In statistics Cronbach's Alpha is used as a measure of internal consistency. This measures how closely items in a set are related. If items in a test seem to be closely related this means that the items are measuring the same thing and are consistent with each other. This is important because if the items are not consistent it cannot be said that they are measuring the same underlying concept. This will make the test results invalid. The closer the items are related the higher the Cronbach's alpha will be.