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CADDIS Glossary

This glossary contains terms and acronyms used in CADDIS. If you can't find what you are looking for here, try EPA's glossary, Terms of the Environment.

Some terms that are complex or controversial are discussed at length in the Causal Concepts section.

CADDIS Glossary of Terms and Acronyms
TERM DEFINITION
Abductive Inference Inference from data to the hypothesis that best accounts for the data. Compare to inductive inference and deductive inference.
Adaptive Management Performance of management actions as experiments. If causation is uncertain, monitoring the results of a management action indicates whether the agent remediated by that action actually caused the effects of concern.
Agent A physical, chemical or biological entity that may affect a biotic system positively or negatively. This term is similar to but more general than stressor. For example, dissolved oxygen and woody debris are agents; low dissolved oxygen and reduced woody debris may be stressors.
Agent Causation See Agent Causation entry under Causal Concepts.
Ambient Waters 1. Waterbodies in the environment in their current state (i.e., polluted or unpolluted).
2. Water samples from such waterbodies.
Analogy A comparison of two things, based on their similarity in one or more respects. In stressor identification, the criterion of an analogy refers specifically to similar causes.
See Analogy entry under Causal Concepts.
Anthropogenic Induced by humans.
Associations Relationships between different types of observations; these relationships become lines of evidence supporting or weakening the case for a candidate cause.
Associationist Causation See Associationist Causation entry under Causal Concepts.
BACI Before-after-control-impact.
BAP Benzo[a]pyrene.
BMPs Best management practices.
BOD Biochemical oxygen demand.
Bioassessment (Biological Assessment) Bioassessment or biological assessment is the evaluation of ecosystem condition(s) using biological surveys and other direct measurements of resident biota.
Biocriteria (Biological Criteria) Numerical values or narrative expressions describing the reference biological condition of aquatic communities inhabiting waters of a given designated aquatic life use. Biocriteria are benchmarks for evaluation and management of water resources.
Biogenic Produced by biological processes. For example, organic acids produced by decomposition of plant litter are biogenic acids.
Biological Gradient A regular increase or decrease in a measured biological attribute with respect to space (e.g., below an outfall), time (e.g., since a flood), or an environmental property (e.g., temperature). Biological gradients are analyzed to generate stressor-response relationships based on field data.
Biological Mechanism The process by which a cause induces a biological effect. A biological mechanism is a causal mechanism emphasizing biological processes.
Biomarker A contaminant-induced physiological, biochemical, or histological response of an organism.
Body Burden The concentration of a contaminant in a whole organism or a specified organ or tissue.
CADDIS The Causal Analysis/Diagnosis Decision Information System, a web-based technical support system for implementing the Stressor Identification process.
CADLit Causal analysis database of literature. This is being replaced by a new database called CADLink.
CADLink A new beta version of a causal analysis database of literature.
CAFO Concentrated animal feeding operation.
CERCLA Comprehensive Environmental Response, Compensation, and Liability Act.
CWA Clean Water Act.
Candidate Cause A hypothesized cause of an environmental impairment, that is sufficiently credible to be analyzed.
Case The situation that is the subject of a causal analysis; for example, the case may be an impaired stream reach and the upstream and downstream reaches, or an estuary and the lower reaches of its tributaries.
Case Study An example illustrating a complete causal analysis or a component of the process.
Categorical Regression Regression analysis in which the dependent variable is defined as a category rather than as a count or continuous variable. In some analyses, the categories are ordered, for example, 1 = enzyme induction, 2 = histological damage, . . . 10 = death of sensitive organisms, . . . 20 = extinction of all metazoan species.
Causal Agent The agent that directly induces the effect of concern when intensity and duration of exposure are sufficient. This term is similar to, but more neutral than proximate stressor.
Causal Analysis A process by which data and other information are organized and evaluated, using quantitative and logical techniques, to determine the likely cause of an observed condition.
Causal Association A correlation or other association between measures or observations of two entities or processes, that occurs because of an underlying causal relationship.
Causal Characterization See Identify Probable Cause.
Causal Considerations See types of evidence.
Causal Evidence Data analysis results that reveal an association between the biological condition and a candidate cause.
Causal Inference The component of a causal analysis that is specifically concerned with the interpretation of evidence to determine the most likely cause. Also see inference.
Causal Mechanism The process by which a cause induces an effect.
Causal Pathway The sequence of processes and states that causally connect a source to exposure to a causal agent, potentially including release, transport, transformation, and direct effects (if the effect of concern is indirect).
Causal Relationship The relationship between a cause and its effect.
Causal Scenario A type of cause that consists of two or more interacting agents. Examples include low pH and metals, or high temperatures and fungal pathogens.
Cause 1. That which produces an effect (a general definition).
2. A stressor or set of stressors that occurs at an intensity, duration, and frequency of exposure sufficient to result in a change in an identified biological effect (a definition specific to stressor identification).
Co-occurrence The spatial or temporal co-location of the candidate cause and the effect. Synonymous with spatial/temporal co-occurrence.
Coherence See reasonable explanation.
Concentration-Response 1. The relationship between the concentration of an agent and the frequency or magnitude of a biological response.
2. A model of that relationship. This term is similar to but more general than stressor-response and exposure-response.
Conceptual Model A graphic depiction of the causal pathways linking sources and effects, that ultimately is used to communicate why some pathways are unlikely and others are very likely.
Consistency of Evidence The degree to which types of evidence in a strength-of-evidence analysis are in agreement in either supporting or weakening the case for a candidate cause.
Control The treatment in a toxicity test or other experiment in which the test chemical or other experimental condition is absent. Reference implies comparison without control. Thus reference, rather than control is the appropriate term for observational studies.
Confounding See Confounding entry under Causal Concepts.
Correlation A statistical relationship between two or more variables such that systematic changes in the value of one variable are accompanied by systematic changes in the other.
Counterfactual Causation See Counterfactual Causation entry under Causal Concepts.
Covering Law See Covering Law entry under Causal Concepts.
Criteria See Criteria, Causal entry under Causal Concepts.
D This acronym means "diagnosed."
DELT This acronym refers to "deformities, erosion, lesions, and tumors."
Deterministic Causation See Deterministic Causation entry under Causal Concepts.
DNR This acronym refers to the Department of Natural Resources.
DO This acronym means "dissolved oxygen."
Deductive Inference Inference from general principles to their consequences. Also see inductive inference and abductive inference.
Define the Case A step in the Stressor Identification process in which the impairment and its spatial and temporal scope are defined. Also see case.
Designated Use Terminology used in the Clean Water Act to describe classes of expectations for waterbodies, including their ability to support aquatic life.
Diagnosis A type of inference in the Stressor Identification process that uses symptomology or a set of specific observations to identify a probable cause.
See also Diagnosis entry under Causal Concepts.
Diagnostic Protocol A standard procedure for diagnosing the cause of illness or impairment.
Dilution Ratio The ratio of stream flow to wastewater flow.
Direct Causation The induction of an effect through a single cause-effect relationship; for example, the direct effect of an herbicide may be reduced algal production. Compare this to indirect causation.
Directionality See Directionality entry under Causal Concepts.
Double-Counting Use of the same association in two different types of evidence.
ECOTOX This acronym refers to the ECOTOXicology database and website.
EMAP This acronym refers to the Environmental Monitoring and Assessment Program.
EPT This acronym refers to "Ephemeroptera, Plecoptera, and Trichoptera."
Ecoepidemiology The study of the nature and causes of past or ongoing effects in ecological systems.
Ecoregion A geographic area having relatively uniform ecological properties.
Effect In general, an effect is something that inevitably follows an antecedent (cause or agent). A biological effect is the biological result of exposure to a causal agent. This term is similar to response, but emphasizes the agent that acts (e.g., the effect of cadmium) rather than receptor that responds to it (e.g., the response of trout).
Elimination Rejection of a candidate cause based on evidence that an expected association between that cause and the effect does not occur.
Endpoint Species A species that is the object of an assessment or test.
Epistemology The branch of philosophy that is concerned with the acquisition of knowledge.
Eutrophication Enrichment of a waterbody with nutrients, often resulting in high levels of primary production and leading to depletion of dissolved oxygen.
Evaluate Data A step in the Stressor Identification process in which data are analyzed to generate associations constituting types of evidence, and these results are then scored.
Event Causation See Event Causation entry under Causal Concepts.
Evidence 1. Knowledge that changes one's degree of belief in a proposition (a general definition). 
2. Results of data analysis concerning associations between the causal agent and the effect, or between sources or steps in the causal chain and the causal agent (a definition specific to stressor identification).
Evidence from Beyond the Case Evidence based on data or observations from laboratory studies or field studies conducted outside the case.
Evidence from the Case Evidence based on data or observations from the impaired system or reference systems that are adjoining or closely spatially related (e.g., reaches in the same stream or watershed).
Evidence of Exposure Evidence indicating that organisms took up or contacted a stressor.
Experiment Manipulation of a candidate cause through elimination of a source or alteration of exposure, to evaluate the candidate causal agent's relationship to an effect.
Expert Judgement A method of inference based on the knowledge and skill of qualified assessors, rather than a formal analysis.
Exposure The co-occurrence or contact of a stressor with the biological resource demonstrating impairment.
Exposure-Response 1. The relationship between the intensity, frequency, or duration of exposure to a stressor and the intensity, frequency, or duration of the biological response.
2. A model of that relationship. This term is similar to concentration-response and stressor-response.
Falsification The rejection of a hypothesis by demonstrating that it does not hold in a case. Falsificationists argue that hypotheses can be rejected but not accepted.
Field Studies Observational or experimental studies carried out in nature.
GLEMEDS This acronym refers to the "Great Lakes embryo mortality, edema, and deformities syndrome."
General Causation See General Causation entry under Causal Concepts.
Goodness of Fit Tests A quantification of how well a statistical model describes a set of observations. Usually, such tests quantify the differences between observed values and those expected under a model of interest and calculate the probability the observed differences would occur if there were actually no difference between the observed and expected values.
Hypothesis A proposed theory concerning a causal relationship; for example, identification of a candidate cause for impairment constitutes a causal hypothesis. Note that these are not statistical hypotheses.
See also Hypothesis Testing entry under Causal Concepts.
IBI This acronym refers to the "Index of Biotic Integrity."
ICI This acronym refers to the "Invertebrate Community Index."
Identify Probable Cause A step in the Stressor Identification process in which the proposed cause is described, the evidence for its causal relationship to the impairment is summarized, and uncertainties are presented.
Impairment A detrimental effect on the biological integrity of a waterbody that prevents attainment of the designated use.
Inclusion Probability (Probability of Inclusion) The probability that a stream segment or other sampling unit is selected for inclusion in a dataset.
Indirect Causation The induction of effects through a series of cause-effect relationships, such that the impaired biological resource may not even be exposed to the initial cause. For example, the direct effect of an herbicide may be reduced algal production, which may indirectly lead to reduced herbivore and predator populations. Compare to direct causation.
Indirect Cause A cause that acts by inducing an effect that, through one or more further cause-effect relationships, ultimately results in the biological effect of concern. Indirect causes eventually lead to the actual cause of the impairment, which is called the causal agent or proximate stressor.
Indirect Effect Change in a resource due to indirect causation.
Inductive Inference Inference from observations to general principles. Compare to abductive inference and deductive inference.
Inference The act of reasoning from evidence.
Inferential Logic A process for reasoning from the evidence to a necessary and specific conclusion.
Initial Response The immediate response of an organism, population, community, or ecosystem to direct exposure to a stressor.
Interaction See Interaction entry under Causal Concepts.
Intermediate Processes Processes that occur between appearance of a stressor in an ecosystem and induction of the effect of concern; for example, reduction in algal abundance is an intermediate process between the introduction of a non-native filter feeder and decreased abundance of native planktivores.
Internal Exposure 1. Exposure to contaminants within the body of an organism.
2. Measures of such exposure, including body burdens or activities of metabolic enzymes.
Iteration Repetition of a process; in particular, repetition of the causal analysis process with new data or observations after results of prior stressor identifications were inconclusive.
INUS This acronym means "Insufficient but Necessary part of a condition which is, itself, Unnecessary but Sufficient." See INUS entry under Causal Concepts.
KDHE This acronym refers to the Kansas Department of Health and Environment.
LC50 Median lethal concentration. The concentration of a substance needed to kill 50% of the organisms within a specified period of time. This measurement endpoint is most often used in acute laboratory toxicity tests.
LC50 (Median Lethal Concentration) The concentration of a material in an environmental medium that causes 50% mortality of a group of test organisms after a certain period of exposure. This measurement endpoint is most often used in acute laboratory toxicity tests.
Life History Developmental processes and behaviors that sustain and reproduce a species. For example, case formation and net-spinning can be components of the life history of certain caddisflies.
Linear Models

Linear models have the form,

ƒ(x)=ß0 + ß1x1 + ß2x2 + ß3x3...

in which for any number of independent or explanatory variables (i.e., x1, x2, x3 ....), each independent or explanatory variable in the model is multiplied by an unknown parameter (i.e., ß1, ß2, ß3 ....),

  1. there is at most one unknown parameter with no corresponding independent or explanatory variable (i.e., ß0), and
  2. all of the individual terms are summed to produce the final function model.

Although such a function may not describe a straight line, it is said to be linear in the parameters, because the problem can be reduced to system (i.e., one to many) of algebraic (i.e., linear) equations that can be solved for unique values of the unknown parameters (i.e., ß0, ß1, ß2, ß3 ....).

MWH This acronym refers to the "modified warm-water habitat."
Manipulation of Exposure A type of evidence in which human action induces, eliminates, or modifies exposure to a stressor (e.g., shutting down an effluent source, fencing cattle from a stream, or caging fish in a contaminated lake).
See also Manipulationist Causation entry under Causal Concepts.
Matched Data In this context, matched data are values for variables potentially responding to a stressor (usually biological attributes) and values for variables which potentially explain the response (usually a measurements of environmental parameters, particularly those associated with a candidate cause) that are matched in a data set, because they are spatially and temporally associated.
Mechanism The process by which a system is changed. See also Mechanistic Causation entry under Causal Concepts.
Mechanistic Plausibility The ability of a candidate cause to realistically induce the observed effects, given knowledge of its mode of action.
Metaphysics The branch of philosophy that is concerned with the fundamental nature of things.
mg/l This reference means "milligrams per liter."
Model Based Causation See Model Based Causation entry under Causal Concepts.
Multiple Causation See Multiple Causation entry under Causal Concepts.
NA This acronym means "Not applicable."
NE This acronym means "No evidence."
NEP This acronym means the "National Estuary Program."
Network Causation See Network Causation entry under Causal Concepts.
NHD This acronym means the "National Hydrography Dataset."
NOECs This acronym means "No Observed Effects Concentrations."
NPDES This acronym means the "National Pollutant Discharge Elimination System."
Natural A state that occurs in the absence of human actions; natural conditions can be approximated but never achieved in the real world.
Necropsy A post-mortem examination or inspection to determine the cause of death or the nature of pathological changes.
Negative Evidence Evidence that tends to refute or weaken the case for a candidate cause.
OEPA This acronym means the Ohio Environmental Protection Agency.
Opportunistic Able to exploit newly available habitats or resources.
PAH This acronym refers to polycyclic aromatic hydrocarbon.
PCB This acronym refers to polychlorinated biphenyls also known as PCBs.
PEL >This acronym refers to the "permissible exposure limit."
Pathogens Organisms capable of inducing disease in susceptible hosts.
Piece of Evidence A specific data analysis or observation that relates to a type of evidence. For example, the type of evidence 'stressor-response relationships from laboratory studies' may include a chronic value for fathead minnows and an acute species sensitivity distribution for freshwater fish as pieces of evidence.
Plausibility The degree to which a cause-effect relationship would be expected, given known facts.
Pluralism See Pluralism entry under Causal Concepts.
Pollutant Any substance introduced into the environment that adversely affects a resource.
Positive Evidence Evidence that tends to support the case for a candidate cause.
Prediction Interval A type of statistical interval that has a specified probability (commonly 95%) of enclosing the value of a "future" unit that is predicted based on the available data and does not belong to the sample used to generate the prediction, assuming that the future unit is drawn from the same population. In the context of regression analysis, standard prediction intervals relate to uncertainty when the fitted regression is used to predict the response (Y) variable for specific values of X variables. In cases of linear regression with a single X variable, prediction intervals associated with all X values are conventionally depicted as a band enclosing the fitted regression line, bounded by two curves that diverge as X increases in distance from the mean X, in either direction. Prediction intervals address both unit variation (e.g., as evaluated using sample quantiles) and statistical error in estimating unknown population parameters (e.g., in estimating a regression slope and intercept) and therefore can be distinguished from confidence intervals which only address the statistical error in parameter estimation.
Predictive Performance See verified predictions and also Predictive Performance entry under Causal Concepts.
Principal Cause The cause that makes the largest contribution to the effect.
Probability Design A sampling scheme such that the probabilities of including selected units in the sample are known, and all population units have a positive (non-zero) probability of selection. This implies that the target population is represented by the sample and that the target population is explicitly defined.
Probable Cause The cause that is most likely to be the true cause of an effect.
Probabilistic Causation See Probabilistic Causation entry under Causal Concepts.
Process Connection See Process Connection entry under Causal Concepts.
Proximate Cause The cause that induces the effect through direct exposure. Compare to an indirect cause.
Proximate Stressor The stressor that directly induces the biological effect of concern. This is equivalent to causal agent, but emphasizes the negative consequences of exposure.
Pseudoreplication The treatment of multiple samples from the same sample unit as replicates for statistical purposes. For example, multiple benthic invertebrate samples taken in a single channelized stream are pseudo-replicates because the stream channel (the hypothesized cause) has not been replicated. True replicates would be taken from multiple channelized streams.
Publicly-Owned Treatment Works (POTW) A water treatment facility, as defined by Section 212 of the Clean Water Act, that is used in the storage, treatment, recycling, and reclamation of municipal sewage or industrial wastes of a liquid nature, and is owned by a municipality or other governmental entity. It usually refers to sewage treatment plants.
QHEI This acronym refers to the "qualitative habitat evaluation index."
Quantile Regression A statistical technique used to estimate an exposure-response relationship when more than one agent may be affecting the receptor, but the other agents are unknown or unmeasured.
R This acronym mean "refuted."
RM This acronym refers to a "river mile."
Reasonable Explanation The final consideration in a strength-of-evidence analysis. If the results of a strength-of-evidence analysis are not consistent, a mechanistic, conceptual, or mathematical model reasonably may explain the apparent inconsistencies. This concept is called coherence in the Stressor Identification guidance document.
Receptor A population, community, or ecosystem that is exposed to a contaminant or other stressor.
Reference 1. A reference site or set of reference sites.
2. An environmental attribute of a reference site or a set of reference sites; for example, dissolved oxygen concentrations at a reference site represent reference concentrations.
Reference Site A location or waterbody selected for comparison with the impaired location or waterbody being assessed. The type of sites selected and the type of comparative measures used will vary with the purpose of the comparisons. References that lack a source, stressor, or impairment are termed negative or clean references; references that have well-defined and elevated levels of a stressor or well-characterized sources or impairments are referred to as positive or dirty references.
Refutation The logical process of demonstrating the impossibility of a candidate cause, thus allowing it to be eliminated from further consideration. See Rejectionist Causation entry under Causal Concepts.
Regional Reference A set of sites within a region that represent the best conditions of some environmental characteristic (e.g., a biological index or a pollutant concentration).
Regularity See Regularity entry under Causal Concepts.
Rejectionist Causation See Rejectionist entry under Causal Concepts.
Replicate 1. One of a set of independent systems that have been randomly assigned a single treatment.
2. The process of generating a set of such systems.
Response The biological result of an exposure. This term is synonymous with effect, but emphasizes the receptor that responds (e.g., the response of trout) rather than the agent that acts upon it (e.g., the effect of cadmium).
SI This acronym refers to "Stressor Identification."
Scoring Categorization of the results of analyses based on types of evidence for a particular candidate cause, based on text descriptors and +, -, and 0 symbols.
Screening SI Use of the SI process to narrow the range of candidate causes and to guide future data collection, rather than to identify the probable cause.
Simulation Model Mathematical representation of the entities and processes in a system.
Site A specific location or body of water (e.g., a stream reach, a pond, an embayment of a lake, or an area of an estuary).
Site Media Water, sediment, fish tissue, or other materials collected from an impaired site or reference sites for testing or analysis.
Source An origination point, area, or entity that releases or emits an agent that may be an indirect cause or a proximate cause.
Spatial Gradient A graded change in the magnitude of some quantity or dimension measured along a transect.
Spatial/Temporal
Co-occurrence
A type of evidence that involves observation of two entities or conditions at the same place or time; it is sometimes shortened to co-occurrence.
Species Sensitivity Distribution The statistical distribution of species based on their sensitivity to exposure to an agent.
Specificity The degree to which an effect is known to result from one or very few possible causes, or a cause is known to have a distinct effect. Specificity is a causal consideration in the Stressor Identification guidance. In CADDIS, the concept is incorporated into the Symptoms type of evidence.
Specific causation See Specific Causation entry under Causal Concepts.
Stakeholders People or organizations with an interest in the outcome of an assessment, including causal analyses for bioassessments.
Stream Reach A segment of a stream delimited in some way (e.g., by occurrence of tributaries or effluents).
Strength-of-Evidence Analysis A structured inferential process that uses multiple lines of evidence to identify the most likely cause or causes of a biological impairment.
Stressor Any physical, chemical or biological entity that can induce an adverse effect. A change in the level of a stressor may be the proximate cause of the biological effect under investigation, or may be one event of several required to produce the effect, or may not contribute to causation. Stressor Identification focuses on stressors that can induce biological effects. Also see agent.
Stressor Identification A methodology for determining the most probable cause of an observed biological impairment, using elimination, diagnosis, and strength-of-evidence analysis. The CADDIS website is based on the Stressor Identification process, which is described in a U.S. EPA guidance document.
Stressor-Response 1. The relationship between the intensity, frequency, or duration of exposure to a stressor and the intensity or frequency of a biological response.
2. A model of that relationship. Equivalent to exposure-response and concentration-response.
Structural Equation Modeling A family of multivariate statistical methods that use covariance analysis to estimate parameters associated with a series of structural equations that express the hypothetical relationships among several variables that can be either directly observed or manifest variables or unobserved hypothetical or latent variables. It is similar to multiple regression, but uses assumptions concerning the causal network to structure the relationships into one or more equations. It is, in effect, a means of quantifying the links in a conceptual model.
Symptom A property of affected organisms, populations, communities, or ecosystems that is indicative of a specific cause or a very few causes.
Symptomatology A set of signs indicating the action of a specific causal agent on organisms.
TDS This acronym refers to "total dissolved solids."
Teleological Causation See Teleological Causation entry under Causal Concepts.
TIE This acronym refers to "Toxicity Identification Evaluation."
TMDLs This acronym refers to "Total Maximum Daily Loads."
TP This acronym refers to "total phosphorus."
TSS This acronym refers to "total suspended solids."
Targeted Sampling Design Targeted sampling designs have a purpose other than obtaining random samples from a statistical population. In particular, they target locations or events with some attribute of interest. For example, one might take a sample below every sewage treatment plant in a watershed to determine if they can cause impairments where effluents are least diluted, sample upstream of an impaired location at 100 m intervals to determine the extent of an impairment, or sample all tributaries to a stream to determine which is the source of a contaminant.
Temporal Gradient A graded change in the magnitude of some quantity or dimension measured over time.
Temporal Sequence A type of evidence based on the relationship between the time of occurrence of a candidate cause and the effect of concern. See Temporality entry under causal concepts.
Tests of Significance An evaluation of a null hypothesis that there is no relationship between two variables or no difference between them. They are not appropriate for observational studies due to pseudo replication and non-random assignment of sites to treatments. Furthermore, testing of null hypotheses is not part of causal analysis.
Tolerance Measure of degree to which a particular taxon can persist in anthropogenically-disturbed systems. We expect to find highly tolerant taxa at severely degraded sites.
Tolerance Value A numerical value that represents the relative sensitivity of a species or other taxon to a particular agent.
Total Maximum Daily Load (TMDL) The total allowable pollutant load to a receiving waterbody, such that any additional loading will produce a violation of water quality standards.
Toxic Units The concentration of a chemical divided by a standard measure of its toxicity.
Toxicant A chemical with known toxic properties.
Toxicity Identification and Evaluation (TIE) A process that identifies the toxic components of an effluent or ambient medium by chemically manipulating the effluent or medium and testing the resulting material.
Toxicity Reduction Evaluation (TRE) A site-specific study conducted in a stepwise process, designed to identify the causative agent(s) of effluent toxicity, isolate the source(s) of toxicity, evaluate the effectiveness of toxicity control options, and then confirm reductions in effluent toxicity.
Transforming Data Data transformation is a mathematical treatment which alters the measurement scale of the observations. Data are transformed to linearize data for analysis by linear regression or to alter the distribution prior to parametric analyses when data do not meet the requirements of normality and homogeneity of variance. The type of transformation necessary is determined by the type of data (count, continuous or proportion) and the direction in which the data are skewed. Kutner et al. (2004) includes several chapters which cover diagnostics and remedial measures for regression analysis.
Type of Evidence A category of relationships that provides a logically distinct way to support, weaken, or refute the case for a candidate cause. A type of evidence may contain multiple lines of evidence. It is synonymous with causal consideration in the Stressor Identification guidance document.
UV This acronym refers to "ultra-violet."
Ultimate Cause The action or policy that is responsible for creating or sustaining a source.
Uncertainty Lack of knowledge concerning an event, state, model, or parameter. Uncertainty, unlike variability, may be reduced by research or observation.
Variability Differences among entities or states of an entity attributable to heterogeneity. Variability is an inherent property of nature and may not be reduced by measurement.
Variable, Types of A functional relationship between two variables is expressed by a mathematical formula: Y=f(X). Y denotes the dependent or response variable and X denotes the independent, explanatory, or predictor variable. When the X variable is the assumed or actual cause of the response, it is referred to as the causal variable.
Verified Predictions A type of evidence in which predictions about conditions in the receiving system, based on knowledge of the mode of action of the candidate cause, are confirmed by observation or measurement; in the Stressor Identification guidance document, this is referred to as predictive performance.
WET This acronym refers to "whole-effluent toxicity."
WQS This acronym refers to the Water Quality Standards.
WQSDB This acronym refers to the Water Quality Standards Database.
WWH This acronym refers to "warm-water habitat."
Watershed An area of land from which any released or deposited water flows into the same waterbody. Equivalent to catchment.

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