Levels of Measurement
Definition of Measurement: Measurement consists of rules for assigning numbers to attributes of objects based upon standardized, well-established, recognized rules.
You need to know your variables to avoid wrong calculations. The physical and biological scientists generally have well-established, standardized, systems of measurement. A scientist knows, for example, what is meant when a "fish" is described as 10.23 centimeters long and weighing 34.23 grams.
What calculation are permitted at different levels of measurements?
| Levels of Measurement and Computational O.K. | Nominal | Ordinal | Interval | Ratio |
| frequency distribution | yes | yes | yes | yes |
| median and percentiles | no | yes | yes | yes |
| add or subtract | no | no | yes | yes |
| mean, standard deviation, standard error of the mean | no | no | yes | yes |
| ratio, or coefficient of variation | no | no | no | yes |
Identify your level of measurement first. Find out what calculations are O.K. and what calculations are not O.K. for your variable. For example: In general you do not have permission to calculate mean, standard deviation, standard error of the mean for a Nominal or Ordinal data set.
SPSS - methods