In an ideal world, each time a measurement is taken it will be the same as the true value and so be completely accurate.

In the real world, measurements are not exactly the same as the true value and the difference between the measured and true values is called error.

There are several types of error, but the one which affects precision is called random error. Random error is cause by factors outside of our control which cause repeat readings to vary.

An example of this is measuring the mass of an object using a top pan balance. If each repeat reading is slightly different then there is a random error in the top pan balance, but you cannot control this.

Below is an example of three readings of the same mass, but the reading on the top pan balance has varied slightly with each repeat:

This variation between repeat readings is due to random error in the top pan balance.