Many measures such as weight, age, psychological reaction time or the frequencies of an event are examples of ratio measurement scales. You will learn later that different statistical techniques apply to Nominal, Ordinal, Interval and Ratio data. For example, you can't generate a meaningful mean average from Nominal or Ordinal data. Reliability and Validity Reliability is a measure of how well repeated measurements of the same thing agree with each other. A stopped watch is highly reliable.
Validity is a measure of how well the measure measures what it sets out to measure.
Good measures should be both reliable and valid. We will see later that problems with reliability can, in some cases, be minimized by taking repeated measurements. Problems with validity may necessitate finding a new measuring device altogether. Nominal, Invalid, and Unreliable Comets were categorized as celestial swords by Johannes Hevelius in his Cometographia published in 1668. This categorization is unreliable since it likely that different observers or repeated observations would not agree on cometary classification.