The number of units to be used to
establish bias is arbitrary,
but the general rule is the more independent “accuracy” trials, the better. The improvement, as measured by the width of the confidence interval for the mean, follows the square root of the number of trials. Once past 8-10 values, improvement comes slowly. To fully contribute, the values must be conducted independently, i.e., nonsimultaneously, throwing in as many environmental or spontaneous differences as possible, such as different analysts, instruments, sources of reagents, time of day, temperature, barometric pressure, humidity, power supply voltage, etc. Each value also contributes to the within-laboratory precision as well. A reasonable compromise is to obtain 10 values from a reference material, a spiked matrix, or by the method of standard addition scattered over several days or in different runs as the basis for checking bias or recovery. By performing replicates, precision is obtained simultaneously. Precision obtained in such a manner is often termed “intermediate precision” because its value is between within-laboratory and among-laboratory precision. When reported, the conditions that were held constant and those that were varied must be reported as well.
The number of units to be used to establish bias is arbitrary, but the general rule is the more independent “accuracy” trials, the better. The improvement, as measured by the width of the confidence interval for the mean, follows the square root of the number of trials. Once past 8-10 values, improvement comes slowly. To fully contribute, the values must be conducted independently, i.e., nonsimultaneously, throwing in as many environmental or spontaneous differences as possible, such as different analysts, instruments, sources of reagents, time of day, temperature, barometric pressure, humidity, power supply voltage, etc. Each value also contributes to the within-laboratory precision as well. A reasonable compromise is to obtain 10 values from a reference material, a spiked matrix, or by the method of standard addition scattered over several days or in different runs as the basis for checking bias or recovery. By performing replicates, precision is obtained simultaneously. Precision obtained in such a manner is often termed “intermediate precision” because its value is between within-laboratory and among-laboratory precision. When reported, the conditions that were held constant and those that were varied must be reported as well.
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