
PROFICIENCY TESTName:
describes a methodology for performing either a onesample or a twosample proficiency test. Proficiency testing is the use of interlaboratory comparisons for the determination of laboratory testing or measurement performance. The methods in the E 2489 06 standard provide direction for assessing and categorizing the performances of individual laboratories based on the relative likelihood of occurence of their test results. The standard recommends a minimum of 10 laboratories and states that it is desirable to have 30 or more participating laboratories. Proficiency testing programs typically have a larger number of participants than an interlaboratory test. This results in a wider variation of test conditions, so a proficiency test can provide more information regarding the precision of test results that may be expected when a test method is used in the general testing community. In this standard, the median is used as the consensus value. The measure of variablility is the interquartile range. In this standard, the interquartile range is defined as the difference between the upper hinge and the lower hinge (this is slightly different than the standard definition of the interquartile range as the difference between the 75th percentile and the 25th percentile). The lower hinge is the median of the points less than or equal to the median and the upper hinge is the median of the points greater than or equal to the median. The inner fence is the value equal to the upper (lower) hinge of the data set plus (upper) or minus (lower) 1.5 times the interquartile range. The outer fence is the value equal to the upper (lower) hinge of the data set plus (upper) or minus (lower) 3.0 times the interquartile range. A test result that is between the lower inner fence and upper inner fence is labeled as "typical". A test result that is between the inner and outer fence values is labeled as "unusual". A test result that is beyond the outer fence values is labeled as "extremely unusual". These statistics are used because they are both simple and robust. Note that the above values are used in generating a box plot.
For a onesample proficiency analysis, each lab reports a single test result. This E 2489 onesample proficiency analysis generates the three tables documented in the above document:
In addition to the tables, the standard also recommends complementing the tables with a dot plot. These are also known as dot diagrams or strip plots. In Dataplot, these are referred to as strip plots. Enter the command HELP STRIP PLOT for details on generating these plots in Dataplot. The first program example below demonstrates this plot. A strip plot is an alternative to a histogram for displaying univariate data. The xaxis contains the value of the test result and the yaxis is simply a constant value. If two or more test results have the same value, the points are stacked vertically. You can draw the points are drawn as filled circles. Alternatively, you can draw the points drawn as the labid (this is useful for identifying outlying labs). You can also generate the strip plot with the data divided into bins (you can specify the bin width and the starting and ending bin limits). In this form, the vertical axis will represent the number of occurences. This form of the strip plot is essentially a histogram. Although the E 2489  06 standard does not explicitly talk about box plots, these can also be a useful complement to the tables since the box plot is a graphical representation of table 2.
For a twosample proficiency analysis, each lab reports exactly two test results (i.e., a single measurement on each sample). The random error quantity is defined as
where X and Y denote the test results for sample one and sample two, respectively, and X_{med} and Y_{med} denote the medians of sample one and sample two, respectively. This E 2489 twosample proficiency analysis generates the three tables documented in the above document:
The standard also recommends complementing the tables with a Youden plot. This is demonstrated in the second program example below.
<SUBSET/EXCEPT/FOR qualification> where <y> is a response variable; <labid> is a lab id variable; and where the <SUBSET/EXCEPT/FOR qualification> is optional.
<SUBSET/EXCEPT/FOR qualification> where <y1> is the first response variable; <y2> is the second response variable; <labid> is a lab id variable; and where the <SUBSET/EXCEPT/FOR qualification> is optional.
TWO SAMPLE PROFICIENCY TEST Y1 Y2 LABID
. . Read the data . SKIP 25 READ E2489A.DAT LABID Y . . Generate the tables to the screen . ONE SAMPLE PROFICIENCY TEST Y LABID . . Now generate the tables in RTF format (for import into Word) . CAPTURE RTF ONE.RTF ONE SAMPLE PROFICIENCY TEST Y LABID END OF CAPTURE . . Generate the strip plot for the raw (unbinned) data . LABEL CASE ASIS TITLE CASE ASIS Y1LABEL Number of Occurences X1LABEL Data Values TITLE Dot Diagram for Original Data Y1TIC MARKS OFF Y1TIC MARK LABELS OFF Y2FRAME OFF X2FRAME OFF YLIMITS 0 2 MAJOR YTIC MARK NUMBER 3 MINOR YTIC MARK NUMBER 0 SET STRIP PLOT STYLE STACK SET STRIP PLOT INCREMENT 0.1 LET STRING SYMBL = CIRCLE IF SYMBL = CIRCLE CHARACTER CIRCLE ALL CHARACTER FILL ON ALL CHARACTER HW 1 0.75 ALL END OF IF IF SYMBL = LABID CHARACTER AUTOMATIC LABIDSRT END OF IF LINE BLANK ALL . STRIP PLOT Y . Y1TIC MARKS ON Y1TIC MARK LABELS ON . . Generate the strip plot for the binned data . CLASS LOWER 0.5 CLASS UPPER 5 CLASS WIDTH 0.1 FRAME CORNER COORDINATES 15 40 85 70 Y1LABEL Number of Occurences X1LABEL Data Values TITLE Dot Diagram for Binned Data Y2FRAME OFF X2FRAME OFF LET MAXFREQ = MAXIMUM Y2 LET NUMTIC = MAXFREQ + 1 YLIMITS 0 MAXFREQ MAJOR YTIC MARK NUMBER NUMTIC MINOR YTIC MARK NUMBER 0 Y1TIC OFFSET 1 1 SET STRIP PLOT STYLE STACK SET STRIP PLOT INCREMENT 1 CHARACTER CIRCLE ALL CHARACTER FILL ON ALL CHARACTER HW 1 0.75 ALL LINE BLANK ALL . STRIP PLOT Y2 X2 . FRAME CORNER COORDINATES CHAR FILL OFF ALL CHAR BLANK ALL CHAR HW LINE SOLID ALL LIMITS FRAME ON MAJOR TIC MARK NUMBER MINOR TIC MARK NUMBER . . Now generate a box plot . Y1LABEL Test Results X1LABEL TITLE Box Plot for Proficiency Data XLIMITS 0 2 XTIC MARKS OFF XTIC MARK LABELS OFF CHARACTER BOX PLOT LINE BOX PLOT FENCES ON . BOX PLOT Y XLIMITS X1TIC MARKS ON X1TIC MARK LABELS ONView the output from this command. Program 2: . . Read the data . SKIP 25 READ E2489B.DAT Y1 Y2 LABID . . Generate the tables to the screen . TWO SAMPLE PROFICIENCY TEST Y1 Y2 LABID . . Now generate the tables in RTF format (for import into Word) . CAPTURE RTF TWO.RTF TWO SAMPLE PROFICIENCY TEST Y1 Y2 LABID END OF CAPTURE . . Generate the Youden plot . LET STRING SYMBL = LABID IF SYMBL = CIRCLE CHARACTER CIRCLE ALL CHARACTER FILL ON ALL CHARACTER HW 1 0.75 ALL END OF IF IF SYMBL = BOX CHARACTER BOX ALL CHARACTER FILL ON ALL CHARACTER HW 1 0.75 ALL END OF IF IF SYMBL = DIAMOND CHARACTER DIAMOND ALL CHARACTER FILL ON ALL CHARACTER HW 1 0.75 ALL END OF IF IF SYMBL = LABID LET LABIDSRT = LABID LET YSORT = SORTC Y LABIDSRT CHARACTER HW 2 1.50 ALL CHARACTER AUTOMATIC LABIDSRT END OF IF LINE BLANK ALL . LABEL CASE ASIS TITLE CASE ASIS TITLE OFFSET 2 X1LABEL Test Results for Sample One Y1LABEL Test Results for Sample Two TITLE Youden Plot of Test Results . LIMITS 0 5 TIC OFFSET UNITS DATA TIC MARK OFFSET 0 0.5 . YOUDEN PLOT Y2 Y1 LABID . PROBE FX1MIN LET AX1 = PROBEVAL PROBE FX1MAX LET AX2 = PROBEVAL PROBE FY1MIN LET AY1 = PROBEVAL PROBE FY1MAX LET AY2 = PROBEVAL . LINE SOLID DRAWDATA AX1 MEDX AX2 MEDX DRAWDATA MEDY AY1 MEDY AY2 LINE DASH DRAWDATA AX1 AY1 AX2 AY2 LINE SOLIDView the output from this command.
Date created: 1/14/2009 