ISKRIT / Laboratory & metrology / QC · Shewhart charts
Laboratory & metrology
Internal laboratory quality control and Shewhart control charts for accredited laboratories
Shewhart control charts with zones, both checked against the instability criteria, freezes limits from a baseline period, computes repeatability with outlier screening and an uncertainty budget. Built for internal laboratory quality control.
Data current as of 2026-08-24
Standards
Charts per ISO 8258, repeatability and trueness per ISO 5725 parts 2 and 6, coverage factor per the GUM. The chart factors A2, A3, B3, B4, D3, D4, d2 and c4 are the ISO 8258 table 2 values.
Input
Result series labelled with a control level, pairs of duplicate determinations, or control against a reference material.
Output
Charts with zones, verdict on the criteria, a level-by-level control summary, repeatability, uncertainty, report.
What it does
- X̄-R, X̄-S, X-mR charts and control against a reference material
- Both charts checked: eight criteria and the out-of-limit rule
- Several control levels per method: every concentration gets its own chart and its own limit
- Limits frozen from a baseline period, method requirements
- Repeatability with outlier screening and an uncertainty budget
The tool and the platform
The tool
- Shewhart charts with zones, a verdict, repeatability and uncertainty
- Load series from Excel, export to XLSX, printable control report
- Methods and series are stored on the server and reopen next time
- Works instantly, no sign-up required
ISKRIT·Lab made to order
This platform does not exist yet: we build it around your workflow. Here is what goes into it.
- Sample and method journals of the laboratory with roles and history
- A yearly quality-control plan with completion marks and reminders
- Charts are built from the laboratory result journal
- Linked to the instrument fleet: a chart relies on a calibrated device
Questions
Are both charts checked?
Yes. The means chart against all eight criteria, the range chart against the out-of-limit rule, because zone criteria do not apply to the skewed distribution of the range.
Can limits be frozen from a baseline period?
Yes, limits are computed from the first N stable series and later points are checked against them.
Is repeatability calculated?
Yes, s_r iteratively with screening of pairs above the limit r = 2.77·s_r, with a verdict for each pair.
How is a method controlled at several concentrations?
Every series carries a control level. The chart, its limits, the baseline period, the σ_r requirement and the repeatability are computed for each level on its own, and a summary shows the verdict for all levels at once.