Interval Variables are measured on a scale such that a one unit change represents the same difference throughout the scale. Examples include IQ or SAT scores. Interval variables do not have true zero points. Ratio Variables have the same property as interval variables, plus the additional property of a true zero. Because of this, proportions or ratios between values are meaningful measures.
Examples are age, education in years, or the number of hours of television watched each day.
For most analytical purposes, interval and ratio variables can be considered identical, yielding three types of measurement scales. For each type of variable a particular measure of central tendency is most appropriate. By central tendency we mean one value that most effectively summarizes a variable's complete distribution. The value that appears most often in a distribution. The arithmetic average of a distribution.