Errors in Measurement

  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

Examples:

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

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

When the value could be between 7 and 9

8 ±1

The error is ±1

 

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 Error
Measured 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

So:

Absolute Error = 0.05 m

And:

Relative Error =   0.05 m   = 0.004
12.5 m

And:

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°

So:

Absolute Error = 1°

And:

Relative Error =     = 0.0263...
38°

And:

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

So:

Absolute Error = 0.5 cm

And:

Relative Error =   0.5 cm   = 0.00625
80 cm

And:

Percentage Error = 0.625% 

 

Area

When working out areas you need to think about both the width and length ... they could both be the smallest possible 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.

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 for Areas

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

  • From 41.25 to 48 = 6.75
  • From 48 to 55.25 = 7.25

Answer: pick the biggest one! So:

Absolute Error = 7.25 m2

Relative Error =   7.25 m2   = 0.151...
48 m2

Percentage Error = 15.1% 

(Which is not very accurate, is it?)

Volume

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

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:

  • 24 ±1 cm
  • 24 ±1 cm
  • 20 ±1 cm

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

And so we get:

10051 ≤ V < 13125

 

Absolute, Relative and Percentage Error for Volumes

Absolute error:

  • From 10051 to 11520 = 1469
  • From 11520 to 13125 = 1605

Pick the biggest one:

Absolute Error = 1605 cm3

Relative Error =   1605 cm3   = 0.139...
11520 cm3

Percentage Error = 13.9%