Random error Information and Courses from MediaLab, Inc.
These are the MediaLab courses that cover Random error and links to relevant pages within the course.
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|The gradual deterioration of an instrument's light source would most likely be reflected in the quality control results as:||View Page|
|Which of the following best describes random error?||View Page|
|Levey-Jennings Quality Control Charts|
Quality control charts are used to record the results of measurements on control samples, to determine if there are systematic or random errors in the method being used. The most common type of chart is the Levey-Jennings chart.There should be a separate control chart for each method being monitored, and separate charts for normal and abnormal controls. The mean and standard deviation of the control being used should be noted on the chart. These should be determined based on at least 20 measurements over 20 days. Here is an example of a Levey-Jennings chart. Each time the control is tested, the result is marked on the chart at the appropriate standard deviation level. For instance, if the mean for a control is 15 and the standard deviation 5, if you test a control, and get a value of 22.5, the chart is marked at +1.5 SD for that day.
|Westgard Multi-Rule Approach|
Quality control charts are examined to see if there are problems in the measuring system. The Westgard multi-rule approach can help to determine whether there is a problem, and whether that problem is due to random or systematic error. If two controls are used, the Westgard Rules that may be considered for rejecting an analytical run are: 13s: This rule applies when a control result falls outside of the 3s limit, either above or below the mean. The run should be rejected. Usually, this indicates that a random error has occurred. 22s rule: This rule applies when two consecutive results exceed the +2 or the -2 standard deviation limit. The controls could be normal or abnormal (across runs), or one of each (within a run and both outside the same 2SD). A violation of this rule usually indicates a systematic error. The run is rejected. R4s rule: This rule applies when the difference between the highest and lowest result of a run exceeds 4 standard deviations. This rule detects random errors and only applies within a run (ie, not across runs) The run is rejected. 41s rule: This rule applies when four consecutive control samples all exceed the +1 or the -1 limit. The controls could be normal, abnormal, or a combination of the two. This rule detects systematic errors. The run is rejected. 8xrule: This rule applies when 8 consecutive controls all fall on the same side of the mean, either above or below. The rule could also apply if 4 consecutive controls fall on the same side of the mean with both controls. This rule detects a systematic error. The run is rejected.
|Random and Systematic Error|
There are two types of error that one should look for when examining quality control charts. Random errors are statistical fluctuations (in either direction) in the measured data due to the precision limitations of the measurement device.Systematic errors, by contrast, are reproducible inaccuracies that are consistently in the same direction. Systematic error is more subtle and harder to detect.
|Sources of Laboratory-Related Errors||View Page|
|Types of Error|
Faulty performances that cause an error in test results fall into two large categories: random error and systematic error. Both types of errors must be investigated and resolved for accurate and precise testing.
Random error affects the precision of a test (reproducibility). Factors that contribute to random errors include: Bubbles in reagents or reagent linesInstrument instabilityTemperature variationsOperator variability, such as variation in pipetting
|Westgard Rule 13S|
Westgard rule 13s states that if a control is greater than ± 3 standard deviations from the mean, it should be rejected and rerun. This is because either a random error or a very large systematic error has occurred, as less than 1% of all test values exceed ± 3SD. In the accompanying example, the control for Day 13(noted by the arrow) is greater than +3SD from the mean. Consequently, the 13s rule applies and the run is rejected. Troubleshooting must be performed before further testing can be done.
|What is a Cumulative Summation Limit?|
Like the Westgard Rules, the Cumulative Summation Limit or Rule (CUSUM for short) has different approaches. The CUSUM type used on the following pages is more sensitive to systematic than random error. Nevertheless, it does provide an easy means to detect impending problems. CUSUM is calculated on worksheets like the one below. Basically CUSUM works in the following manner: a decision limit is predetermined (See section E. on the right side of the chart, where CUSUM limit is defined as SD x 2.7), and when the CUSUM of control observations exceed this limit, one must look for error in the testing process. The right side of the worksheet is used to determine the mean, standard deviation (SD), and CUSUM limit.
|What is a Youden Plot?|
In the late 1950's, Dr. William Youden (1900-1971) developed what has now become known as the Youden Plots. This statistical technique involves both normal and abnormal controls and graphically helps to differentiate between systematic and random errors. The inner square of the plot (yellow) represents one standard deviation (1SD). The next larger square (green) represents 2SD, and the outer square (blue) represents 3SD. A horizontal median line is drawn parallel to the X-axis and a second median line is drawn parallel to the Y-axis. The intersection of the two median lines is called the Manhattan Median. One or two 45-degree lines are drawn through the Manhattan Median. The results of at least two different levels of controls (e.g. Level 1/Level 2 or Normal/Abnormal) are then plotted on the chart as X-axis versus Y-axis.
|Using a Youden Plot|
Controls are run and plotted. Plots that lie near the 45-degree reference line and within the one and two standard deviation squares show acceptable results. Points that lie near the 45-degree reference lines but outside the 2SD square indicate a systematic error. Points that lie far from ether 45-degree reference line indicate a random error.
|Possible Corrective Actions, continued|
Rerun the control that is out-of-range.Random errors in sampling may be resolved by simply running the test again using the same control and a fresh testing device.
|Examining the Youden Plot to the right, you would:||View Page|
|Referring to the Levey-Jennings chart shown on the right, what is the most likely quality control problem that occurred starting with day five?||View Page|
|In this example glucose run. possible random errors occurred on days:||View Page|
|Indicate which of the problems in the list below are more likely to be random errors or systematic errors.||View Page|
|In the accompanying Youden plot, what conclusions can be drawn about the data?||View Page|