Incident-rate measures limited usefulness

This small plant size presented a problem for safety measurement in the company. In 1993 the company was well along with the implementation of a total quality management implementation. Based on the recent training in variation and the use of control charts, the safety managers had begun to use U charts when looking at incident-rate measures. The wide limits of variation encountered with small plant sizes convinced them that incident rates would not be particularly useful as a measure of performance in the company, at least for small sites and for short time periods. 

The upper control limit is three standard deviations from the mean, or 3 + (3 X 2.74), or 11.22. This is based on one year of data. If the observation period shrinks to a month, then the units of exposure go from. 4 to. 033, and the upper control limit rises to 31.60. This looks high, but how many incidents can happen in a month for the measure to remain within control limits The answer is that one accident in a month will yield a rate of 30.3 for that month, just inside the control limits. At this site, if we had two recordables in the month, we would exceed control limits. The point here is that comparing either the recordable rates of small sites or rates of larger sites computed over short periods is not very useful. If you measure two plants with 40 employees for over a year and one has a rate of 8 and the other of 0, there is no statistical basis for concluding they are different. This difference is within the normal random variation of measure being used. The incident rate of 8 took only 3 recordable events to reach. 

Higher than normal flow rates, measured at 201b/min, have been used without incident. Still higher flow rates estimated as possibly 40 Ib/min (but not measured accurately) produced on a rare windless day a combustible gas indication of 34 of the lower explosive limit on the outside of the building 6 ft above ground level. 

Establishing the absence of a radionuclide becomes important when radioan-alytical chemistry is used to demonstrate that a radiological incident, such as a threatened terrorist act, did not occur. Absence of a substance in a sample is a relative conclusion that could be altered by a more sensitive measurement. A radionuclide is reported as measured or as less than the lower limit of detection. A more sensitive detection method may replace the less than description with an actual value, or continue to report less than, but at a lower value. Such less than values are based on net count rates that may be zero or a positive value sufficiently near zero that is too uncertain, as discussed in Section 10.4, to be reported. This net count rate can be the difference between the gross count rate and (1) the detector background count rate or (2) the count rate in the sample attributed to background from various sources, as defined above. 

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