Data Collection for Research: Nonprobability Sampling Techniques

Sampling data for various researches is a tedious job. Once our study deals with a large population, we should expect to consider large number of samples. Scientific researches are more of this nature. On the other hand, social researches may not require several samples or that the selection of samples may not be very meticulous. These data may not fully represent the characteristics of population under study, but they are still accepted because of their validity and significance for in-deep analysis and generalizations. This method of selection of samples is called as Nonprobability Sampling.

Nonprobability sampling happens if there is no way of estimating the probability that each individual or element will be included in the sample; hence, probability of any particular member of the population being chosen is unknown. In this method, not all the sampling elements in the population are given a chance of being included in the sample. It violates scientific principles, so findings obtained from nonprobability sampling techniques are not projectable to the population.

Here are the different nonprobability sampling methods:

a.      Judgment Sampling – selects a sample in accordance with an expert’s judgment or by a researcher’s personal judgment
-  inexpensive, little time to administer
- subjective, lack of generalizability

b.      Convenience Sampling – chooses units which come to hand or are convenient; selecting sample items that are close at hand or otherwise easy to obtain
- inexpensive, little time to administer, convenience
- biased, lack of generalizability

c.       Accidental or Incidental Sampling

d.      Quota Sampling – the proportions of the various subgroups in the population are determined and the sample is drawn (usually not at random) to have the same percentages in it
-   a specific  number of particular types of elements are selected
- the researchers determine proportion of target population that possesses the characteristics of interest,  then specify the number of these individuals to be included in the sample to reflect their proportion in the population.
- can be used to examine groups with certain traits
- subjective

e.      Volunteer Sampling – collected from those elements of the population which chose to contribute the needed information on their own initiative

f.        Snowball Sampling - Initial respondents are chosen at random. They provide names of additional respondents to include in a sample (via referral). Used when potential respondents are difficult to locate because they are a tiny part of the entire population.
- can be used to examine unusual groups, low cost
- a lot of time to administer

g.      Purposive Sampling – sets out to make a sample agree with the population in regard to certain characteristics
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