| Other Laboratory Operations Food and Drug Administration |
| DOCUMENT NO.: III-04 | VERSION NO.:1.2 | Section 4 - Basic Statistics and Presentation | EFFECTIVE DATE: 10/01/2003 | REVISED: 06/27/2008 |
4.6 Control Charts
A control chart is a graph of test results with limits established in which
the test results are expected to fall when the instrument or analytical procedure
is in a state of "statistical control." A procedure is under statistical
control when results consistently fall within established control limits. There
are a variety of uses of control charts other than identifying results that
are out of control. A chart will disclose trends and cycles which will allow
real time analysis of data and information for deciding corrective action prior
to say an entire analytical system goes out of control. The use of control
charts is strongly encouraged in regulatory science.
4.6.1 Definitions
- Central line: mean value of earlier determinations, usually a minimum
of twenty results
- Inner control limit: the mean value ± 2 standard deviations
- Outer control limit: the mean value ± 3 standard deviation
4.6.2 Discussion
Control charts are frequently used for quality control purposes in the laboratory.
Control charts serve as a tool that determines if results performed on a routine
basis (e.g. quality control samples) are acceptable for the intended purposes
of the data.
The mean control chart consists of a horizontal central line and two pairs
of horizontal control limits lines. The central line defines the mean value,
the inner control limit (mean ± 2 standard deviations), and outer control
limit (mean ± 3 standard deviations). Results are plotted on the y-axis
against the x-axis variable (e.g. date, batch number).
Results fall within the inner control limits 95% of the time. Results falling
outside the inner control limit serve as a warning that the results may be
biased. Results falling outside the outer control limit indicate the results
are biased and corrective action should be taken.
4.6.3 Quality Control Sample Example
The control chart for a laboratory instrument often plots the results of the
calibration result (y-axis) against the date (x-axis).
Mean control chart:
- Calculate the mean calibration value
- Calculate ±2 standard deviation, ± 3 standard deviation values
- Draw horizontal lines above and below the mean value at ±2
deviations and the mean value ± 3 standard deviations
- Plot calibration results against the date or batch number
- Define corrective actions if the calibration results fall outside
the inner and outer control limits.
4.6.4 References
- Pecsok, Shields, Cairns. (1986). Modern methods of analysis (2nd
Ed.). New York: John Wiley and Sons.
- Steinmeyer, K. P. (1994). Mathematics review for health physics technicians.
Hebron, CT: Radiation Safety Associates Publications. (Also 2nd Ed. in 1998.)
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