Table A13: Extracted precision data [This method assumes that samples themselves were replicated at each location, so that the variation measured represents the sum of analytical and sampling variation.]

A

99.30

102.14

100.27

A2

100.70

97.86

99.73

B

93.69

101.69

94.03

B2

106.31

98.31

105.97

C

91.89

93.48

97.56

C2

108.11

106.52

102.44

D

93.33

96.21

93.40

D2

106.67

103.79

106.60

Mean (%)

100

100

100

Standard deviation (%)

6.6

4.3

4.9

95% error (%) [The samples taken are not the site, but they do represent it. Student's t-test 95% error is the best method to establish whether or not we can say that the underlying population mean for the site (that a given sample was collected from) is distinguishable from the guideline value. Normality can be assumed as data here describe variation between the normalised replicates.]

5.5

3.6

4.1

Guideline value (mg/kg)

30

370

300

Lower than guideline value: any value below (mg/kg) ... [Example for arsenic: calculated as 30 mg/kg minus 5.5% of 30 mg/kg.]

28.4

356.7

287.6

Higher than guideline: any value above (mg/kg) ...

31.6

383.3

312.4

Indistinguishable from guideline (mg/kg)

28.4 to 31.6

357 to 383

287 to 312