When a scientist repeats an experiment with a different group of people or a different batch of the same chemicals and gets very similar results then those results are said to be reliable. Reliability is measured by a percentage – if you get exactly the same results every time then they are 100% reliable.

Try holding a ruler above a friend’s open hand and dropping it – they have to catch the ruler but may not move until they see the ruler start to move. Note down the measurement where the ruler was caught. Do this ten times and calculate the mean (average) result.

Is the ‘dropping a ruler’ experiment a reliable measure of reaction time?


Validity describes whether the results of an experiment really do measure the concept being tested. Does seeing how far a ruler can drop through someone’s hand really measure reaction time? What other variables may be influencing the results?

Is the ‘dropping a ruler’ experiment a valid measure of reaction time?


Accuracy describes how well a measuring instrument determines the variable it is measuring. It can be employed in two ways

An accurate measuring instrument, say a thermometer, is one whose readings confirm a known result.The level of accuracy of a measuring instrument determines the detail to which it can measure. A micrometer measures length to a greater level of accuracy than a ruler which in turn measures length to a greater level of accuracy than a ‘clicker’ wheel.

In order to be accurate in their work scientists need to first select a measuring instrument that allows an appropriate measure of accuracy eg a micrometer for the diameter of a piece of wire and a ruler marked in mm for its length and then to calibrate it. Calibrating an instrument involves measuring already known quantities too assess how accurately it is working.

Is the ‘dropping a ruler’ experiment an accurate measure of reaction time?