Measurements and Errors (Lecture 4) Flashcards
What does accuracy describe?
how close a measured value is to the “true” value
What does precision describe?
the reproducibility of a result
How can you demonstrate accuracy?
Analyse a certified reference material under identical conditions to your experimental sample. You will find the value of the reference material within the precision of your method.
Compare results from two or more different analytical methods. They should agree within their expected precision.
Analyse a blank sample spiked with a known addition of analyte. E.g. standard additions.
What does repeatability describe?
the spread in results when one person uses one procedure to analyse the same sample by the same method multiple times
What does reproducibility describe?
the spread in the results when different people in different labs using different instruments each try to follow the same procedure
What is error?
the difference between the true and the measured values
What is systematic (or determinate) error?
It arises from a flaw in equipment or the design of an experiment. It is reproducible if you conduct the experiment in exactly the same way again.
What is random (or indeterminate) error?
It arises from uncontrolled (and maybe uncontrollable) variables in the measurement.
What is gross error (blunder)?
They are extreme instances of random or systematic error, due to accidental but significant departures from procedure.
What does absolute uncertainty express?
the margin of uncertainty associated with a measurement
What do we use to calculate the errors in an addition or subtraction?
absolute uncertainty
What do we use to calculate the errors in a multiplication or division?
percentage relative uncertainty
