Background

Test validity versus reliability

When selecting exercise tests, we want the test to be valid and reliable. It is possible for tests to be one but not the other. If a test is valid, it measures what we want it to measure. If a test is reliable, the results are consistent and stable.

For example, someone could decide they want to measure relative body fat to estimate people’s fitness. They could use an established method to measure relative body fat, repeat the test three times, and produce results of 18.1%, 17.9%, and 18.1%. These results have little variability, and for a measurement like body fat we would consider these to be reliable results—these measures are consistent. In addition, people who are fit generally have lower relative body fat than people who are not fit. Therefore, there is a relationship between relative body fat and fitness.

However, measuring someone’s relative body fat does not provide us with information to determine the fitness level of the individual. We all probably know someone who meets exercise recommendations, but has higher relative body fat than someone else who is thin and does not exercise very much. This means the example of using relative body composition is a reliable measurement (the results were consistent and stable), but it is not a valid test to determine fitness. However, it would be a valid (and reliable!) test of body composition—and indeed we do use relative body fat as a measure of body composition.

relative versus absolute

You probably noticed in the text above the term “relative body fat”, as opposed to the shorter “body fat”. There is a distinction between a measurement that is relative or absolute, and especially in exercise science it can make a difference in interpreting results and prescribing an exercise training plan. When we are using a test or prescription that is in reference to some other physiology this is a relative measurement. In the example above, 18.1% body fat is relative to the whole person (100%). If we know the person’s body weight, we can also express body composition in absolute terms. If the person who is 18.1% had a body weight of 80kg, then they have 14.48kg of body fat. The 14.48kg measure is an absolute term—it is not relative to anything else. In exercise prescription, we frequent use relative load guidelines. For example, prescribing someone to walk on a treadmill at 50% of heart rate maximum.

Class activity

Activity 1: Validity, reliability, relative, and absolute

Equipment: Two items provided by the instructor (items will vary by group)

Participant: One person in your group will be measured

Instructions: You will receive two items from your instructor, and will be instructed to measure something. Use each item to measure the something three times. Record your values in absolute or relative terms (you decide as a group which is best). Once all groups have completed the activity, the instructor will lead a discussion on the measures.

Activity 2: Calibrate treadmill

Equipment: Treadmill, measuring tape, chalk

1. Measure the length of the entire belt (not just the length on the top of the deck!)
2. Turn the treadmill on a low speed
3. Time how long it takes for the treadmill to complete 10 revolutions
4. Noticeably increase the speed of the treadmill
5. Time how long it takes for the treadmill to complete 10 revolutions at this second speed
6. Calculate the speed of the treadmill (see equation below)
7. Compare the calculated speed with the programmed speed of the treadmill

To calculate the speed of the treadmill:

The distance of the TM belt should be IN METERS. Multiply distance in meters*number of revolutions, then divide this number by the time in SECONDS. This will give you the speed in m/s, but the TM is in mph. To covert m/s to mph, multiple your number by 2.23694

(distance in meters * # of revolutions) / time in sec = (TM speed in m/s)

(TM speed in m/s)* (2.23694) = TM speed in mph