Journal of the ICRU Vol 6 No 2 (2006) Report 76 doi:10.1093/jicru/ndl029Oxford University Press

4 MEASUREMENT ANALYSIS METHODS

4.1 CONTROL CHARTS

Graphical presentation of data can provide usefulinformation about variations and trends. This typeof presentation is also well suited for demonstrationsto laboratory personnel and others who might needto examine the stability of calibration measurementsas a function of time. In addition, a large amountof data can be presented graphically in a controlchart format. Comparisons between measurementsperformed many years ago can easily be comparedwith recent data. Although most of these samecharacteristics can be attributed to the format ofdata presented in the tabular form, it is often easierto detect small problems with the data when presen-ted in a graphical form.

In a control chart, data can be plotted either asratios relative to a reference value or simply as thenumerical values of the measured quantity. Nor-mally, these values are plotted as a function oftime. Graphical indicators of deviation levels thatshould trigger investigation or action can be plottedabove and below an expected value for the measur-and. A typical control chart is shown in Figure 4.1.As an example, two indicators can be placed atpositions that represent expected statistical uncer-tainty for the measurement. For instance, indicatorscan be placed at set percentage variations fromthe reference value. A data point that exceeds thisindicator is easily visible and can trigger an invest-igation of a possible change in response. A secondset of indicators can be placed at a larger variationvalue, and data points falling outside these indicat-ors might require a work-stoppage and corrective-action plan. It is appropriate to record notes as tothe cause of the deviation and the rectification ofthe problem that caused that deviation.

In some situations, the indicators or control levelsmight not be symmetric with respect to the meanor reference value. This could occur when it isimportant not to allow the measurement to fallbelow a certain value. For example, the dose neededto sterilize a medical product is a critical parameter,and serious consequences result if the dose isnot delivered.

4.2 LONG-TERM STATISTICALANALYSIS OF DATA

The quality of a routine measurement systemcan be monitored, and where necessary corrected orimproved, by the statistical analysis of measurementresults over long periods of time (vanDijk, 1998). Themean values, variances, and shapes of distributionsfor monthly, quarterly, and/or annual samples ofresults for the total data set, or for subsets, can beanticipated to be less variable than the mean andvariance for the total data set. The analysis mightalso be carried out for different dose levels or activityconcentrations.

In addition to the use of statistical analysis forexamining the dosimetric data for quality-assurancepurposes, there can also be a need to demonstrate toauthorities that the dosimetry provider has main-tained the required level of consistencywith nationalstandards over an extended time period. The calib-ration records will also indicate any deviationsfrom the control limits and notations as to how thedeviations were dealt with (see Sections 4.4 and 4.5).

4.3 TREATMENT OF OUTLIERS

During the performance of a series of measure-ments, something unexpected or unknown mighthappen that causes incorrect measurements. If thereis no obvious indication of a problem at the time ofthe measurement, there might be no reason forrejecting the result of the measurement at thatpoint. However, when the data are plotted in theform of a control chart or when a table of data isexamined, it might be observed that one or morepoints differ significantly from the others. Individualdata points that are clearly different from theexpected distribution for the measurand are calledoutliers. If the unexpected event causing the incor-rect measurement has been observed, e.g., a transi-ent in the line voltage or a significant change in roomtemperature, then the measurement affected by thisevent can be classified as an outlier. Outliers of thistype can be considered for removal from a data setbefore identifying the cause, because inclusionmight

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distort calculations of the mean and standarddeviation.

If no causative event can be associated with anapparent outlier and no deviation from standard testconditions of temperature, pressure, etc. is evident,then additional data should be taken. Statisticalmethods should be used to test whether the resultof measurement can be considered an outlier; see forexample, Huber (1981) and Rousseeuw and Leroy(1987).

4.4 ERROR (QUALITY ANOMALY)CORRECTION PROCEDURES

One of themost important activities of a facility forthe calibration and measurement of ionizing radi-ation is the awareness and minimization of errors. Itcannot be assumed that mistakes will never happen.In fact, it is a more realistic viewpoint to assume thaterrors will occur, but that every effort will be takento minimize their frequency of occurrence and theirpotential for negative effects. A consequence of thisassumption is the need to develop techniques fordealing with mistakes, correcting erroneous results,and revising procedures so that the problem doesnot recur.

An advisable method to be followed in case ofmistakes that have been discovered in writtendocumentation is that the record of the mistakeshould be crossed out, not erased or made illegible.

For example, in a report the correct value shouldbe entered alongside the value that has beendetermined to be incorrect. All such alterations torecords should be signed and dated by the personmaking the correction. Similar techniques can beapplied to electronic records. Many software pro-grams contain features that allow the strikeoutof a word or a number to remain visible and to bestored along with the modified data. Records oferrors or, in general, of malfunctions in any part ofthe process of measurement should be retained andperiodically checked, in order to avoid recurrencesof the same error. These records can be used laterfor investigations or in-house reviews.

The laboratory must have procedures in place fordealing with mistakes. These procedures shouldensure that responsibilities of staff members aredefined and that action is taken when a problem isuncovered. Staff members should have the authorityto stop work when a significant problem comes totheir attention. Once a mistake has been uncovered,either within the facility or by a client or customerof the measurement facility, corrective actionsshould be taken, retrospectively, as necessary.When appropriate, the clients or customers shall benotified. Appropriate verifications of measurementquality should be performed before work is author-ized to continue.

Several methods have been found to be useful inreducing the occurrence of errors. Informal auditsof measurement quality performed by a member of

Figure 4.1. Thermoluminescent dosimeter (TLD) readings recorded as a function of time. The four plots indicate readings from the fourTLD chips used in a particular design of personal dosimeter. Readings exceeding control limits may bear investigation. (Plot kindlysupplied by B.A. Rathbone and S.E. Huneycutt of the Pacific Northwest National Laboratory).

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the laboratory are effective in identifying potentialproblems. One staff member can be designatedwith the responsibility for reviewing proceduresand requesting measurements (as if that personwere a client or customer). Periodic round-robincomparisons with similar calibration facilities alsoserve as useful checks on measurement quality,and these interactions also encourage communica-tion of common problems and successful solutions.

4.5 REVIEW AND APPROVAL OF RECORDS

It is essential thatmeasurement records be subjectto rigorous quality-assurance procedures and, whenrequired, are compatible with national regulationsand national data record-keeping requirements. Thestandard quality-assurance procedures of checkingrecords for completeness, conformity, document con-trol, duration of storage, safety, and back-up shouldbe applied. Procedures must be clearly defined inorder to be checked and approved by supervisorystaff, and the procedure with themost current date istaken as the official version.

A record is a document that states results thathave been achieved or provides evidence of activitiesthat have been performed. A record does not undergothe periodic review and updating activities usedfor procedures. However, it is subject to additionalspecial requirements to ensure correctness. A pro-cedure should be established to control technicalrecords and quality records so that their identifi-cation, archiving, storage, and eventual disposalafter specified retention times are assured. Qualityrecords include reports from internal and externalaudits along with management reviews and docu-mentation of corrective and preventive actions.Records are to be kept legible, secure, and held inconfidence.

Usually, records shall not be changed. If a recordis subsequently found to have been inaccurate,a clearly identified corrected record is issuedwhile retaining the original that has been markedas incorrect. The correction should not make theoriginal data illegible or result in its deletion. It isadvisable that an identification of the person respons-ible for the correction be placed alongside the correc-tion. This also applies to records stored electronically.Configuration-control software can be used to makechanges in a record without erasing the originalentry, and electronic notes can be entered to identifythe person who has made the correction.

Technical records include calibration reports, staffrecords and copies of issued reports, original obser-vations, derived data, and sufficient informationto establish an audit trail. The records retainedshould be sufficient to facilitate the identificationof factors affecting the uncertainty and to enablethe test to be repeated under conditions as close aspossible to the original test. Therefore, all the dataneeded to reconstruct the calculations to achievethe result of the measurement should be systemat-ically archived and kept for a predetermined timeperiod in a secure location. The records shouldinclude the identification of persons responsible forsampling, performing each test or calibration, andchecking results. Record keeping should fulfill therequirements for good practices of data management(ANSI/HPS, 1999). The main principles relating todata quality are that data are processed correctlyin an unbiased manner, are collected for specificpurposes, are adequate to fulfill these purposes, andare kept up to date.

4.6 QUALITY AUDIT AND QUALITYSYSTEM REVIEW

Properly conducted internal and external auditsand reviews are an essential part of any qualitysystem. Their purpose is to check the effectiveness ofthe quality assurance system, which the proceduresare clearly documented and are being followed, andto identify areas for improvement. As indicated inFigure 2.1, even in the early stages of developing thequality system an internal audit should be per-formed to ensure that the procedures developed foruse in the laboratory are understood and are beingfollowed by the staff.

Quality audits shall be carried out regularly toensure that the quality system as detailed in thedocumentation is fully implemented as described.Quality audits might need to be carried out followingthe investigation of an error. Internal audits shouldbe carried out reasonably frequently or at leastannually. External audits can be less frequent.

Reviews are surveys of the quality system todetermine if the system as described meets thequality-assurance policy requirements. Reviewswould normally be carried out annually and give anopportunity to assess the overall effectiveness of thequality assurance system and to make modificationsand revisions.

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