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SamplingIn statistics and survey methodology, sampling is concerned with the selection of a subset of individuals from within a population to estimate characteristics of the whole population....
is the use of a subset of the population to represent the whole population. Probability sampling, or random sampling, is a sampling technique in which the
probabilityProbability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...
of getting any particular sample may be calculated.
Nonprobability sampling does not meet this criterion and should be used with caution. Nonprobability sampling techniques
cannot be used to infer from the sample to the general population. Any generalizations obtained from a nonprobability sample must be filtered through one's knowledge of the topic being studied. Performing nonprobability sampling is considerably less expensive than doing probability sampling, but the results are of limited value.
Examples of nonprobability sampling include:
 Convenience, Haphazard or Accidental sampling
Accidental sampling is a type of nonprobability sampling which involves the sample being drawn from that part of the population which is close to hand. That is, a sample population selected because it is readily available and convenient...
 members of the population are chosen based on their relative ease of access. To sample friends, coworkers, or shoppers at a single mall, are all examples of convenience sampling.
 Snowball sampling
In sociology and statistics research, snowball sampling is a nonprobability sampling technique where existing study subjects recruit future subjects from among their acquaintances. Thus the sample group appears to grow like a rolling snowball...
 The first respondent refers a friend. The friend also refers a friend, etc.
 Judgmental sampling or Purposive sampling  The researcher chooses the sample based on who they think would be appropriate for the study. This is used primarily when there is a limited number of people that have expertise in the area being researched.
 Deviant Case  Get cases that substantially differ from the dominant pattern (a special type of purposive sample).
 Case study  The research is limited to one group, often with a similar characteristic or of small size.
 ad hoc quotas  A quota is established (say 65% women) and researchers are free to choose any respondent they wish as long as the quota is met.
Even studies intended to be probability studies sometimes end up being nonprobability studies due to unintentional or unavoidable characteristics of the sampling method. In public opinion polling by private companies (or other organizations unable to require response), the sample can be selfselected rather than random. This often introduces an important type of error: selfselection bias. This error sometimes makes it unlikely that the sample will accurately represent the broader population. Volunteering for the sample may be determined by characteristics such as submissiveness or availability. The samples in such surveys should be treated as nonprobability samples of the population, and the validity of the estimates of parameters based on them unknown.
See also
 Sampling (statistics)
In statistics and survey methodology, sampling is concerned with the selection of a subset of individuals from within a population to estimate characteristics of the whole population....
 Cluster sampling
Cluster Sampling is a sampling technique used when "natural" groupings are evident in a statistical population. It is often used in marketing research. In this technique, the total population is divided into these groups and a sample of the groups is selected. Then the required information is...
 Judgment sample
Judgment sample is a type of nonrandom sample, which is selected based on the opinion of an expert. Results obtained from a judgment sample are subject to some degree of bias, due to the frame and population not being identical. The frame is a list of all the units, items, people, etc that define...
 Multistage sampling
Multistage sampling is a complex form of cluster sampling.Advantages * cost and speed that the survey can be done in* convenience of finding the survey sample* normally more accurate than cluster sampling for the same size sampleDisadvantages...
 Simple random sample
In statistics, a simple random sample is a subset of individuals chosen from a larger set . Each individual is chosen randomly and entirely by chance, such that each individual has the same probability of being chosen at any stage during the sampling process, and each subset of k individuals has...
 Systematic sampling
Systematic sampling is a statistical method involving the selection of elements from an ordered sampling frame. The most common form of systematic sampling is an equalprobability method, in which every kth element in the frame is selected, where k, the sampling interval , is calculated as:k =...
 Stratified sampling
In statistics, stratified sampling is a method of sampling from a population.In statistical surveys, when subpopulations within an overall population vary, it is advantageous to sample each subpopulation independently. Stratification is the process of dividing members of the population into...