What is the most common reason for a control value to fall outside of 3 standard deviations (3SD)?

A control value falling outside of 3 standard deviations (3SD) is predominantly associated with random error. Random errors are inherent variations that occur in measurement processes and can arise from a variety of unpredictable factors, such as fluctuations in the environment or slight variations in technique. These errors do not exhibit a consistent pattern and can lead to occasional results that lie outside the accepted control limits, such as the 3SD threshold, which is set to capture 99.7% of data points for normally distributed processes.

Understanding that random error can cause a control value to deviate unexpectedly helps in identifying the importance of maintaining strict quality control measures in laboratory processes. Regularly assessing control values ensures that any deviations can be promptly investigated, allowing for the maintenance of accuracy and reliability in test results. Such awareness of random error assists technologists in distinguishing between common variations in data and more systematic or significant issues that may require different corrective actions.