Errors in Measurement

questions   Error?
No ... you didn't measure it wrong ... this is about accuracy.

Measuring instruments are not exact!

Degree of Accuracy

Accuracy depends on the instrument you are measuring with. But as a general rule:

The degree of accuracy is half a unit each side of the unit of measure


When your instrument measures in "1"s
then any value between and is measured as "7"
accuracy 1: 6.5 to 7.5
When your instrument measures in "2"s
then any value between 7 and 9 is measured as "8"
accuracy 2: 7 to 8

Notice that the arrow points to the same spot, but the measured values are different!

Plus or Minus

We can show the error using the "Plus or Minus" sign:   ±

When the value could be between and :

7 ±0.5

The error is ±0.5

accuracy 1: 6.5 to 7.5

When the value could be between 7 and 9:

8 ±1

The error is ±1

accuracy 2: 7 to 8



Example: a fence is measured as 12.5 meters long, accurate to 0.1 of a meter

Accurate to 0.1 m means it could be up to 0.05 m either way:

Length = 12.5 ±0.05 m

So it could really be anywhere between 12.45 m and 12.55 m long.


Absolute, Relative and Percentage Error

The Absolute Error is the difference between the actual and measured value.

But ... when measuring we don't know the actual value! So we use the maximum possible error.

In the example above the Absolute Error is 0.05 m

What happened to the ± ... ? Well, we just want the size (the absolute value) of the difference.

The Relative Error is the Absolute Error divided by the actual measurement.

We don't know the actual measurement, so the best we can do is use the measured value:

Relative Error =  Absolute ErrorMeasured Value

The Percentage Error is the Relative Error shown as a percentage (see Percentage Error).

Let us see them in an example:


Example: fence (continued)

Length = 12.5 ±0.05 m


Absolute Error = 0.05 m


Relative Error =  0.05 m12.5 m  = 0.004


Percentage Error = 0.4% 

More examples:


Example: The thermometer measures to the nearest 2 degrees. The temperature was measured as 38° C

The temperature could be up to 1° either side of 38° (i.e. between 37° and 39°)


Temperature = 38 ±1°


Absolute Error = 1°


Relative Error =  38°  = 0.0263...


Percentage Error = 2.63...% 

Example: You measure the plant to be 80 cm high (to the nearest cm)

This means you could be up to 0.5 cm wrong (the plant could be between 79.5 and 80.5 cm high)


Height = 80 ±0.5 cm


Absolute Error = 0.5 cm


Relative Error =  0.5 cm80 cm  = 0.00625


Percentage Error = 0.625% 



When working out areas you need to think about both the width and length ... they could possibly both be the smallest measure or both the largest.

Example: Alex measured the field to the nearest meter, and got a width of 6 m and a length of 8 m.

Measuring to the nearest meter means the true value could be up to half a meter smaller or larger.

area 6x8 measure error 41.25,48,55.25

The width (w) could be from 5.5m to 6.5m:

5.5 ≤ w < 6.5

The length (l) could be from 7.5m to 8.5m:

7.5 ≤ l < 8.5

The area is width × length:

A = w × l

The smallest possible area is: 5.5m × 7.5m = 41.25 m2
The measured area is: 6m × 8m = 48 m2
And the largest possible area is: 6.5m × 8.5m = 55.25 m2

41.25 ≤ A < 55.25


Absolute, Relative and Percentage Error

The only tricky thing here is ... which is the absolute error?

Answer: pick the biggest one! So:

Absolute Error = 7.25 m2

Relative Error =  7.25 m248 m2  = 0.151...

Percentage Error = 15.1% 

(Which is not very accurate, is it?)


And volume has three measurements: width, length and height!

Each measurement could possibly be the smallest possible measure, or the largest.

Example: Sam measured the box to the nearest 2 cm, and got 24 cm × 24 cm × 20 cm

Measuring to the nearest 2 cm means the true value could be up to 1 cm smaller or larger.

The three measurements are:

Volume is width × length × height:

V = w × l × h

The smallest possible Volume is: 23cm × 23cm × 19cm = 10051 cm3
The measured Volume is: 24cm × 24cm × 20cm = 11520 cm3
The largest possible Volume is: 25cm × 25cm × 21cm = 13125 cm3

volume measure error 10051, 11520,13125

And so we get:

10051 ≤ V < 13125


Absolute, Relative and Percentage Error

Absolute error:

Pick the biggest one:

Absolute Error = 1605 cm3

Relative Error =  1605 cm311520 cm3  = 0.139...

Percentage Error = 13.9%