease or outcome), by looking at their (historical or current) exposures. The starting point occurs after the onset of the disease or outcome, and looks back to find postulated causal factors. It’s like taking snapshots of some sick and some healthy family members, and then paging through the family album looking for previous snapshots of those same family members, looking for clues as to why they got sick or not.
A study in which people with a disease (cases) are compared to people without the disease (controls) to see if their past exposures to chemicals or other risk factors were different.
 Cohort study. An analytic observational study where a well-defined group of disease free people who have had a common experience or exposure are followed up through time, looking for disease outcomes. The starting point occurs before the onset of the disease or outcome, having already identified postulated causal factors. It’s like a number of snapshots taken at periodically of all family members who could be present, looking for clues as to why some got sick.
A study in which a group of people with a past exposure to chemicals or other risk factors are followed over time and their disease experience compared to that of a group of people without the exposure.
 Cross-sectional study. (Prevalence study.) A study that examines the relationship between exposures and outcomes prevalent in a defined population, at a defined point in time.  The start and endpoint are the same, and disease and postulated causal factors are measured simultaneously. It’s like a single snapshot of everyone in the family who could be gathered for the photo, looking for clues as to why some are sick.

Variable.
Any characteristic or attribute that can be measured and can have different values. 

Numbers
Denominator.
The lower portion of a fraction. Epidemiologists use fractions to calculate rates, or ratios. The denominator is usually the population at risk, although it may also be a measure, such as person-time, that quantifies the population's exposure. 

Incidence.
A rate that measures the frequency with which a health problem, such as a new injury or case of illness, occurs in a population.
 Incidence proportion. The number of new cases occurring in a population during a given period of time, divided by the total population at risk during that time.
 Incidence rate. The number of new cases occurring in a population, divided by the person-time at risk. 

Mortality rate.
A measure of the frequency of occurrence of death in a defined population during a specified time interval. Mortality rates can be given as age-adjusted, age-specific, cause-specific, or crude. The crude mortality rate is a population's mortality rate from all causes of death. 

Numerator.
The upper portion of a fraction. Typically the number of people with disease.

Odds ratio.
A measure of association used in comparative studies to quantify the relationship between an exposure and a health outcome; also known as the cross-product ratio. 

Prevalence.
The number or proportion of cases or events or conditions in a given population, as measured at a point in time, e.g. if measured now, the number or proportion of people now who have diabetes. 

Proportion.
A ratio in which the numerator is included in the denominator; the ratio of a part to the whole, expressed as a decimal fraction (e.g. 0.2), a fraction (1/5), or a percentage (20%). It’s a special ratio, where the value is between 0 and 1. 

Rate.
A ratio with an implicit time-element, often multiplied by 10n. 

Ratio.
The value obtained by dividing any one quantity by another, e.g. female: male ratio. Percentages, proportions and rates are all ratio's.

Risk.
The probability that an individual will be affected by, or die from, an illness or injury within a stated time or age span. 

Two-by-two table.
A table with 4 cells, which is the basic numerical method used to compare occurrence of disease in different groups. The groups are simply divided into those with and without exposure and those with and without disease. The 4 cells are labeled a, b, c and d. Cell a contains those with both exposure and disease, and cell d contains those free of both exposure and disease.