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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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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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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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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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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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