Fundamentals of Statistics contains material of various lectures and courses of H. Lohninger on statistics, data analysis and chemometrics......click here for more. 
See also: Observations and Variables, Variance, Descriptive Statistics, Distributions  
Variability
Whenever some measurements are taken, one has to deal with variability (or dispersion) in the data. As a matter of fact, virtually all natural processes vary  even those which yield "constant" results show up fluctuations if either the observation time is long enough or the scale of the measurement is zoomed in (see random experiment).
Examples:


Last Update: 20121008