Representative sample - meaning

Representative sample means a sample that consists of a number of units that are drawn based on rational criteria such as random sampling and... 

One of the most important uses of specific surface determination is for the estimation of the particles size of finely divided solids the inverse relationship between these two properties has already been dealt with at some length. The adsorption method is particularly relevant to powders having particle sizes below about 1 pm, where methods based on the optical microscope are inapplicable. If, as is usually the case, the powder has a raiige of particle sizes, the specific surface will lead to a mean particle size directly, whereas in any microscopic method, whether optical or electron-optical, a large number of particles, constituting a representative sample, would have to be examined and the mean size then calculated. 

The random nature of most physieal properties, sueh as dimensions, strength and loads, is well known to statistieians. Engineers too are familiar with the typieal appearanee of sets of tensile strength data in whieh most of the individuals eongregate around mid-range and fewer further out to either side. Statistieians use the mean to identify the loeation of a set of data on the seale of measurement and the variance (or standard deviation) to measure the dispersion about the mean. In a variable x , the symbols used to represent the mean are /i and i for a population and sample respeetively. The symbol for varianee is V. The symbols for standard deviation are cr and. V respeetively, although a is often used for both. In this book we will always use the notation /i for mean and cr for the standard deviation. 

Due to its nature, random error cannot be eliminated by calibration. Hence, the only way to deal with it is to assess its probable value and present this measurement inaccuracy with the measurement result. This requires a basic statistical manipulation of the normal distribution, as the random error is normally close to the normal distribution. Figure 12.10 shows a frequency histogram of a repeated measurement and the normal distribution f(x) based on the sample mean and variance. The total area under the curve represents the probability of all possible measured results and thus has the value of unity. 

The value 0,5 per cent is in fact the difference between the sample mean x and the actual value p. If this value is represented by E, then equation (2) may be written as... 

Figure 2.3. Carbon and nitrogen isotope values of human bone collagen at Maya sites in Belize. The value for the modem sample has been corrected for collagen-hair spacing and the Industrial Effect. Boxes represent isotopic means one standard deviation.

Whole cell OPH activity was measured by following the increase in absorbancy of p-nitrophenol from the hydrolysis of substrate (0.1 mM Paraoxon) at 400 nm (sm = 17,000 M cm ). Samples of culture (1 ml) were centrifuged at 10,000 g and 4 C for 5 min. The cells were washed, resuspended with distilled water, and 100 pi was added to an assay mixture containing 400 pi 250 mM CHES [2-(N-cyclohexylamino)ethane-sulfonic acid] buffer, pH 9.0, 100 pi 1 mM Paraoxon, and 400 pi distilled water. One unit of OPH activity was defined as pmoles Paraoxon hydrolyzed per min. Each value and error bar represents the mean of two independent experiments and its standard deviation. 

Table 1.2 gives a representative sampling of eqnilibrinm constants for additions to varions types of carbonyl compounds. Notice that there are nnmerons gaps in the table. This means that mnch remains to be done in the stndy of carbonyl addition reactions. In trying to devise schemes for predicting the eqnilibrinm constants for snch reactions, the scarcity of experimental data is a serions handicap. There are many fewer equilibrium constants for additions to imines, and even fewer cases where... 

Subset of a population that is collected Frank and Todeschini [1994] in order to estimate the properties of the underlying population , e.g., the sample parameters mean x and standard deviation s. In the ideal case of representative sampling, the sample parameter fit the parameter of the population ji and a, respectively. 

To compute the results shown in Table 34-5 for production samples, the accuracy of each set of replicates for each sample, method, and location was individually calculated using the root mean square deviation equation as shown in equations 34-5 and 34-6 in standard symbolic and MathCad notation, respectively. The standard deviation of each set of sample replicates yields an estimate of the accuracy for each sample, for each method, and for each location. The accuracy is calculated where each y is an individual replicate measurement GM is the Grand Mean of the replicate measurements for each sample, both methods, both locations and N is the number of replicates for each sample, method, and location. The results found in Table 34-5 represent samples 1-3. Note Each sample had a Grand Mean computed by taking the mean for all measurements made for each of the samples 1-3. 

A survey of the radon concentrations in a representative sample of more than 2000 dwellings in the UK has been completed and provisional results are now available. On average, concentrations are 29% lower in bedrooms than in living areas. The mean radon concentration weighted for room occupancy is 22 Bq m 3. Assuming an equilibrium factor of 0.35 and a mean occupancy of 75%, the mean annual exposure in UK homes is assessed as 0.08 Working Level Months (WLM) and the mean annual effective dose equivalent as 0.43 mSv. 

Fig. 4 Left the mean 1961-1990 monthly temperature for the Ebro catchment. Part (a) shows the annual cycle, each line representing a different RCM simulation and the bold line representing the CRU observed series. The shading represents the 95% confidence interval for the estimate of the observed 30-year sample mean. Part (b) represents the individual monthly model means as an anomaly from the CRU mean with 95% confidence interval superimposed. Part (c) represents the mean absolute annual error for each of the RCMs. Right-, as for left column but for mean precipitation (d) for the Gallego catchment. Model anomalies in parts (e) and (f) are expressed as a percentage relative to the CRU monthly mean. Model numbers correspond to experiments shown in Table 1. Figure from [35]...

Figure 2.9 Cumulative release of heparin from silica xerogel matrix with varying drug concentration. Each data point represents the mean of three samples and error bars are standard deviations. (Reproduced from ref. 4, with permission.)...

XRF characteristics that can limit its usefulness are the surface area observed and surface contamination. In XRF, the surface area measured is small, meaning that a large number of determinations must be made in order to obtain a representative sample of the elements present. In addition, transport and storage of uncovered soil samples can lead to surface contamination that will subsequently appear as part of the soil constituents. 

A sample is a portion of a larger bulk system that has all the characteristics of the bulk system and is acquired for the purpose of testing it in a laboratory. To obtain a sample means to carry out a process by which a sample is acquired from the bulk system such that the sample truly represents the bulk system and is not altered in any way before the analysis takes place in the laboratory. 

Figure 5. Inhibitory effect of NO on Fe -induced lipid peroxidation. Shown is the decreased generation of an oxidative marker (thiobarbituric acid reactive substances, TBARS) as a result of 0.9 iM NO. HL-60 cells (5 x loVral) were placed in an O2 monitor and at the designated time points, butylated hydroxytoluene was added and samples were quick frozen for determination of TBARS. The values represent the mean and standard error of 3-5 independent determinations. Also shown for comparison is the residual concentration of O2 after exposure to the the same conditions. This shows a decrease in utilization of O2 in the presence of NO. We conclude that NO reduces TBARS, and the percent inhibition is similar to the poeent inhibition of O2 consumption. (Modified from our data in Kelley, E.E., Wagner, B.A., Buettner, G.R., and Bums, C.P., 1999, Arch. Biochem. Biophys. 370 97-104).

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