8/19/2023 0 Comments Z score standard deviationLonger is usually better because the estimates will include more operators and more method changes, such as pre and post maintenance performance, changes in reagent lot numbers, sample probes or pipettes, etc., thus even one month might be too short a period. Too short a period leads to too small an estimate of the standard deviation. You do this by including control materials as part of your daily work for a long enough period to observe the variation expected in your laboratory. The rule of thumb is to collect at least 20 measurements over at least 2 weeks or 10 working days, and preferably over at least 4 weeks or 20 working days. How many control measurements should be collected before making these calculations? It's also common to express the standard deviation in percent by calculating a coefficent of variation, or CV. What statistics need to be calculated to establish my own control limits? You need to calculate the mean and standard deviation from the control results that have been collected for each control material. Note that US CLIA regulations require that the laboratory determine it's own mean and standard deviation. If the control limits are too wide, you won't be able to detect problems in your own laboratory. Therefore, those limits are likely to be too wide for an individual method in your laboratory. Values and limits found on assay sheets often describe the performance observed by a specific method in several different laboratories, which means that the figures are likely to include variations that occur between laboratories. Yes, you still need to collect your own control measurements and calculate the control limits that apply in your own laboratory. PLEASE NOTE: an updated version of this lesson is available in Basic QC Practices, 3rd Edition What Calculations?Īre any calculations necessary if the control material has an assay sheet that lists the range of acceptable values for my method? Westgard discusses the terms Mean, SD, CV, Control Limits, z-scores and SDI's, explaining what they are, giving the equations, and demonstrating how to calculate them. Despite the age of computers, we still have to crunch the numbers ourselves sometimes. This lesson discusses the math involved with QC practice.
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