ASTM D5791-95(2006)
(Guide)Standard Guide for Using Probability Sampling Methods in Studies of Indoor Air Quality in Buildings
Standard Guide for Using Probability Sampling Methods in Studies of Indoor Air Quality in Buildings
SIGNIFICANCE AND USE
Studies of indoor air problems are often iterative in nature. A thorough engineering evaluation of a building (1-4)3 is sometimes sufficient to identify likely causes of indoor air problems. When these investigations and subsequent remedial measures are not sufficient to solve a problem, more intensive investigations may be necessary.
This guide provides the basis for determining when probability sampling methods are needed to achieve statistically defensible inferences regarding the goals of a study of indoor air quality. The need for probability sampling methods in a study of indoor air quality depends on the specific objectives of the study. Such methods may be needed to select a sample of people to be asked questions, examined medically, or monitored for personal exposures. They may also be needed to select a sample of locations in space and time to be monitored for environmental contaminants.
This guide identifies several potential obstacles to proper implementation of probability sampling methods in studies of indoor air quality in buildings and presents procedures that overcome those obstacles or at least minimize their impact.
Although this guide specifically addresses sampling people or locations across time within a building, it also provides important guidance for studying populations of buildings. The guidance in this document is fully applicable to sampling locations to determine environmental quality or sampling people to determine environmental effects within each building in the sample selected from a larger population of buildings.
SCOPE
1.1 This guide covers criteria for determining when probability sampling methods should be used to select locations for placement of environmental monitoring equipment in a building or to select a sample of building occupants for questionnaire administration for a study of indoor air quality. Some of the basic probability sampling methods that are applicable for these types of studies are introduced.
1.2 Probability sampling refers to statistical sampling methods that select units for observation with known probabilities (including probabilities equal to one for a census) so that statistically defensible inferences are supported from the sample to the entire population of units that had a positive probability of being selected into the sample.
1.3 This guide describes those situations in which probability sampling methods are needed for a scientific study of the indoor air quality in a building. For those situations for which probability sampling methods are recommended, guidance is provided on how to implement probability sampling methods, including obstacles that may arise. Examples of their application are provided for selected situations. Because some indoor air quality investigations may require application of complex, multistage, survey sampling procedures and because this standard is a guide rather than a practice, the references in Appendix X1 are recommended for guidance on appropriate probability sampling methods, rather than including expositions of such methods in this guide.
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Designation:D5791–95 (Reapproved 2006)
Standard Guide for
Using Probability Sampling Methods in Studies of Indoor Air
Quality in Buildings
This standard is issued under the fixed designation D5791; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope D1356 Terminology Relating to Sampling and Analysis of
Atmospheres
1.1 This guide covers criteria for determining when prob-
ability sampling methods should be used to select locations for
3. Terminology
placement of environmental monitoring equipment in a build-
3.1 Definitions—For definitions of terms used in this guide,
ing or to select a sample of building occupants for question-
refer to Terminology D1356.
naire administration for a study of indoor air quality. Some of
3.2 Definitions of Terms Specific to This Standard:
the basic probability sampling methods that are applicable for
3.2.1 census—survey of all elements of the target popula-
these types of studies are introduced.
tion.
1.2 Probability sampling refers to statistical sampling meth-
3.2.2 cluster sample—a sample in which the sampling
ods that select units for observation with known probabilities
frame is partitioned into disjoint subsets called clusters and a
(including probabilities equal to one for a census) so that
sample of the clusters is selected.
statistically defensible inferences are supported from the
3.2.2.1 Discussion—Data may be collected for all units in
sample to the entire population of units that had a positive
each sample cluster or, when a multistage sample is being
probability of being selected into the sample.
selected, the units within the sampled clusters may be further
1.3 This guide describes those situations in which probabil-
subsampled.
ity sampling methods are needed for a scientific study of the
3.2.3 compositing samples—physically combining the ma-
indoor air quality in a building. For those situations for which
terial collected in two or more environmental samples.
probability sampling methods are recommended, guidance is
3.2.4 expected value—the average value of a sample statis-
provided on how to implement probability sampling methods,
tic over all possible samples that could be selected using a
including obstacles that may arise. Examples of their applica-
specified sample selection procedure.
tion are provided for selected situations. Because some indoor
3.2.5 multistage sample—a sample selected in stages such
air quality investigations may require application of complex,
that larger units are selected at the first stage, and smaller units
multistage, survey sampling procedures and because this stan-
are selected at each subsequent stage from within the units
dard is a guide rather than a practice, the references in
selected at the previous stage of sampling.
Appendix X1 are recommended for guidance on appropriate
3.2.5.1 Discussion—Forassessingtheindoorairqualityina
probability sampling methods, rather than including exposi-
population of office buildings, individual buildings might be
tions of such methods in this guide.
selected at the first stage of sampling, floors selected within
2. Referenced Documents sample buildings at the second stage, and monitoring locations
2 (for example, rooms or grid points) selected on sampled floors
2.1 ASTM Standards:
at the third stage.
3.2.6 population parameter—a characteristic based on or
calculated from all units in the target population.
This guide is under the jurisdiction of ASTM Committee D22 on Air Quality
3.2.6.1 Discussion—The purpose of selecting a sample is
and is the direct responsibility of Subcommittee D22.05 on Indoor Air.
usually to estimate population parameters. Population param-
Current edition approved Oct. 1, 2006. Published October 2006. Originally
eters cannot actually be calculated unless data are available for
approved in 1995. Last previous edition approved in 2001 as D5791 - 95 (2001).
DOI: 10.1520/D5791-95R06.
all units in the population.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
3.2.7 probability sample—a sample for which every unit on
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
the sampling frame has a known, positive probability of being
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. selected into the sample.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D5791–95 (2006)
3.2.7.1 Discussion—The terms probability sampling and sampling frame from which population elements can be
random sampling are sometimes used interchangeably. selected. Examples include:
3.2.8 sampling frame—a list from which a sample is se- 4.2.1 A list of all offices or work stations in a building,
lected. 4.2.2 A grid of potential monitoring locations that effec-
3.2.8.1 Discussion—An ideal sampling frame contains each tively covers the entire population of interest, and
member of the target population exactly once and contains no 4.2.3 A list of all persons who work in a specific building.
units that are not members of the target population. In practice, 4.3 Since environmental concentrations usually vary con-
the sampling frame may miss some members of the target tinuously in time, spatial frame units like those listed in 4.2
population (for example, new employees in a building) and often must be crossed with temporal units, such as seasons,
include some individuals who are not members of the target weeks, days, or hours, to form sampling frame units (for
population(forexample,individualswhonolongerworkinthe example, building-seasons, office-weeks, or person-days). Spe-
building). However, no member of the population should be cific issues that must be considered when constructing these
listed more than once on the sampling frame. types of sampling frames are discussed in Section 7.
3.2.9 simple random sample—a sample of n elements se- 4.4 In addition to constructing sampling frames, a random-
lected from the sampling frame in such a way that all possible ization procedure is necessary so that units can be selected
samples of n elements have the same chance of being selected. from the frame with known probabilities. Some basic consid-
3.2.10 statistic—a sample-based estimate of a population erations for and methods of selecting probability samples for
parameter. studies of indoor air quality are presented in Section 8.
3.2.11 stratified sample—a sample in which the sampling 4.5 Finally, Section 9 discusses considerations for statistical
frame is partitioned into disjoint subsets called strata, and analysis and reporting that are peculiar to data collected using
sample units are selected independently from each stratum, probability sampling designs. Special statistical analysis meth-
possibly at different sampling rates. ods are necessary when the sampling design includes stratifi-
3.2.12 systematic sample—a sample selected by choosing cation, clustering, multistage sampling, or unequal probabili-
oneofthefirstkelementsonthesamplingframeatrandomand ties of selection.
then including every kth element thereafter.
5. Significance and Use
3.2.13 target population—the set of units or elements (for
example, people or locations in space and time) about which a
5.1 Studies of indoor air problems are often iterative in
sample is designed to provide inferences.
nature.Athorough engineering evaluation of a building (1-4)
3.2.13.1 Discussion—The target population is sometimes
is sometimes sufficient to identify likely causes of indoor air
referred to as the population or universe of interest.
problems. When these investigations and subsequent remedial
3.2.14 unbiased estimator—a statistic whose expected
measures are not sufficient to solve a problem, more intensive
value is equal to the population parameter that it is intended to
investigations may be necessary.
estimate.
5.2 This guide provides the basis for determining when
probability sampling methods are needed to achieve statisti-
4. Summary of Guide
cally defensible inferences regarding the goals of a study of
4.1 When the objectives of an investigation of indoor air
indoor air quality. The need for probability sampling methods
quality include extending inferences from a sample of units to
in a study of indoor air quality depends on the specific
the larger population from which those units were selected,
objectives of the study. Such methods may be needed to select
probability sampling methods must be used to select the
a sample of people to be asked questions, examined medically,
sample units to be observed and measured. Examples include:
or monitored for personal exposures.They may also be needed
4.1.1 Estimating the distributions of health and comfort
to select a sample of locations in space and time to be
symptoms experienced by the employees in a particular build-
monitored for environmental contaminants.
ing during a specific week.
5.3 This guide identifies several potential obstacles to
4.1.2 Estimating the distribution of hourly average concen-
proper implementation of probability sampling methods in
trations of specific substances in the breathing zone air in a
studies of indoor air quality in buildings and presents proce-
particular building during the working hours of a specific
dures that overcome those obstacles or at least minimize their
week.
impact.
4.1.3 Estimating the relationship between measures of en-
5.4 Although this guide specifically addresses sampling
vironmental conditions in a building and the health or comfort
people or locations across time within a building, it also
symptoms experienced by the occupants.
provides important guidance for studying populations of build-
4.1.4 Thus, the study objectives are always a key consider-
ings. The guidance in this document is fully applicable to
ation for determining if probability sampling methods are
sampling locations to determine environmental quality or
necessary.Potentialobjectivesforindoorairstudiesthatwould
sampling people to determine environmental effects within
require probability sampling methods are discussed explicitly
each building in the sample selected from a larger population
in Section 6.
of buildings.
4.2 Guidance is provided regarding the appropriate prob-
ability sampling methods to address these and other goals that
require extending inferences from a sample to a specific
The boldface numbers in parentheses refer to the list of references at the end of
population. Those sampling methods require construction of a this guide.
D5791–95 (2006)
6. Study Objectives That Require Probability Sampling depend upon the burden it imposes, pre-survey publicity (for
Methods example,newslettersorunionendorsements),andfollow-upof
nonrespondents. The survey should be conducted in such a
6.1 Inferences beyond the units actually observed in a
manner that people are sufficiently motivated to participate but
sample are not rigorously defensible unless the units observed
not unduly alarmed about a potential air quality problem.
are a probability sample selected from the population to which
Finally, residual nonresponse is inevitable, and survey data
inferences will be extended. Thus, probability sampling meth-
analysis procedures that utilize weighting or imputation to
ods are needed whenever inferences will be extended from the
compensate for nonresponse are recommended.
units observed in a sample to a larger population. The need for
such inferences depends directly on the objectives of the study.
6.3 Environmental Monitoring:
The study objectives may include characterizing a building’s
6.3.1 Since air quality characteristics generally exhibit both
occupants using a survey, or characterizing a building’s air
spatial and temporal variability, each air quality measurement
quality using environmental monitoring, or a combination of
(for example, temperature, humidity, or concentrations of
both.
specific substances) is generally representative of a specific
6.2 Occupant Survey:
location and time (or period of time). If the objective is to infer
6.2.1 A sample of building occupants may be asked to
information about the distribution of the measured character-
complete a questionnaire or to submit to a physical examina-
istics (for example, the mean or the range) for a target
tion. If the intention is to make inferences from the sample
population of times and places, then probability sampling of
regarding the health and comfort symptoms of all the employ-
both locations and times is required to justify that inference.
ees of the building, a census of all building occupants or a
6.3.2 Specific study objectives that require inferences to a
probability sample selected from them is required. The occu-
population of units defined in time and space include the
pants would typically be asked about their health and comfort
following:
symptoms for a specific period of time (for example, the day
that the survey is administered, the previous week, month, or 6.3.2.1 Estimate the distribution of hourly average concen-
year, and so forth). Developing a valid and reliable question- trations of specific substances in a building during a specified
naire is a complex process and is not directly addressed by this time frame either before or after implementing remedial
guide (5).
measures, or as a measure of the magnitude of a potential
6.2.2 Specific study objectives that require inferences to a
indoor air problem.
population of building occupants include the following:
6.3.2.2 Estimate the distribution of hourly average concen-
6.2.2.1 Estimate the distribution of health and comfort
trations of specific substances in a building with suspected
symptoms in a building either before beginning air quality
problems and in another building studied for comparison
measurements, after implementing remedial measures, or as a
purposes. In each case, the target population would be defined
measure of the magnitude of a potential indoor air problem.
asaspecificsetofbuildinglocationscrossedwithaspecificset
6.2.2.2 Estimate the distribution of health and comfort
of time points. Inferences to the population would require that
symptoms in a building with reported problems and in another
data be collected for a probability sample of the population
building studied for comparison purposes.
units.
6.2.2.3 Estimate the relationship of health and comfort
6.3.3 Temporal variations in air quality must always be
symptoms with worker characteristics, such as age, sex, work
consi
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