The correct answer is: C.

To answer the question, use the TAK method for breaking down the information.

T -- What is this question telling me? Mean is 80, Standard Error of Measurement (SEM) is 5, Score is 95

A – What is this question asking me? Determine the 95% confidence interval.

K – Ask yourself, “what information do I already know about calculating a confidence interval?”

To calculate a confidence interval, it may help to draw the normal curve and plot the individual’s true score (95) in the middle of the curve (see graphic below). Once you have this, add and subtract the appropriate number of SEMs to the curve. Remember, there is a 68% chance a score will fall between +1 and -1 SEM of the person’s true score. There is a 95% chance a score will fall between +2 and -2 SEMs of the individual’s true score. There is a 99% chance the individual’s score will fall between +3 and -3 SEM’s of the person’s true score. This information doesn’t change, so once you remember this, all you need to do is plug in the numbers from the question!

You’re looking for the 95% CI, so you will add 10 to and subtract 10 from the true score (95). Where did we get 10? It’s 2 SEMs (adding 5 twice and subtracting 5 twice). If we were looking for the 99% CI, it would be 15 (3 SEMs) and 68% would be 5 (1 SEM).

As a result, we are 95% confident that the individual’s true score falls between 85 and 105.

NOTE: In this scenario, we do not need the mean to calculate a confidence interval, so the mean of 80 is extra information.

EPPP Practice Question!