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Nuclear Safeguards Education Portal

Calibrations and Sampling

Safeguards measurements have important legal and political ramifications.  Because of this physical standards are used extensively to calibrate measurement equipment and provide a basis for determining the accuracy of measurements.  We use these standard reference materials to determine the measurements systems characteristics with respect to accuracy and precision.

Accuracy characterizes the measurement systems ability to provide a result close to the true value when a sample is measured.  This is related to systematic error.

Precision characterizes the measurement system's probability of providing the same result every time a sample is measured.  This is related to random error.

Sampling is an important part of safeguards measurements because it is unreasonable to expect all items to be measured.  When sampling, an appropriate sample plan must be implemented to select a subset of items to be evaluated to gain information about the whole group.  This requires the collection of representative samples (samples which are typical in respect of certain characteristics of the population from which they are collected).

There are two main sampling strategies, random and systematic.

Random Sampling consists of selecting samples using random number lists or random number generators such that all items in a population have the same probability of being selected.

Systematic Sampling consists of selecting samples in a repeated pattern, such as every 5th item or at fixed time intervals.  This method can result in a bias.

Taking representative samples is often more difficult than you would think since sampling is generally done using some automated, mechanical, or repeated process.

We need to know how many samples are needed to be taken in order to draw conclusions.  The following equation can be used to determine the number of samples needed:

n = N(1-β x/M)

where, n is the number of samples to be selected from each stratum, N is the number of items in the stratum, β is the non-detection probability, M is the mass detection goal, and x is the average nuclear material weight of an item in the stratum.

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