Understanding Errors in Chemical Analyses

mean, arithmetic mean, and average

the quantity obtained by dividing the sum of replicate measurements by the number of measurements in the set

median

is the middle result when replicate data are arranged in order of size

precision

the closeness of results to others that have been obtained in exactly the same way; describes the reproducibility of measurements

accuracy

the closeness of a measurement to the true or accepted value and is expressed by the error; measures agreement between a result and its true value but precision describes the agreement among several measurements

absolute error

the difference between the measured value and the true value; E = xi - x

relative error

the absolute error divided by the true value

random or indeterminate errors

errors that affect the precision of measurement; causes data to be scattered more or less symmetrically around a mean value

systematic or determinate errors

errors that affect the accuracy of a result; this type of error causes the mean of a set of data to differ from the accepted value

gross errors

usually occur only occasionally, are often large, and may cause a result to be either high or low; this error causes the result to differ significantly from the rest of the results

bias

measures the systematic error associated with an analysis; has a definite value, an assignable cause and are about the same magnitude for replicate measurements

instrumental errors

are caused by the imperfections in measuring devices and instabilities in their components

method errors

arise from nonideal chemical or physical behavior of analytical systems

personal errors

results from the carelessness, inattention, or personal limitations of the experimenter

constant errors

error that does not depend on the size of the quantity measured

proportional errors

errors that decrease or increase in proportion to the size of the sample taken for analysis

standard reference materials

are substances sold by the National Institute of Standard and Technology and Certified to contain specified concentrations of one or more analytes

blank determinations

are useful for detecting certain types of constant errors; reveal errors due to interfering contaminants from the reagents and vessels employed in analysis