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N Glossary Alternative hypothesis: the hypothesis that observations are influenced by some nonrandom cause; contrast to null hypothesis. Analytical study: a study designed to examine associations often concerned with measuring the effect of a risk factor; contrast to descriptive study. Association: the relationship between two or more events, characteristics or other variables; does not necessarily imply cause and effect. Absolute risk: in the context of a disease such as cancer is the observed or calculated probability that a person will develop a disease over a certain period of time, as contrasted with the relative risk. Background radiation: ionizing radiation to which a person is exposed from natural sources, such as terrestrial radiation, cosmic radiation, and naturally occurring radionuclides deposited in the body. Becquerel (Bq): the international (SI) special name for the unit of activity; one Bq is equal to one disintegration per second, or 2.7 × 10–11 curies (Ci). Bias: tendency for an estimate to over- or underpredict an actual event due to a systematic error in an epidemiologic study. Biological plausibility: the criterion that an observed association could be causal based on existing biological knowledge. Biomarker: a substance or molecular/cellular event that is used as an indi- cator of a specific biologic state and which can link a specific environ- mental exposure to a health outcome. 401
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402 APPENDIX N Carcinogenesis: the process by which normal cells are transformed into cancer cells. Case-control study: the epidemiologic observation of a group of persons with a disease of interest and a group of persons without the disease; cases and controls are compared for the frequency of the factor that is believed to be associated with the disease. Causality: the relationship between an event (e.g., an exposure) and a sec- ond event (e.g., the disease) in which the second event is explained as a consequence of the first. Census: the enumeration of an entire population that includes demographic information. Census tract: a geographic area for which details on population structure are tabulated at a given census. Census tracts typically contain 1,200 to 8,000 people (with a target of 4,000 people). Centroid: the geographic or population center for a geographic unit. Classification of diseases: arrangement of diseases into categories based on shared characteristics such as body site that they occur, etiology, histology, and others. Cluster: a grouping of health related events that are related in time, space, or both. Cohort study: the epidemiologic observation of a group of persons with the exposure hypothesized to be associated with a disease of interest and a group of persons without the exposure; exposed and unexposed persons are often followed with time until the disease of interest devel- ops and the frequency of disease occurrence by exposure is calculated. Cold shutdown: a state of a nuclear reactor in which it is deemed subcritical and its coolant system is at atmospheric pressure and at a temperature less than 200 °F. Confounder: a variable that is associated with both an exposure of inter- est and disease of interest and may result in statistically false cause or prevent detection of a cause-effect relationship between the exposure and outcome of interest. Confidence intervals: the computed range with a particular confidence level, commonly set up at 95 percent, intended to give the assurance that if a statistical model is correct, the true value of the parameter (for example risk estimation) is within the range indicated; if the 95% CI range does not include 1, then the estimated risk is significantly different from that of a comparison group. Correlation: a statistical measurement of the relationship between two variables. Correlation can be positive (as one variable goes up, the other variable goes up), or negative (as one variable goes up, the other variable goes down).
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403 APPENDIX N Covariate: a variable that is associated with the outcome of interest. For example, in a study of cancer risks, covariates of interest may be age, race/ethnicity, socioeconomic status, smoking status, and others. Curie (Ci): a special name for the unit of radioactivity equal to 37 billion decays per second. Decommission: removal of a nuclear facility from service. Descriptive study: a study concerned with reporting the existing distribu- tion of variables, e.g., cancer registry data analyses that often occur in ecologic studies; contrast to analytical study. Distribution: the frequency of the values or categories of a measurement made on a population. For example, the age distribution of a popula- tion may be summarized as how many people in this population are 0-15, 16-25, 26-45 years of age, and so on. Dose dose-rate effectiveness factor (DDREF): a factor by which the effect caused by a specific dose or dose rate of radiation changes at low doses or dose rates. Ecologic fallacy: error in inference associated with ecologic studies due to extrapolating correlations observed at the group level to individuals; e.g., it has been shown that countries with high dietary fat intake have high incidence of breast cancer (the fallacy would be to infer from this observation alone that it is the individuals that have a high fat diet are those that develop breast cancer). Ecologic study: a study in which the unit of analysis is a population or group (countries, states, counties, communities) and not individuals. Effluent: solid, liquid, or gaseous release from a nuclear facility. Epidemiology: the study of the distribution of diseases and other health- related conditions in populations and the application of this study to address health. Excess risk: an estimate of the amount of risk due to the exposure of inter- est when the effects of other risk factors are removed. Can be relative or absolute risk. Experimental study: a study in which the conditions are being directed by the investigator, e.g., a clinical trial in which patients are separated in two groups where some receive a new drug and some receive a placebo. Follow-up: observation over a period of time of an individual or a popu- lation to retrieve new information and record changes in the health status. Geocoding: the process of finding geographic coordinates (often expressed as latitude and longitude) from other geographic data such as address.
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404 APPENDIX N Gray (Gy): the international (SI) special name for the unit of absorbed dose; one Gy is equal to 1 joule per kilogram, or 100 rad. Hazard: an act or phenomenon (e.g., ionizing radiation) that has the po- tential to produce harm or other undesirable consequences to humans or what they value (NCRP Report No. 139). Half-life: the time required for half the atoms of a radioactive isotope to decay. Healthy worker effect: the notion that an individual must be relatively healthy to be employable in a workforce; therefore, both disease and mortality rates are typically lower among workers than in the general population. Within the workforce studies, healthier workers are more likely to stay employed for longer periods of time compared to the relatively unhealthy workers which would have the shortest duration of employment. Incidence: the number of persons that have developed a disease of interest in a specified population in a specific period of time. Information bias: a flaw in estimating risk because of the difference in qual- ity or accuracy of information collected for comparison groups. Latency period: the lag time between exposure to a disease-causing agent and clinical recognition of disease. In terms of cancer due to exposure to radiation, the concept of minimum latency period is important and is often considered to be 2 years for leukemia and 10 years for solid cancers. Lifetime risk: the risk to an individual that a given health effect or disease such as cancer will occur, without consideration of time elapsed since exposure. Matching: the process during epidemiologic study design of making com- parison groups similar to one or more extraneous factors so that the factor of interest is examined by eliminating the “noise” of other factors. Misclassification: the erroneous attribution of a value into a category other than that it should be assigned. Mortality rate: the number of deaths from all causes or a specific cause in a specified time period. Multiple comparison: a problem in detection of a likely false positive as- sociation due to chance alone that arises when too many comparisons are made. Multivariate analysis: a method used to study the effect of variation of many variables simultaneously.
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405 APPENDIX N Nested case-control study: a case-control study in which the study subjects are selected from a cohort study; presents a number of advantages over case-control studies, notably less inherited bias. Null hypothesis: the hypothesis that one variable and another variable are not associated, e.g., a risk factor and a disease; in statistics, equivalent to test hypothesis, which the investigator will reject or accept based on available data; contrast to alternative hypothesis. Observational study: a study in which the investigator does not have con- trol of the conditions, but observes and reports information as nature takes its course. Odds ratio (OR): the ratio of the odds of an event occurring in one group to the odds of the event occurring in a comparison group. Population mixing hypothesis: proposes that childhood leukemia can be caused by a yet unidentified infectious agent transmitted due to the influx of people into rural areas where susceptible individuals are more prevalent than the average results in epidemics of this infection. Prevalence: the number of people with a disease in a given population at a designated time; often used to describe incidence rate. Prospective study: a cohort study that follows individuals that differ with respect to a factor of interest over time. P (probability) value: a measure of the compatibility of data with the null hypothesis; traditionally, P < 0.05 is considered sufficiently unlikely for the association to have occurred by chance and justifies the designation “statistically significant.” Radiation: the energy that comes from a source and travels through some matter or through space. Two types of radiation are commonly dif- ferentiated in the way they interact with matter: ionizing and non- ionizing radiation. Ionizing radiation, which includes alpha particles, beta particles, gamma rays and x-rays, and neutrons, is considerably more energetic compared to nonionizing radiation such as that found in microwaves. In general, ionizing radiation is far more harmful to living organisms per unit of energy deposited than nonionizing radia- tion, since it has the potential to cause DNA damage and consequently cancer. Radiation exposure: the absorption of ionizing radiation by an object; this absorption can impact health. Radioactivity: the property or characteristic of an unstable atomic nucleus to spontaneously transform with the emission of energy in the form of radiation.
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406 APPENDIX N Relative risk (RR): in the context of a disease such as cancer is the prob- ability of the disease occurring in an exposed group relative to the probability occurring in a nonexposed group. Rad: special name for the unit of absorbed radiation dose; one rad is equivalent to 1/100 Gy. Reference group: the group to which the population under study is compared. Release: a discharge to the environment of radioactive materials, either during normal operations or due to an accident. Rem: special name for the unit of radiation dose equivalent; the product of absorbed dose (measured in rads) and a weighting factor which ac- counts for biological damage caused by radiation (1 rem = 1/100 Sv). Retrospective study: a study in which past exposures related to past or cur- rent disease is explored; can be case-control or cohort in design. Risk assessment: An analysis of the potential adverse impacts of an event (e.g., releases of radioactive material from a nuclear facility) on the health or well-being of an individual or population. Risk assessment is a process by which information or experience concerning causes and effects under a set of circumstances is integrated with the extent of those circumstances to quantify or otherwise describe risk (NCRP Report No. 139). Risk communication: an interactive process of exchange of information and opinion among individuals, groups, and institutions; often involves multiple messages about the nature of risk or expressing concerns, opinions, or reactions to risk messages or to legal and institutional ar- rangements for risk management.1 Risk management: The process by which results of risk assessments are integrated with other information (e.g., results of cost-benefit analysis, judgments about acceptable risk, and other societal concerns) (NCRP Report No. 139). Sample size: the number of individuals selected from a population to be the subjects of an epidemiologic study. Selection bias: a flaw in estimating real risk because of systematic differ- ences in characteristics of those that participate in the study and those that do not. Sievert (Sv): the international (SI) special name for the unit of dose equiva- lent radiation measured in J/kg, calculated by multiplying the absorbed dose (in Gy) with a weighting factor; 1 Sv = 100 rem. 1 NRC (National Research Council) (1989). Improving Risk Communication. Washington, DC: National Academy Press.
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407 APPENDIX N Standardization: method for removing the effect of potential confounders such as age, gender, race from risk estimations. Standardized incidence rate (SIR): the ratio of incident cases observed in the study group or population in a time period to the number of ex- pected deaths if the study population has the mortality experience of the standard population. Standardized mortality rate (SMR): the ratio of deaths observed in the study group or population in a time period to the number of expected deaths if the study population has the mortality experience of the standard population. Standby mode: nuclear facilities available for operation but not currently operating. Statistical power: the probability that a test will reject a null hypothesis when the hypothesis is actually false. Statistical significance: refers to a result that is unlikely to be caused by chance; see “P (probability) value.” Stratification: the process of separating a sample into categories according to a specific criterion, e.g., age, gender, smoking status. Susceptibility: the risk of becoming afflicted by something that can impact health. Temporality: the issue associated with specific study designs (e.g., cross- sectional studies, case-control studies) that makes it difficult to under- stand if exposure or disease came first. Uncertainty: Lack of sureness or confidence in predictions of models or results of measurements (NCRP Report No. 158).
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