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: samples and population, experimental design, random sampling  
Representative Samples
Drawing representative samples can be quite demanding. An experimenter
should always ask himself whether the drawn samples are representative
of the population he is interested in. A few examples should clarify the
situation:
A gardener changed the method of cultivation of tulips. In order to know whether the new method was successful, some statistical tests were performed. As the size of the population of tulips (= all available tulips) was approx. 4000, she decided to draw a selection of 100 flowers to calculate an estimate of the average length of the new cultivation population. How could she select 100 out of about 4000 flowers, without distorting the measurement by subjective influences? Note: sampling by personal "standards" almost always causes errors due to psychological reasons. Maybe she was convinced of the new method, or rejected it for some reason. Even if she tried to be objective, it is questionable whether an unconscious manipulation of the sampling occurred anyway. A usual method for creating representative samples is to use random
numbers for the selection of individual test objects:


Last Update: 20121008