In order to facilitate communication and achieve compatible and comparable
designs and analyses the following terms are defined for use by the Design
& Analysis Teams and Clients.
Skip to A-C, D-F, G-I,
J-N, O-R, S-T, U-Z
305(b): Refers to section 305 subsection (b) of the
Clean Water Act. 305(b) generally describes a report of each states water
quality, and is the principle means by which EPA, Congress, and the public
evaluate whether US waters meet water quality standards, the progress made in
maintaining and restoring water quality, and the extent of the remaining
Assessment: Evaluation and
interpretation of scientific results for the purpose of assisting policy
development and establishing management plans for aquatic resources. This often
includes the description of the fraction of the target population that meets or
exceeds a quality criteria, characterization of the aquatic resource conditions
and description of the association between indicators of resource conditions
and environmental stressors.
Base Samples: The number of sites (sample size) that
will fulfill the monitoring program requirements for precision and uncertainty
(generally, +\- 10% precision at 90% confidence). See
Sample size FAQ
Continuous Population: Continuous populations or
resources generally can be thought of as resources that cover an geographical
area. As an example assume that a study of one of the Great Lakes, e.g., Lake
Ontario, requires an estimate of the percent of the lake area with an
unacceptable concentration of dissolved oxygen. In this case the target
population is the entire surface area of Lake Ontario and the elements of the
target population are all points within the lake. Conceptually, dissolved
oxygen can be measured everywhere on the lake (an infinite number of points).
See: Target Population FAQ
Change: Providing information on change requires the ability
to compare the resource status at two different time intervals, e.g., the
current number of stream kilometers that meet their designated uses compared to
a 1990 estimate.
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Discrete Population: Discrete (from the
Latin discretus, meaning to separate) is an adjective meaning separate and
individually distinct. In a discrete population, features can be spatially
distinguish one from another (i.e., number of lakes within a State) and contain
a finite number of units. See: Target
Ecological indicator: A characteristic of an ecosystem
that is related to, or derived from, a measure of a biotic or abiotic
attributes that can provide quantitative information on ecological condition,
structure and function. An indicator can contribute to a measure of integrity
Ecoregions: Ecoregions is a term used to denote regions
with general similarity in ecosystem characteristics. This includes types,
quality and quantity of natural resources. The classification is designated to
serve as a spatial framework for researchers to conduct monitoring, assessment
and management programs.
EMAP: Environmental Monitoring and Assessment Program -
an EPA Office of Research and Development long term research program. EMAP was
developed to establish the scientific basis for monitoring programs that
measure the current and changing conditions of the nation's ecological
resources. See: EMAP Background
Frame Materials: Maps, GIS files, Ecoregions, Political
Boundaries (State, county), watershed boundaries, RF3, NHD, DEM's are examples
of frame materials that may be relevant to development of the Sample Frame. The
frame materials provide a spatial representation for all units of the target
population and associated auxiliary information within the study area.
Generally, several frame materials are combined to develop the Sample Frame
during the design process for a monitoring program.
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Inclusion Probability: The probability
that a unit of the target population has of being selected (included) during a
random draw from the population. For example, in a normal deck of cards, each
card has a 1/52 probability of being drawn from the deck.
Index Period: The time period within a year selected for
measurement (ecologically based). Measurements may be taken once or more
often during the time period, with the response design providing the protocol
for obtaining a single value for each indicator. Indicator variability
within Index period contributes to non-survey sampling error.
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Known Confidence: Refers to the
estimate of uncertainty or confidence limits associated with a survey result.
Usually the 90% confidence limits are estimated and presented along with the
survey results. See: Known
Non-Target Population: Sample Frame may contain
non-target elements, e.g., misidentified sample units.
Probability Sample: A probability sample is a sample
where every element of the target population has a known, non-zero probability
of being selected. That is, it is possible for every element of the target
population to be in the sample. Two important features of a probability sample
are that the probability selection mechanism (1) guards against site selection
bias and (2) is the basis for scientific inference to characteristics of the
entire target population. See: Probability
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Over Samples: When known or suspected
circumstances are likely to prevent sampling at some base sample sites, e.g.,
frame errors, denied access, hazardous site conditions, etc. prior additional
sample sites can be identified. These Over-Sample sites are sampled whenever a
Base-Sample site cannot be sampled. Alternate terminology: Replacement Samples
Response Design: The process of obtaining a response at a
site. Can be in a single index period during a year, or multiple periods
during a year. Detailed guidance and protocols to be followed when
measuring the response at a site.
Rotating Basin: A survey or monitoring strategy that
identifies panels or subsets of basins within an area that are sampled on a
periodic frequency. Usually one panel is sampled each year, after 4 or 5 years
all panels have been sampled, and then the sampling sequence is repeated. Each
year estimates are available for the Basins surveyed. See:
Rotating Basins FAQ
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Sample Frame: Refers to the list or map
that identifies every unit within the target population of interest, a physical
representation of the target population. Such a list is needed so that every
individual member of the population can be identified unambiguously. The
individual members of the target population whose characteristics are to be
measured are the sampling units. See: Sampling
Sampled Population: A conceptual population that is a
subset of the intersection of the Target Population and the Sample Frame.
It excludes portion of the Target Population within the Sample Frame that could
not be sampled (conceptually) due to access problems, lost samples, or other
reasons a sample could not be collected.
It doesn't include part of the Sample Frame that is determined to not include
elements of the Target Population.
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Status: often seen as a "snapshot" of resource
conditions, e.g., the number of stream kilometers in Region III that meet their
Strahler Order: Hierarchical ordering of streams based
on the degree of branching. A first-order stream is an unforked or unbranched
stream. Two first-order streams flow together to form a second-order stream,
two second-order streams combine to make a third-order stream, etc. (Strahler
1957). This ordering method starts at zero at each terminal segment, and
proceeds towards the root stream. Each time a bifurcation node is encountered,
if both daughter branches have the same order, then the order is increased by
one, otherwise the largest order is used.
Subpopulation: A subset of the target population that
has been identified for a specific purpose, usually requires the ability to
estimate an attribute of the subpopulation (i.e., a fish IBI for all lotic
streams with a Strahler order of < 4).
Survey Design: The process of selecting sites at which a
response will be determined. Includes a probability model for inference
based on the randomized selection process.
Target Population: Target population denotes the aquatic
resource about which information is wanted. Requires a clear, precise
definition of the resource. This includes definition of the elements that make
up, or are associated with, the target population (i.e., perennial streams and
rivers, lakes or estuaries). Usually includes a description of the area of
interest or study area (i.e., State, conterminous States of EPA Regions 8, 9,
and 10). See: What is a Target
Trend: Trend evaluations require several estimates of conditions
often over longer time periods, e.g., the trend in nitrate concentration in the
Santiam River at its confluence with the Willamette River during the last 4
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Unequal Probability: Often it is
desirable to select units from the target population with different (unequal)
inclusion probabilities. For example, a random sample of lakes within a State
might well select more small lakes (more numerous in the target population)
compared to large lakes than desired to meet program objectives. One strategy
for addressing this is to select large and small lakes with unequal
probabilities during the random selection process.
USGS Hydrologic Units (HUCs):
Hierarchical subdivision of land area of United States based on
"Provide a standardized base for use by water-resource organizations in
locating, storing, retrieving, and exchanging hydrologic data, in indexing and
inventorying hydrologic data and information, in cataloging water-data
acquisition activities, and in a variety of other applications" (USGS
Water-Supply Paper 2294, 1987)
A cataloging unit is a geographic area representing part or all of a surface
drainage basin, a combination of drainage basins, or a distinct hydrologic
Almost all cataloging units are larger than 1800 sq km except in special
National maps available for 1, 2, 3, and 4th fields: regions (21), subregions
(222), accounting units (352), cataloging units (2150).
Being extended to 5th and 6th fields.
Approximately 50% of units at any level correspond to "true"
See: USGS NHD
Watershed Definitions for Streams:
"A region or area bounded peripherally by a water parting and draining
ultimately to a particular watercourse or body of water." [Webster's]
Common to apply definition at confluences.
Definition is hydrologic but typical to use terrain elevation to define
Definition applies to any point on a stream network.
See: Selecting a Watershed
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