Choice test limitation

The most common format of achievement test items is the multiple-choice question. This is certainly true of commercially developed tests and is becoming true of more and more classroom tests, especially at the college level. Multiple-choice items certainly have a number of limitations, and there is no doubt that students have more freedom of expression on open-ended questions. It is well understood that items requiring generation and construction of responses by students usually produce richer data than simple recall and recognition items. However, multiple-choice tests can be effective if they are carefully developed, and they may be especially... 

Table 14. Effective antifeedant doses (EC ) and 95% confidence limits (lower, upper) of compounds 27-31 against S. littoralis sixth-instar (L6) larvae and L. decemlineata adults in choice tests [20].

The most common procedure is to run an indium standard under the normal test conditions and measure the heat of fusion value and melting onset temperature. These values are then compared with literature values and a check made against accepted limits. For many industries limits of 0.5°C for temperature or 1% for heat of fusion maybe accepted, though tighter limits of 0.3°C and 0.1% may also be adopted. The choice of limits depends on how accurate you need to be. Indium is the easiest standard to use because of its stability and relatively low melting point of 156.6°C, which means it can often be reused, provided it is not heated above 180°C. 

In contrast to a direct injection of dc or ac currents in the sample to be tested, the induction of eddy currents by an external excitation coil generates a locally limited current distribution. Since no electrical connection to the sample is required, eddy current NDE is easier to use from a practical point of view, however, the choice of the optimum measurement parameters, like e.g. the excitation frequency, is more critical. Furthermore, the calculation of the current flow in the sample from the measured field distribution tends to be more difficult than in case of a direct current injection. A homogenous field distribution produced by e.g. direct current injection or a sheet inducer [1] allows one to estimate more easily the defect geometry. However, for the detection of technically relevant cracks, these methods do not seem to be easily applicable and sensitive enough, especially in the case of deep lying and small cracks. 

When constmction is complete, the pipeline must be tested for leaks and strength before being put into service industry code specifies the test procedures. Water is the test fluid of choice for natural gas pipelines, and hydrostatic testing is often carried out beyond the yield strength in order to reHeve secondary stresses added during constmction or to ensure that all defects are found. Industry code limits on the hoop stress control the test pressures, which are also limited by location classification based on population. Hoop stress is calculated from the formula, S = PD/2t, where S is the hoop stress in kPa (psig) P is the internal pressure in kPa (psig), and D and T are the outside pipe diameter and nominal wall thickness, respectively, in mm (in.). 

The information obtained during the background search and from the source inspection will enable selection of the test procedure to be used. The choice will be based on the answers to several questions (1) What are the legal requirements For specific sources there may be only one acceptable method. (2) What range of accuracy is desirable Should the sample be collected by a procedure that is 5% accurate, or should a statistical technique be used on data from eight tests at 10% accuracy Costs of different test methods will certainly be a consideration here. (3) Which sampling and analytical methods are available that will give the required accuracy for the estimated concentration An Orsat gas analyzer with a sensitivity limit of 0.02% would not be chosen to sample carbon monoxide... 

Guidance on the choice of respiratory protection for selected environments is given in Figure 13.6. All persons liable to use such protection should be fully trained this should cover details of hazards, limitations of apparatus, inspection, proper fitting of facepiece, testing, cleaning etc. 

Subcontracts enable you to choose the degree of control exercised over your subcontractors. With suppliers, your choices are often limited as you have no privileges. Control over your suppliers is therefore exercised by the results of receipt inspection or subsequent inspections and tests. If your confidence in a supplier is low, you can increase the level of inspection and if high you can dispense with receipt inspection and rely on in-process controls to alert you to any deterioration in supplier performance. 

The woven fabrics have various yarn patterns for different spacings between the yarn fabrics (Table 4-14). There is a wide variety of choices for not only the materials of construction but the tightness of weave and the size of the yarn. All of these factors along with the others noted earlier, make the selection of bag fabric an art that requires manufacturer s and plant s actual field tests. Woven fabrics have a low ratio of weave openings for yarn area and generally have a limited face velocity for air flow of about 1.5 to 3.0 cu ft/min/sq ft [47]. 

We were surprised to find that no method existed that was sensitive enough to detect TCDD in animal tissues even after administration in some species of lethal doses. An example is the guinea pig, the most susceptible species of the few that have been tested, and therefore a good choice for establishing desirable limits of detection. The lethal single oral dose (LD50) in males of this species is 0.6 /xg/kg body weight... 

The choice of materials from which to make the holder is important. Materials must be durable enough to ensure satisfactory completion of the test. It is good practice to select very resistant materials lor the test assembly. Insulating materials used are plastics, porcelain. Teflon, and glass. A phenolic plastic answers most purposes its principal limitations are unsuitability for use at temperatures over 150°C (300°F) and lack of adequate resistance to concentrated alkalies. 

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