Chemistry

12.3 - Evaluating & Improving a Method

3 min

note 1 of 2

Sources of error and improvements

A source of error is something in the method that makes a reading less accurate.
An improvement must directly target and reduce that specific error.

Common errors and their fixes

Source of error	Improvement that fixes it
Heat lost to the surroundings in a temperature-change experiment	Insulate around the cup, fit a lid on top, stir the mixture, and keep it away from draughts
Parallax error when reading a scale (e.g. on a gas syringe or measuring cylinder)	Put your eye level with the scale and read it straight on
Judging a titration end point colour change by eye	Add the last few cm³ drop by drop while swirling constantly, and stand the flask on a white tile

note 2 of 2

Anomalies and repeats

An anomalous result is a result that doesn't fit the pattern of the others.
When calculating a mean, identify any anomalous results and exclude them so they don't skew the average.

Why repeat readings and calculate a mean?

It helps you identify anomalous results.
It reduces the effect of random errors, so the mean is likely to be closer to the true value.

Representative samples

A representative sample is used when the material being tested varies across different locations, times, or batches.
For example, test water samples taken from several points along a river, or test multiple tablets from across a batch of medicine.

Concordant titres

Concordant titres are results that agree closely with each other, usually within 0.10 cm³.
Only the mean of the concordant titres is used in titration calculations; any non-concordant titres are left out.

key terms

TermDefinition

Source of errorSomething in the method or equipment that makes measurements differ from the true value.

Anomalous resultA result that does not fit the pattern of the other results.

Representative sampleA sample made up of portions taken from different places, times, or batches, so that it shows what the whole material is like, including how it varies.

ConcordantDescribes titration results (titres) that agree closely with each other, usually to within 0.10 cm³ to 0.20 cm³.

end of notes

Key points to remember

Sources of error and improvements

A source of error is something in the method that makes a reading less accurate.

An improvement must directly target and reduce that specific error.

Common errors and their fixes

Source of error	Improvement that fixes it
Heat lost to the surroundings in a temperature-change experiment	Insulate around the cup, fit a lid on top, stir the mixture, and keep it away from draughts
Parallax error when reading a scale (e.g. on a gas syringe or measuring cylinder)	Put your eye level with the scale and read it straight on
Judging a titration end point colour change by eye	Add the last few cm³ drop by drop while swirling constantly, and stand the flask on a white tile

Anomalies and repeats

An anomalous result is a result that doesn't fit the pattern of the others.

When calculating a mean, identify any anomalous results and exclude them so they don't skew the average.

Why repeat readings and calculate a mean?

It helps you identify anomalous results.

It reduces the effect of random errors, so the mean is likely to be closer to the true value.

Representative samples

A representative sample is used when the material being tested varies across different locations, times, or batches.

For example, test water samples taken from several points along a river, or test multiple tablets from across a batch of medicine.

Concordant titres

Concordant titres are results that agree closely with each other, usually within 0.10 cm³.

Only the mean of the concordant titres is used in titration calculations; any non-concordant titres are left out.