Limits, action warning

QCC contain - as quality target values Q - standard or reference values, x0, resp. optimum values as well as their limits. The inner pair of limits are called warning limits and the outer pair control limits (action limits). When... 

Method validation provides information concerning the method s performance capabilities and limitations, when applied under routine circumstances and when it is within statistical control, and can be used to set the QC limits. The warning and action limits are commonly set at twice and three times the within-laboratory reproducibility, respectively. When the method is used on a regular basis, periodic measurement of QC samples and the plotting of these data on QC charts is required to ensure that the method is still within statistical control. The frequency of QC checks should not normally be set at less than 5% of the sample throughput. When the method is new, it may be set much higher. Quality control charts are discussed in Chapter 6. 

Define statistical control, control chart, warning limits, action limits. 

For a Gaussian distribution, 95.5% of all observations are within 2oVV/t from the mean and 99.7% are within a 3o-/V/, where n is the number of tablets (=25) that are averaged each hour. The 2oVVn limits are warning lines and the 3cr/Vn limits are action lines. We expect —4.5% of measurements to be outside the warning lines and —0.3% to be outside the action lines. It is unlikely that we would observe two consecutive measurements at the warning line (probability = 0.045 X 0.045 = 0.002 0). 

Procedures for calibration of various instruments lacked some or all of the following information persons responsible for the calibration specihcations or limits action taken if a test fails and a periodic review by management. [FDA Warning Letter, 2001]... 

Control charts often have a center line and two control lines with two pairs of limits a warning line at m 2s and an action line at m 3s. Statistics predict that 95.45% and 99.7% of the data will fall within the areas enclosed by the 2s and 3s limits. The center line is either the mean or the true value. In the ideal case, where unbiased methods are being used, the center line would be the true value. This would apply, for example, to precision control charts for standard solutions. 

Statistical data can be employed to construct control charts (Shewart charts) which provide action limits and warning limits for values of mean and standard deviation. Charts can be used for measurement of variability (size, volume, hardness, etc.) or attributes (good/bad or acceptable/rejectable). Values calculated from routine samples are plotted onto the relevant chart, and provided the results fall within the acceptance area, manufacture can continue until the next sample is called for. 

Upper action limit Upper warning limit... 

Lower warning line Lower limit (action line)... 

Control charts often have a center line and two control lines with two pairs of limits a warning line at 2 a and an action line at 3 o. Statistics predict that 95.45 and 99.7 % of the data will fall... 

The conventional control chart is a graph having a time axis (abscissa) consisting of a simple raster, such as that provided by graph or ruled stationary paper, and a measurement axis (ordinate) scaled to provide six to eight standard deviations centered on the process mean. Overall standard deviations are used that include the variability of the process and the analytical uncertainty. (See Fig. 1.8.) Two limits are incorporated the outer set of limits corresponds to the process specifications and the inner one to warning or action levels for in-house use. Control charts are plotted for two types of data ... 

In a modem laboratory, automatic sensors are often used to detect unwanted changes in laboratory conditions and warn laboratory staff. Basic laboratory conditions, such as temperature, humidity and particulates, can all be monitored continuously using sensors. The results can either be fed to chart recorders, or into computer-controlled laboratory management systems, which can take corrective action or sound alarms in the event of the limit for a particular condition being exceeded. 

We do this by using the statistical ideas outlined above. First of all, the QC sample is measured a number of times (under a variety of conditions which represent normal day-to-day variation). The data produced are used to calculate an average or mean value for the QC sample, and the associated standard deviation. The mean value is frequently used as a target value on the Shewhart chart, i.e. the value to aim for . The standard deviation is used to set action and warning limits on the chart. 

When using control charts, you should take action on any points which fall outside the action limits and be alert when points exceed the warning limits. There are three other situations which normally indicate a problem with the system, as follows ... 

If action and warning limits are added to a moving average chart, they should be plotted at 31+Jn and 2l+Jn units of standard deviation, respectively. 

Figure SAQ 6.2 (a) Shewhart chart with warning and action limits at 2 and 3 standard deviations, respectively, (b) Moving average chart (n = 5) with warning and action limits at 2 Jn and 3 Jn standard deviations, respectively, (c) CUSUM chart, (d) CUSUM chart with V-mask.

Any device routinely checked for calibration can be monitored in this way. For example, an analytical balance can be tested with a known weight, the value of the known weight being the desirable value and the expected range of precision dictating the warning and action limits (Experiment 1). 

A procedure or method maybe checked by the use of a quality control solution (often called a control), a solution that is known to have a concentration value that should match what the procedure or method would measure. The known numerical value is the desirable value in the control chart. The numerical value determined for the control by the procedure or method is charted. The warning and action limits are determined by preliminary work done a sufficient number of times so as to ascertain the population standard deviation. 

A process, such as a manufacturing process, may also be monitored with a control chart. In this case, the desirable value, warning limits, and action limits for the product of the manufacturing process is determined over time using materials and equipment that the scientists and engineers are confident provide an accurate picture of the product. 

The narrower limits are usually known as the warning limits. Failure to meet these limits implies that the method must be investigated and any known weakness, such as unstable reagents, temperature control, etc., should be rectified. However, results obtained at the same time as the control result can still be accepted. Probably the first step in a case like this is to repeat the control analysis. If the original result was a valid random point about the mean then the repeat result should be nearer to the mean value. If the repeat analysis shows no improvement or the original control result lay outside the wider control limits (known as action limits) then it must be assumed that all the results are wrong. The method must be investigated, the fault rectified and the analysis of samples and controls repeated. 

Flammable gas detection systems are typically used to initiate an alarm at a concentration level below the lower flammable limit (LFL). Two gas alarm levels (low and high) are often utilized to allow early warning prior to taking automatic actions. Detection systems may also be used to stop electrical power and initiate process shutdown. The low alarm setpoint should be —20% LFL and the high alarm level set point should be between 40%-60% LFL. Where these devices are used to initiate process shutdown or activate fire protection systems, it is common practice to use some form of voting, typically 2 out of 2, such that the frequency of spurious shutdowns or system activation is minimized. 

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