Validity measurement is important, because it measures the target behavior, measures the duration of the target behavior and any questions that are directed towards that behavior. Validity also makes sure the data was tracked at appropriate times and are relevant to the target behavior(Cooper, Heron & Heward, 2007, p. 103). If none of these elements add up, then the resulted data is compromised, therefore the data is meaningless.
Reliability measurements are based on measurements being consistent to one another. Basically saying measurements should yield the same value when the events are similar to one another. Reliability is consistent measurements(Cooper,