Wastewater Lab · Reading A QC Control Chart
How Do You Read a QC Control Chart?
Plot the result, read the pattern, and use the laboratory's documented chart rules before making a QC decision.
Read The Whole Pattern
A control chart places repeated QC results in time order. The center line represents the established center of the chart, while warning or action lines show where the laboratory's documented rules call for closer review or a response. The chart does not replace the method or SOP; it makes changes in performance easier to see.
Start With The Current Point
Confirm that the new point belongs on this chart: the correct test, QC check, units, method, and chart version. Plot it in the correct sequence and compare it with the applicable lines. A point that appears normal on the wrong chart provides false reassurance.
Review The Recent Trend
After plotting the new result, compare it with the recent points on the chart. Look for a steady move in one direction, several points grouped on one side of the center line, a sudden shift, or results spreading farther apart. A developing pattern can deserve attention even when the newest point remains inside an action line. Use only the pattern rules established by the laboratory; do not invent one rule for every chart.
Do Not Rebuild The Chart Around A Bad Result
Do not erase a point, change a date, or recalculate the limits simply because the chart looks uncomfortable. Keep the original QC result and any associated sample results traceable. New limits require the documented basis and approval described by the laboratory's quality system.
Connect The Pattern To The Work
A chart can show that performance changed; it does not automatically identify why. Review the QC preparation, reagents or standards used, equipment records, analyst notes, method steps, and recent changes that apply. Follow the laboratory response before reporting affected results or repeating work.
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