See how reliability affects the trust you can place in individual scores

To see why psychologists don't like to put a great deal of weight on the exact score that a person gets on a test, play around with the different test reliabilities and confidence intervals.

• Note how big the range has to be if your reliability is .70 and you want to say that 99% of the time, the person's true score will be in that range.
• Changing the standard deviation will also affect the range.
• IQ tests tend to have a mean of around 100, a standard deviation of around 15, and reliabilities between .85 and .95.
  Personality tests tend to have lower reliabilities than that.
  For example, the scales for the Minnesota Multiphasic Personality Inventory (MMPI) have test-retest reliability coefficients that range from 0.50 to 0.80.