Types of sampling error

If our sample is in fact not representative of the underlying population, then instead of throwing light on the situation, it will positively mislead us. So, we will next consider the various and nefarious ways in which a sample might misrepresent the population. 

The purpose of a sample is to estimate properties of the broader population, such as its mean value, but we need to be alert to the possibility that a sample may 

There are two distinct types of error that may arise - bias and random error. 

It is all too easy to take samples, which are virtually guaranteed to produce an average value that is predictably above (or predictably below) the true population mean. In some cases the problem is blindingly obvious and in others more subtle. A couple of examples follow  

A consistent form of mis-estimation of the mean. Either most such samples would over-estimate the value or most would under-estimate it. 

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Flow rate variations cause three types of sampling errors ... 

It is well accepted that two types of sampling errors are possible when removing small masses of powder from bulk [5]. 

Other problems occur in the measurement of pH in unbuffered, low ionic strength media such as wet deposition (acid rain) and natural freshwaters (see Airpollution Groundwatermonitoring) (13). In these cases, studies have demonstrated that the principal sources of the measurement errors are associated with the performance of the reference electrode Hquid junction, changes in the sample pH during storage, and the nature of the standards used in caHbration. Considerable care must be exercised in all aspects of the measurement process to assure the quaHty of the pH values on these types of samples. 

Analytical measurements are fundamentally subject of uncertainty where various types of deviations (errors) can appear and these may be influenced to varying degree. Even when instrument readings are sufficiently accurate, repeated measurements of a sample lead, in general, to measured results which deviate by varying amounts from each other and from the true value of the sample. 

Data analysis was reduced to a separate one-way analysis of variance on the data from individual laboratories in order to examine the difference between types of sampling bottle on a single (common) hydrowire, and to determine the influences of the three types of hydrowire using a single type of sampling bottle (modified GO-FLO). Samples were replicated so that there were, in all cases, two or more replicates to determine the lowest level and analytical error. 

Semisolid samples. As with liquid samples, methods (B) and (C) are the best choices for this type of sample. The specific choice will depend on fhe rheological properties (viscosity, density, air retention) of the particular preparation. These samples are best measured in the transflectance mode. Liquid and semisolid samples may contain a mixture of solvents of disparate volatility which may evaporate separately during the measurement process. Differences in solvent volatility can alter the sample matrix and lead to errors in the determination which are best avoided by using a set of calibration samples spanning an expanded range of solvent proportions. ... 

The strict regulations of the pharmaceutical industry have a significant effect on the quality control of final products, demanding the use of reliable and fast analytical methods. The capacity that the technique has for the simultaneous determination of several APIs with no need of, or with minimum, sample preparation has considerably increased its application in pharmaceutical analytical control. The main limitation of NIR is the relatively low sensitivity that limits the determination of APIs in preparations when their concentration is less than 0.1%. Nevertheless, instrumental improvements allow the determination below this limit depending on the nature of the analyte and the matrix, with comparable errors to the ones obtained with other instrumental techniques. The reference list presents an ample variety of analytical methodologies, types of samples, nature of analyte and calibration models. A detailed treatment of each one is beyond the scope of... 

As the considered error only applies for a specified concentration of analyte isolated from a specified type of sample or matrix, sampling errors and matrix variation effects are not included here [4]. 

Since trace analysis also includes air or gas samples, it is appropriate to point out that proper addition of an internal standard to this type of sample is difficult. This difficulty lies, not in the mechanical problem of transfer, but in the difficulty of knowing that the precisely intended volume has properly been transferred. However, the internal standard technique is still not widely used here for the same reason it is not generally used in trace analysis. This reason again is because the analyst normally has no prior knowledge of the variation in composition from sample to sample. The continual risk exists that any given sample in a series will have a component, not present in others, which elutes with the internal standard. This occurrence would introduce significant error into the quantitative calculations which result. 

Most of these potential sources of error have actually been eliminated, or at least minimized, in commercial analyzers by proper hardware and software implementation, e.g., proper washing and cleansing procedures, signal stability criteria, corrections of the residual potential or simply by declaring limits , i.e., the types of sample, interfering substances, vial types to be avoided, by limiting the number of reports per hour, adjusting within-day and between-day imprecision and the lifetime of the electrodes. 

If the sample mean concentration is close in value to the action level, the decision, however, becomes uncertain, and two types of decision errors may take place ... 

Errors in timing. For the determination of elements yielding short half-life indicator radionuclides (such as in the determination of oxygen via 7.3 sec16N), accurate timing is extremely important. For these cases electronic scaler timers are to be preferred over electromechanical types of timers. Errors due to variable detector dead-time must also be considered when the gross activities of the sample and the standard differ appreciably and the indicator radionuclide is short-lived. 

Ultrasonic energy is frequently used to accelerate the dissolution of solid samples under soft conditions of temperature, pressure and chemical reagents. Similar to direct dissolution by agitation, US-assisted soft digestion is not used to the same extent as other operations of the analytical process such as leaching, derivatization or detection. The simplicity of this operation with some types of samples and the operator s lack of awareness of its error contribution are responsible for the absence of optimization studies for this process. Inappropriately conducted soft digestion can result in major errors and affect the quality of the results. 

The method of drying, type of samples, Mean Absolute Deviation, Mean Absolute Percentage Error, Mean Squared Deviation of these models used for moisture content change with time are presented in Table 1. 

Prepare a list of all samples reqmred for the test and their frequency. Discuss those samples with the lab supervisors to determine the sample containers suitable for each analysis (e.g., bombs, glass bottles, plastic bottles) and the best way of sealing the containers (plugs, corks, screwed tops). Determine which samples need special handling (e.g., refrigeration, stabilization). Attempt to use as few types of sample containers as possible to minimize errors. 

The analysis of solids is time consuming and is prone to many sources of error. Analytical chemists have been trying for many years to analyze solid samples directly, without having to dissolve than. For some types of samples, this can be done by AAS. Solids can be analyzed using a glow discharge (GD) atomizer, by inserting small pieces or particles of sample directly into the flame or furnace or by the use of laser ablation. 

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