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Accuracy is not the same thing as precision. Accuracy is about measurements being as close to the true value as possible. Precision is the measurement being the same value each time the experiment is performed. A measurement system can be accurate but not reliable (precise) or vise versa.

Precision is written as a percentage. It tells you how precisely you can use a piece of equipment to take a reading. It is calculated by subtracting the maximum percentage error from 100%.

Take for example a balance that is set to read in grams. If you take a reading of 10 grams, it could in fact be anywhere between 9.5 and 10.5 grams. That is one gram difference on the reading of 10 grams - that is a 10% error so in this case, the precision of the reading would be 100% minus 10% i.e. **90% precision**.

If the balance is now set to read in tenths of a gram, a reading of 10 grams could be as low as 9.9 grams or as high as 10.1 grams. The error could be plus or minus 0.1 grams. Now this is only a total of two hundredths (i.e. 2%) of the 10 gram reading so the precision is now 100% minus 2% or 98% precision.

Precision is also applied in a similar way to a set of results or data. You first work out the mean value. Then you take the difference between the lowest and highest result and calculate it as a percentage of the mean value. Subtract that from 100% to find the precision of the experiment.

Precise measurements are ones in which there is very little spread about the mean value. Precision depends only on the extent of random errors which is why it gives no indication of how close the results are to the true value and is not a measure of accuracy.

Is it precise results or accurate ones which can be repeated? Try this quiz see how well you understand the precision of data.

1.

Another word for reliable.

Precise

Accurate

Anomalous

Unreasonable

Precision and reliability are the same thing in terms of scientific experiments

2.

Precision means that the measurements are...

anomalous

unreliable

repeatable

inaccurate

It is measured as a percentage

3.

Resolution of a measuring device is...

the largest change giving a result

the smallest change possible which can give a result

any change leading to a result

the best result possible

Resolution of the human eye is the minimum distance of two dots so the eye can see them as two dots. For most people this is 0.2mm

4.

If there is a systematic error within the experiment, this will always be a problem.

Precision

Reliability

Factuality

Accuracy

Errors such as poor measuring techniques cannot be improved simply by repetition. The measurements will always lack accuracy

5.

Precise data will always have a value very close to the...

mean

range

anomaly

edge

This does not necessarily indicate accuracy, it simply means that the method of an experiment produces repeatable results

6.

Increasing the sample size can increase this.

Accuracy

Results

Validity

Precision

Repeating an experiment many times is a technique used to improve precision

7.

Unreliable data will have a large...

mean

data

range

value

A large range gives a lower percentage for the precision

8.

Reliable results are just like reliable people, they are always...

at the same time

pleasant

the same

anomalous

Not quite the same but very consistent

9.

Accuracy is...

the same as reliable

the same as precision

unreliable

close to the true value

Results can be of high precision but low accuracy

10.

When measurements are both accurate and reliable they are described as...

invalid

valid

correct

incorrect

In the evaluation of your investigations you are expected to say how precise, accurate and valid you think that your results are

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