Math consepts

• x can represent a variable or a set of observations. Then in $x_i$ i represents exact observation.
• Absolute frequency - amount of same observations.

  If 157 respondents have answered "28 days", than the absolute frequaency for the category '28 days' is 157.
• Relative frequency - part from all possible meanings

  Relative frequency of the group 28 days of vacation is 31,1% of the sample.
• Frequency distribution is all possible values of a data-set, combined into classes together with absolute and relative frequencies.
• How to standardize a variable?

  Subtract the mean and divide by the sample's standard deviation.