Method of Evaluation
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Known as the foundation for monitoring performance
Quality Control
Sources of Analytic Variability
Operator technique Instrument differences Test accessories Contamination
Measures of Center
Mean Median Mode
Obtained through dividing the sum of the data points (Σ) by the total number of data points (n)
Mean
Rarely used, but often used to describe data that seemed to have two centers (bimodal)
Mode
Measures of Spread
Range Standard Deviation Coefficient of Variation
The difference between the largest value and smallest value in a dataset
Range
Represents the “average” distance from the center of the data (the mean) and every value in the dataset
Standard Deviation
Most frequently used measure of variation
Standard Deviation
Compares SDs with different units
Coefficient of Variation
Bell-shaped curve/Normal distribution curve/Normal frequency curve
Gaussian Curve
Describe the 68–95–99 Rule of Gaussian distribution
≈68% of the data fall between ±1 SD from the mean ≈95% of the data fall between±2 SDs from the mean ≈99% fall between ±3 SDs from the mean
Describe as the difference between test and reference method results
Errors
Type of error that is present in all measurements and can be either positive or negative
Random Error
Error that occur due to chance
Random Error
Factors contributing to random error
Instrument Operator Reagent Environmental variations (pipetting error, mislabeling in samples, temperature of analyzer, and improper mixing of sample or reagent)
Error that can influence the observations consistently in one direction
Systemic Error
Corrective measures for systemic errors
Check on equipment/ machines Check reagents Check quality control materials Check standard solutions Review standard operating procedures
Factors contributing to systemic error
Calibration problems Deterioration of reagents and control materials Improperly made standard solutions Contaminated solutions Unstable and inadequate reagent blanks Leaky ion selective electrode Poorly written procedures.
Error that exists when there is a continual difference between the test method and the comparative method values, regardless of the concentration
Constant Systematic Error
Cause of systematic constant error
Interference or contamination
Error that exists when the differences between the test method and the comparative method values are proportional to the analyte concentration
Proportional Error
Cause of proportional error
Poor recovery of analyte during an analysis