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Quality control is an umbrella of activities that certify a system is under statistical control. It includes quality control charts, proficiency testing, documentation, and archiving. These activities track data, visualize trends, and identify variations that may affect analysis accuracy, ensuring reliable and consistent laboratory results.

Quality control charts plot one analytical characteristic over time, comparing each measured value to the known value of reference material. When a measured value falls outside controlled limits, typically defined as standard deviations from the reference value, an inspection of analysis conditions is required. This enables early detection of deviations affecting accuracy.

Proficiency testing analyzes performance across multiple laboratories, comparing each lab's result to the mean of all results or a certified reference material. The z score characterizes variations from different laboratories, estimating how far individual lab values deviate from the reference value. This standardized metric enables fair comparison of laboratory performance.

Documentation and archiving ensure transparency and traceability in laboratory processes. Every quality control activity, including sample custody, instrument calibration, and results, must be documented with signatures and dates for accountability. Accurate archives allow past data retrieval and assessment, supporting informed decisions about future analyses and identifying quality variations.

When a measured value falls outside the range of controlled limits, an inspection of analysis conditions is required. Modifications to analysis conditions may be necessary to maintain statistical control of the system. This corrective action prevents continued deviations and ensures the analytical system returns to acceptable performance standards.

Quality control activities monitor and verify that analytical characteristics remain within acceptable ranges throughout testing. Proper sample preparation for analysis advanced techniques ensures samples are correctly prepared before analysis, and quality control then tracks whether the prepared samples yield consistent, reliable results within established control limits.

The three cyclical quality assurance activities are quality control, which includes quality control charts and proficiency testing; documentation and archiving of all laboratory activities; and ongoing monitoring to maintain statistical control. Together, these activities create a comprehensive system that ensures analytical reliability, traceability, and continuous improvement in laboratory operations.