Convenience sampling

Changes in the distribution of organic compounds in a seawater sample can be due to physical, chemical, or biological factors. As a physical factor, we might consider the absorption of surface-active materials on the walls of the sample container. While this effect cannot be eliminated it can be minimised by the use of the largest convenient sample bottle, and the avoidance of plastic (especially Teflon) containers. Another possible method of eliminating this source of error would be to draw the sample directly into the container in which the analytical reaction is to be run. 

Convenience sampling involves selecting samples on the basis of availability and/or accessibility. 

Modification of the apparatus to accomplish automation is allowed by <711>. One example is hollow shaft sampling as illustrated in Figure 10 (15). This method is theoretically within the stated sampling location of the text of <711>, although there may be question about the concentration of sample surrounding the shaft. This and other sampling techniques, for example in-residence probes, are convenient sampling tools but should be properly validated. 

For simplicity and convenience, sampling at evenly spaced intervals over a population is often used in place of random sampling For example, a field may be divided into uniform segments, and a sample taken from the center of each segment This procedure is generally subject to more bias than random sampling Should periodicity in the population be present or suspected, segments to be sampled should be selected with the... 

Disposal pits should be designed for convenient sampling of the liquid contents. The soil doesn t move, so contamination will occur exclusively through the liquid and vapor phase. Containment can be estimated by infrequent sampling of the liquid contents and regular sampling and analyses of the surrounding water taken from shallow wells outside the pit and the air taken from above the pit. 

It is not usually possible to measure the concentration of a drug at its sites of action. Plasma, which can be conveniently sampled, is generally used instead, but drug concentrations may be determined in other bodily fluids, such as saliva and cerebrospinal fluid, as well as, of course, the excreta, urine and faeces. There is often a relationship between plasma concentration and response, although this may sometimes... 

A final issue is that ease of access to populations is not considered an acceptable reason for inclusion and can be considered coercive, especially if the population is vulnerable (i.e., foster children). Institutional review boards and review groups are increasingly concerned about the problems of convenience samples and are requiring full explanation of the appropriateness of certain populations for the study in question. 

Selection bias is the Achilles heel of such samples. Therefore when researchers use convenience samples for assessing population characteristics such as prevalence, incidence, or causal relationships, they must justify the validity of the sample. 

At a minimum, when such convenience samples are reported, the strategy used for recruitment and selection must be made completely transparent and explicit so that scientists can assess the distortions or biases that may result from analyzing measurements in such groups as though they were true population samples. The committee recommends that if convenience samples are chosen, then funders, reviewers, and editors of peer-reviewed journals must insist on complete characterization of how each sample was chosen so that misinterpretation—intentional or not—is less likely. 

A complementary style of nationality-based tourism studies involves the direct assessment of what motivates travellers, how they behave and what they derive from their experiences. Several of these studies are comparative, while others focus on just one nationality group. The comparative studies include work that compares groups on destination image and attitudes, on motivation, on travel patterns, in communication style and on satisfaction levels (Chadee Mattson, 1996 Kozak, 2001 Kozak Neild, 1998 Reisinger Turner, 1999 Richardson Crompton, 1998 Yuan MacDonald, 1990). The nationalities compared are varied and reflect convenience samples rather than systematic research programs. They include Japanese and Americans, British and German, Korean and... 

Samples of the ehlorofluorophosphite are collected in glass ampoules of approximately 1 ml. capacity, which are then sealed and weighed.2 These samples are hydrolyzed in 125 ml. of solution containing sodium hydroxide in slight excess. The difluorophosphite is most conveniently sampled as a gas in a weighing bulb of 500 ml. capacity (AB, Fig. 16, synthesis 45).8... 

The PSOC coals were especially convenient samples for our x-ray studies, because in the data bank, there is a considerable amount of information about these coals obtained by techniques other than x-ray scattering. Without these other results we would not have been able to make such a detailed interpretation of the scattering data. Moreover, since we had studied PSOC coals, we could compare our porosity studies with the work of others, such as that of Gan, Nandi, and Walker—. On the other Jujiui, as the other small-angle x-ray scattering studies of coals— — which we are aware of did not investigate PSOC coals, we could not make a quantitative comparison of our scattering results with those previously published. 

The smallest vertebrates are homogenized whole and most laiger animals can be dissected and different tissues removed for storage in 1.5-ml Eppendorf microtubes at — 70° until homogenized. Heart, kidneys, or liver are sufficient to score more than 20 proteins testes, spleen, brain, and muscle can be sampled for tissue-specific enzymes. Additionally, blood, muscle, and/or saliva from most vertebrates can be conveniently sampled without killing the animal. During all sample preparation steps, keep tubes on crushed ice. 

When determining the percentage composition by mass of a homogeneous sample, the size of the sample does not matter. According to the law of definite proportions, there is a fixed proportion of each element in the compound, no matter how much of the compound you have. This means that you can choose a convenient sample size when calculating percentage composition from a formula. 

The solid supports used in this study were macroporous co-polymers of vinylpyridine and styrene crosslinked with divinylbenzene. Polymers of this type in the form of beads are available commercially (e.g. Reillex 425) and were also prepared for this study by Purolite. For spectroscopic studies, a more convenient sample morphology was required and thin-film polymers of similar stoichiometry were synthesised by the group of Sherrington at the University of Strathclyde. Full details of the methods used to prepare thin film polymers are reported elsewhere.11 To generate the ion exchange resin, the pyridyl functionalities of the polymer were quatemised with methyl iodide (Eq 1). 

Hair washing survey. Results of a convenience sample of 74 middle-class high school students who were asked "How often do you wash your hair ". The distributions were normalized for each sex and ethnicity. The total number of individuals of each type is given in the inset table. (Data from Blank, D. L. and Kidwell, D. A., Forensic Sci. Int., 70, 13, 1995.)... 

Sample Size. A standardized sample size of 0.075 mmole of reactant was chosen. In a binary reaction system the total sample comprises 0.15 mmole. These amounts provide a convenient sample for analysis and are considered to be safe in the event of an explosion in the 2-ml. reaction vessel. 

Mix the beans by stirring or shaking. You will probably have only limited success since smaller, lighter-weight beans will move to the bottom. Decide on a convenient sample weight, and use it for all samples. It s all right if the weights vary a little from sample to sample. 

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