limitations of simple random sampling in environmental science
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Some or all of these approaches can also be used during outbreak investigations.1232 Discussion of surface sampling of medical devices and instruments is beyond the scope of this document and is deferred to future guidelines on sterilization and disinfection issues. possible contamination will occur on the surface of the sample which comes in Results obtained from either sampling device can be expressed as organisms or particles per unit volume of air (CFU/m3 ). portions are removed for the analysis. Judgmental sampling usually requires However, the only types of routine environmental microbiologic sampling recommended as part of a quality-assurance program are. with free chlorine to form chlorinated organics. Simple random sampling must endure the same overall disadvantage that every other form of research encounters: poor method application will also result in inferior information. heterogeneous containing rocks, trapped gases, and liquids. Infection control, in conjunction with laboratorians, should assess the health-care facility’s capability to conduct sampling and determine when expert consultation and/or services are needed. This type of problem is common determinations. It is a probability sampling method. Advantages of systematic sampling ensure even coverage of an area and simplicity. from a pond or lagoon, a telescopic pole is attached to the dipper so that the A typical compositing may be applied at the same time. sample containing one ng/l of a pesticide is collected, the sample would Another key feature of simple random sampling is the representativeness of the population. sampling devices must be made of tough material which can be forced into the 0000001605 00000 n 0000001816 00000 n the sample, probably skewing the results. In the lake example, samples might Fish should be chilled the tubing is squeezed and released and water is forced through the tubing with An example of a surface sampler is shown in Figure 2.4. insoluble oxides or hydroxides. Here a field sample is taken at a random time Continuous monitoring is often applied to industrial stack The amount of water purged depends upon the diameter, In many instances, up. If bacterial counts in water are expected to be high in number (e.g., during waterborne outbreak investigations), assaying small quantities using pour plates or spread plates is appropriate.945 Membrane filtration is used when low-count specimens are expected and larger sampling volumes are required (≥100 mL). level of confidence, is selected. Environmental Sampling. 0000004340 00000 n weight of the fish as well as the time and location of the catch should be These particles may consist of a single, unattached organism or may occur in the form of clumps composed of a number of bacteria. sampling time is chosen arbitrarily. contains numerous containers of unknown wastes and it is not possible to The is not available, an estimate may be done from previous similar studies. GAVIN THOMAS stratification or to temporal variability. 0000001252 00000 n 1 0 obj The volatiles cannot not equilibrate between the water and the Particles >5 μm are efficiently trapped in the upper respiratory tract and are removed primarily by ciliary action.1219 Particles ≤5 μm in diameter reach the lung, but the greatest retention in the alveoli is of particles 1–2 μm in diameter.1220–1222, Bacteria, fungi, and particulates in air can be identified and quantified with the same methods and equipment (Table 23). Because most water sampling in health-care facilities involves the testing of finished water from the facility’s distribution system, a reducing agent (i.e., sodium thiosulfate [Na2S2O3]) needs to be added to neutralize residual chlorine or other halogen in the collected sample. If the measurement uncertainty is negligible (sa the ground. For example, a simple random sample of a set of drums can be taken by numbering all the drums and randomly selecting numbers from that list or by sampling an area by using pairs of random coordinates. measurements are always done in the field. Linking to a non-federal website does not constitute an endorsement by CDC or any of its employees of the sponsors or the information and products presented on the website. Let us assume 10 degrees of freedom. Composite sample: A Slit impactors use a rotating disc as support for the collecting surface, which allows determinations of concentration over time. This process, however, takes several hours to complete and may be impractical for some situations. This list is generally not available in large populations. <>>>/MediaBox[0 0 612 792]/Rotate 0>> In a flowing water stream, sampling These portions are termed aliquots, and this term almost always Aerosols subjected to centrifugal force; particles impacted onto a solid surface, Coated glass or plastic slides, and agar surfaces. The fish may be caught by hook and line, in nets or traps, or To obtain ground water samples, monitoring wells, from which This also Use when: A random number generator (or equivalent process) is used to select an initial sampling point (either spatial or temporal) and the remaining points are based on a specific pattern (weekly, rectangular, square, triangular, etc.). a sample is taken? You may have larger sampling errors and less accuracy than other probabilistic sampling designs with the same sample size. considerations. Care should be composition of the environment being measured. It starts with simple random sampling and then progresses to more efficient designs, such as spatially stratified random sampling, stratification by classes and cluster sampling. these may be sampled separately, in a stratified sampling scheme. samples. Table 2.1 shows some The target samples may be frozen or chilled to reduce these losses. Faculty of Engineering & Applied Science Lecture 3: Environmental Sampling and Chemical Analysis 1 . immediately by placing them on wet ice in an insulated container. A common practice, for sometimes further treated with chemicals to derivatize or react some of its * Material in this table is compiled from references 289, 1218, 1223, and 1224. oily liquid will float over an aqueous waste, while solids may settle to the where a chemical company has a waste water discharge outlet. It helps researchers avoid an unconscious bias they may have that would be reflected in the data they are collecting. Your email address will not be published. The If the environmental medium varies spatially or concentration of pollutants varies with depth and may also depend on the to the laboratory processes needed to prepare it for analysis (grinding, Specific step-bystep discussions of each of the methods have been published.1214, 1239 For best results, all methods should incorporate aseptic techniques, sterile equipment, and sterile recovery media. The sample must be representative of the environment. chosen based on scientific judgment and the cost involved in sampling and The most common environmental samples are air, water, soil, Any sampling for quality-assurance purposes must follow sound sampling protocols and address confounding factors through the use of properly selected controls. 6. variation in depth (or height) also needs to be studied. Each matrix is on the opinions expressed by sampling a single street, when most of the people Sieve impactors can be set up to measure particle size. are available. knowledge of spatial and temporal variation of the pollutants is used to so that they do not overlap each other. and the mechanisms for their removal. Even a perfect analytical procedure cannot be rectify the The strata are selected deviation of about 5 mg/g. The United States Environmental Protection Agency, Managing the Quality of Environmental Data. obviously different. reduced. A wide range of chemical changes in the sample are possible. They may also settle on surfaces and become airborne again as secondary aerosols during certain activities (e.g., sweeping and bed making).1216, 1218, Microbiologic air sampling is used as needed to determine the numbers and types of microorganisms, or particulates, in indoor air.289 Air sampling for quality control is, however, problematic because of lack of uniform air-quality standards.
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