Glossary

The terms used in Cloud QC and this manual. “How this is calculated” under each chart gives the formulas.

Accuracy: How close results are to the true value. Cloud QC checks it with certified reference materials (CRMs).
Action limit: The line beyond which a result is out of control: 3 standard deviations from the expected value on Shewhart charts. For duplicates, twice the check stage’s precision limit.
Analysis suite: The laboratory’s method or package of analyses a result was reported under. Results from different suites aren’t mixed.
Analyte: The element, method and units of a result, as the laboratory names it, e.g. Cu_ME-ICP41_ppm.
Bias: A steady difference between a CRM’s results and its certified value: the laboratory reads high or low. Usually given in standard deviations or as a percentage.
Blank: Material with almost none of the analyte, sent with the samples. A blank that reads high points to contamination.
Certified value: A CRM’s accepted value, with its certified standard deviation, from the CRM’s certificate.
Check stage: The stage a duplicate was taken at: field, crushing, pulverising and so on. Each stage has its own precision limit.
Contamination: The analyte getting into a sample from equipment or from the sample before it. Cloud QC checks it with blanks.
Control lines: The lines on a chart that a result is judged against, such as the warning and action limits.
CRM (certified reference material): A material of known, certified grade sent with the samples to check accuracy. Company standards are bought or made by the company; laboratory standards are the laboratory’s own.
CUSUM: Cumulative sum chart. It adds up each result’s difference from the target, so a small, steady bias builds up and shows before any single result fails.
Despatch: A batch of samples sent to the laboratory together, under one despatch number.
Duplicate: A second analysis of the same sample, taken at a check stage. The pair’s difference measures precision.
EWMA: Exponentially weighted moving average chart. Each point averages the latest result with the ones before it, recent ones counting most, to show gradual drift.
LLD (lower limit of detection): The lowest value the laboratory’s method can report. Results below it are shown with hollow markers.
Lab Confidence: The Overview’s overall rating of a laboratory: High, Moderate, Low or Insufficient data.
MAD (median absolute deviation): A measure of spread that a few extreme results don’t distort. Cloud QC can draw control lines from it instead of the standard deviation.
Nelson rules: Eight tests for patterns that suggest a process is out of control, such as a run of results on one side of the mean, even when every result is within its limits.
Precision: How well repeat analyses of the same sample agree. Cloud QC checks it with duplicates.
RD (relative difference): The difference between a duplicate pair as a percentage of their size. Cloud QC offers several forms: ARD%, CV%, HARD%, MPRD% and RDHYSLOP.
RD Rolling Median: The median relative difference of the duplicates in each despatch and the ones before it, over a rolling window, to show whether precision is changing.
Shewhart chart: Each CRM result plotted in order against its expected value, with warning and action limits.
Standard deviation (SD): The usual measure of spread around the mean.
Threshold factor: Multiplies the LLD to give a threshold. Duplicate pairs below it are greyed out and left out of the statistics.
Thompson-Howarth: A precision chart that plots how the spread of duplicate pairs changes with grade.
ULD (upper detection limit): The highest value the laboratory’s method can report.
Warning limit: 2 standard deviations from the expected value on Shewhart charts. About one result in twenty falls outside it by chance.
Z-Score: How many standard deviations a result is from the expected value: (result − expected) ÷ SD.