Probability samples have the distinguishing characteristic that each unit in the population has a known nonzero probability of being included in the sample.
Probability samples imply the use of random selection mechanism.
Random selection eliminates subjective bias in the selection process and underlies the theories used to infer the sample results to the population. Random selection include either using a lottery procedure with well-mixed numbers, extracting a set of numbers form a listing of random numbers , or using computer program to generate a random list of units from an automated listing. Properly designed sampling methods allow the researchers to improve the reliability and draw informative conclusions about very large group of people. Design and Construction of Sample Frame Sampling frame is the list of all the sampling units in the population. This might be an alphabetic listing of residents of a community, or a list of households arranged by street address.