Quality Control Procedures, Documentation and Interpretation: Basic Terms

Acceptable Standard Deviation

  • A smaller SD represents data where the results are very close in value to the mean. The larger the SD the more variance in the results.
  • Data points in a normal distribution are more likely to fall closer to the mean. In fact, 68% of all data points will be within ±1SD from the mean. 99% of all data points will be within ±3SD.
  • To state it briefly, statisticians have determined that values no greater than plus or minus 2 SD represent measurements that are more closely near the true value than those that fall in the area greater than ± 2SD. Thus, most QC programs call for action should data routinely fall outside of the ±2SD range.

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