Lecture 1 (Precision/Accuracy)

How many replicate samples should be made for “reliable results”?

2-5 replicates

Why is it difficult to determine accuracy of results rather than the precision?

Accuracy assessment involves comparison to a true value, which may be unknown or subjective, whereas precision measures the consistency of results across multiple measurements.

What are the 2 general types of errors found in clinical testing?

random (indeterminate) and systemic (determinate)

What can cause systemic errors?

Uncalibrated or improperly calibrated instruments

What is the difference between constant errors and proportional errors?

constant: are INDEPENDENT of the sample size
proportional: INCREASE OR DECREASE in proportion to the sample size

When is a constant error of serious concern and how can it be fixed?

concerns increase as sample size decreases, keep sample size larger to minimize effects

What is the best way to estimate the bias of an analytical method? What is used as an accepted value to determine accuracy?

using standard reference materials (SRM’s)

What is the definition of a blank solution?

a solution that contains all the components (solvent and reagents) of the sample except for the analyte of interest.

What is the definition of a matrix (in clinical chemistry)?

It is all the components of a sample containing an analyte

When is the use of the median preferred over the mean?

When the data contains an outlier (drastically different value from the rest of the data)

What does an outlier have/not have a significant effect on?

significant: the mean
nonsignificant: the median

What is the definition of precision?

The closeness of results that have been produced under the same conditions.

What are the 3 terms associated w/ precision?

standard deviation, variance, and coefficient of variation

What is the definition of accuracy?

The closeness of a measured value to the “true or accepted value” and is expressed by the error

What is the difference between deviation from the mean and absolute error?

deviation from the mean: d=x_i-x_mean
absolute error: E=x_i-x_t where x(t) = true/accepted value

Does “absolute error” in clinical chemistry include/exclude direction of the error? (±)

Includes the direction of the error

What is the equation for relative error and units can it be expressed in?

Absolute error/true value *100
percent, ppt,ppm

What is the advantage of having relative error?

It gives us an idea of how large an error is relative to the true value even if 2 runs have the same absolute error value

For random and systemic errors, which affect precision/accuracy?

random: precision
systemic: accuracy

What are gross errors the product of and what values do they lead to?

product of human errors, lead to outliers

What do systemic errors lead to?

bias (can be either + or -) like a trend of error that affects all the data the same

What are the 3 sources of systematic errors?

instrumental, method, personal

What can help reduce/eliminate systematic and personal errors?

systematic: calibration

personal: automation and predetermined protocols for biased decisions like rounding

What can help reduce/eliminate method errors?

1. using SRM’s

2. conduct 2nd independent method in parallel w/ current method

3. statistical test to deduce if error was random or due to bias

4. spectrophotometry and atomic absorption spectroscopy

In a graph of relative error (y-axis) vs sample mass (x-axis), what would a graph for proportionate/constant error look like?

constant: exponential curve increases as sample mass decreases
proportional: horizontal line