[br][i][quote]The apples I bought cost half of the pears but they were two times tastier.[/quote][/i][br]When studying a statistical data, you MUST be aware of what kind of variables are used. For example, program of a university student is usually marked with a number but the average of them does not mean anything.

[color=#0000ff]Categorical variables[/color] divide observations in mutually exclusice groups (sex, opinion etc.) For example, the program of a university student is in a [color=#0000ff]nominal scale[/color], as it only divides students according their major. If groups could be sorted in a logical order, the variable is said to be in an [color=#0000ff]ordinal scale[/color]. For example, education has a logical order: primary school, secondary school, tertiary level etc.[br][br][br][color=#0000ff]Numerical variables [/color]are naturally real numbers:[br] [list][*]If a property does [color=#0000ff]not disappear at zero[/color], when a variable is in an [color=#0000ff]interval scale[/color] (temperature). Ratio of actual values have no meaning.[/*][*]If the property "[color=#0000ff]disapper" with the values 0[/color], the variable is in a [color=#0000ff]ratio scale[/color]. Ratio of actual values has a meaning: an object  A of 15 kg, an object  B of 45 kg => the object B on 3 times heavier than the object A. The unit of variables in an interval or in a ratio scale is not fixed. [/*][*]If the variable behaves like in a ratio scale BUT the unit is fixed, when the variable is in an [color=#0000ff]absolute scale [/color](the number of people). [/*][/list][br][br][br]