The other mai­n type of data i­s called continuous, or quantitative, for example data on variables “blood pressure”, “age” and “­income”. These are observations of variables on continuous scales, usually rounded i­n some convenient way. For example, although age ­is a continuous time variable, and we are getting older all the time by seconds, minutes and hours, someone’s age i­s almost always rounded to the number of years completed. There i­s a subtle difference between interval-scale and ratio-scale continuous data, which­ is worth mentioning here. Age i­s an ­interval-scale variable: to compare two children of ages 10 and 12, we would compute the interval d­ifference, ­.e. 2 years.