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Practical Experiment Report Writing

Accuracy and Precision

As with many ideas in science accuracy and precision often cause problems not because they are difficult to understand but because they have 'everyday' and 'scientific' meanings.

You need to have a scientific understanding of the two terms if you are to gain maximum credit for your work.

Accuracy

This refers to results that are close to the 'actual value' - a value that is obtained by using several sets of equipment and is performed by several groups of people and methods.

Precision

This refers to the spread of repeated results in an experiment. We say that the random error is very small if the repeated results are close to each other

Does one imply the other?

Simply put the answer to that must be 'no' since data can be:-

accurate and precise

inaccurate and precise

accurate and imprecise

inaccurate and imprecise

e.g. 1. Measuring the diameter of a sphere to be used in a terminal velocity investigation.

If the true value of the diameter is 1.00 cm and you use a micrometer reading to 0.0005 cm then the precision would be ± 0.05%.

However if the micrometer has a large zero error then the data would be inaccurate.

e.g. 2. If you measure the diameter of the same sphere but this time use a metre rule
calibrated in mm you could estimate to the nearest 0.5 mm which gives a precision of ± 5%.

However you are likely to have a mean value which is very close to the actual value which makes the data accurate.