Non Probability Sampling In Statistics

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1. Overview
Sampling is a procedure of statistical analysis in which a pre-established amount of observations will be taken from a larger population. From the characteristics and features of the sample, we can infer the characteristics and features of the whole population. And the most significant thing is to ensure that the sample must be able to represent the entire population.

2. Sampling Techniques
2.1 Probability Sampling Methods
Probability sampling is sampling method which is capable of being selected in the population of all units are the same. It could be the best method we can pick out a likely sample which delegates for the whole population. As can be calculated due to sampling bias, it will be able to apply the statistical
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The non-probability sampling is entirely dependent on the experience and understanding of population of researcher so that the research outcomes are usually the subjectivity of researcher. On the other hand, it is unable to calculate the bias due to picking out sample, so it is impossible to apply statistical estimation method for extrapolating the results of sample in the population. There are five types of non-probability sampling.
• Quota sampling: For this sampling method, we first start allocating population according to a certain criteria that we 're interested in, just as stratified random sampling; however we will use the convenience sampling or purposive sampling to select the units in each group to conduct investigations. The allocation in research units of each group is divided based on the subjective experience of the researcher.
For example, researcher requires interviewers to interview 800 people aged over 18 in a city. If we apply the quota sampling method, it can allocate by gender and demographic as follows: select 400 people (200 men and 200 women) aged 18 to 40, select 400 people (200 men and 200 women) aged 40 and older. Then investigators can pick out those which close to home or facilitate for their investigation to quickly complete the
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And keep doing this; it will create an enormous social network. This is the reason why it is called snowball, the more rolling the larger the snowball.

• Self-selection sampling is a technique which accepts individual cases of the entire population to be able to voluntarily take part in research. The key element of this method is that the volunteers expect to join the research rather than being approached directly by the interviewer.

• Convenience sampling: It means that sampling based on convenience or accessibility of the object, where investigators are more likely to meet the audience.
For example: researcher can stop anyone they encounter in commercial centers, streets or shops to do interviews. If the interviewee does not agree, then they move on to other subjects.
Convenience sampling is often used in research to discover and determine the practical significance of the research problem; or to check the questionnaire sheet in order to complete it; or when you want to preliminary estimate about concerned issues that you do not want to take too much time and

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