Measurement is an important part of the scientific process. The key aspects concerning the quality of scientific measures are reliability and validity.
Reliability is a measure of the internal consistency and stability of a measuring device.
Validity gives us an indication of whether the measuring device measures what it claims to.
Internal consistency is the degree in which the items or questions on the measure consistently assess the same construct. Each question should be aimed at measuring the same thing. Internal consistency is often measured using Cronbach’s Alpha — a super-correlation of all the items on the scale. If the score is .70 or higher the measurement is acceptable. However, .80 or higher is preferable. It is also important to consider the context when considering the score that reflects internal consistency.