Sampling is a process used to make inferences about a whole population by studying a representative subset. There are two main types of sampling: probability sampling, where every member has a known chance of selection, and non-probability sampling, where the probability of selection is unknown. Common probability sampling techniques include simple random sampling, stratified sampling, systematic sampling, and cluster sampling. Common non-probability techniques are judgment sampling, quota sampling, snowball sampling, and convenience sampling. The advantages of sampling include lower cost, faster results, and greater flexibility compared to studying the entire population.

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Sampling
“Sampling is the process by which inference is made to the whole by examining a part”
The purpose of sampling is to provide various types of statistical information of a
qualitative or quantitative nature about the whole by examining a few selected units. The
sampling method is the scientific procedure of selecting those sampling units which would
provide the required estimates with associated margins of uncertainty, arising from examining
only a part and not the whole. There are two types of sampling, that’s are;
1. Probability Sampling
With probability sampling, every element of the population has a known probability of being
included in the sample. The types of non probability sampling are:
a. Simple random Sampling
A randomly selected sample from a larger sample or population, giving all the individuals in
the sample an equal chance to be chosen.
For example a person researching education levels within a company takes the full employee
list and applies a random number algorithm to this in order to select people to interview. (Or)
When a teacher enters in the class and randomly select two students without knowing him.
b. Stratified Sampling
A stratified sample is a probability sampling technique in which the researcher divides the
entire target population into different subgroups, or strata, and then randomly selects the final
subjects proportionally from the different strata. This type of sampling is used when the
researcher wants to highlight specific subgroups within the population.
For example a high school student who is studying year-ten attitudes in the school uses
registration tuition classes as strata and studies a random selection of students from each of these
classes. (Or)
In a company there are more men than women, but it is required to have each group equally
represented. Two strata are thus created, of men and women, with an equal number in each.
c. Systematic Sampling
Systematic sampling is a random sampling technique which is frequently chosen by
researchers for its simplicity and its periodic quality.

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For example a study of people going to night-clubs first determines that there are about 250-
300 people in the club (due to fire regulations). A sample size of 30 is selected, giving an interval
of 300/30 = 10. A random number between 1 and 10 is generated and comes up with 7. Starting
with the 7th person to enter the club, every 10th person is given a brief interview. (Or)
Other precautions are taken to neutralize any impact on the study of what time of night
people enter the club.
d. Cluster Sampling
In this type of sampling, the total population is divided into clusters and a sample of the
cluster is selected. Then the required information is collected from the elements within each
selected cluster.
For example in a study of the opinions of homeless across a country, rather than study a few
homeless people in all towns, a number of towns are selected and a significant number of
homeless people are interviewed in each one. (Or)
Suppose that the Department of Agriculture wishes to investigate the use of pesticides by
farmers in England. A cluster sample could be taken by identifying the different counties in
England as clusters.
e. Sequential Sampling
A sampling plan in which an undetermined number of samples are tested one by one,
accumulating the results until a decision can be made.
For example if you are interested in the sample of defected element in unit, then select until
your sample is complete.
2. Non probability Sampling
With non-probability sampling, we cannot specify the probability that each element will
be included in the sample. The types of non probability sampling are:
a. Judgment Sampling
In this type of sampling, researcher uses his own judgment to select what seems like an
appropriate sample. For example
A TV researcher wants a quick sample of opinions about a political announcement. They
stop what seems like a reasonable cross-section of people in the street to get their views. (Or)
If you want to check the behavior of people with each other then judge them.

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b. Quota Sampling
In quota sampling the selection of sample is made by researcher, who has been given quotas
to fill from specified sub groups of the population.
For example, an interviewer may be told to sample50 males between ages 45 to 60. (Or)
A researcher in the high street wants 100 opinions about a new style of cheese. She sets up a
stall and canvasses passers-by until she has got 100 people to taste the cheese and complete the
questionnaire.
c. Snowball Sampling
A snowball sample is one in which the researcher collects data on the few members of the
target population he or she can locate, then asks those individuals to provide information needed
to locate other members of that population whom they know.
For example a researcher is studying environmental engineers but can only find five. She
asks these engineers if they know any more. They give her several further referrals, which in turn
provide additional contacts. In this way, she manages to contact sufficient engineers. (Or)
If obtaining subjects for a study that wants to observe a rare disease, the researcher may opt
to use snowball sampling since it will be difficult to obtain subjects. It is also possible that the
patients with the same disease have a support group; being able to observe one of the members
as your initial subject will then lead you to more subjects for the study.
d. Convenience Sampling
In this type of sampling, researcher use people who are available easily.
For example if you want to know how people behave with others then use people in the
street, people you know, people who work with you, customers and so on. (Or)
A group of students in a high school do a study about teacher attitudes. They interview
teachers at the school, a couple of teachers in the family and few others who are known to their
parents.
Advantages of Sampling
 The total cost of a sample will be much less than that of the whole lot.
 With smaller number of observations it is possible to provide results much faster as
compared to the total number of observations.
 Sampling has a greater scope regarding the variety of information by virtue of its
flexibility and adaptability.

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 Actual appraisal of reliability
Limitations of sampling
 Errors due to sampling may be high for small administrative areas.
 Sampling may not be feasible for problems that require very high accuracy.
References
 Basic concepts of sampling, Available at
http://cs.gmu.edu/cne/modules/dau/stat/data/sample_frm.html , Accessed on 11
September 2014
 Statistics and Probability, Available at
http://stattrek.com/statistics/dictionary.aspx?definition=probability_sampling , Accessed
on 11 September 2014
 Systematic sampling, Available at https://explorable.com/systematic-sampling ,
Accessed on 11 September 2014
 Cluster sampling, Available at
https://www.princeton.edu/~achaney/tmve/wiki100k/docs/Cluster_sampling.html ,
Accessed on 11 September 2014
 Sequential sampling, Available at
http://encyclopedia2.thefreedictionary.com/sequential+sampling , Accessed on 11
September 2014
 Judgment sampling, Available at
http://changingminds.org/explanations/research/sampling/judgment_sampling.htm ,
Accessed on 11 September 2014
 Quota sampling, Available at http://www.slideshare.net/sumanto123/quota-sampling ,
Accessed on 11 September 2014
 Snowball sampling, Available at http://sociology.about.com/od/Types-of-
Samples/a/Snowball-Sample.htm , Accessed on 11 September 2014
 convenience sampling, Available at
http://changingminds.org/explanations/research/sampling/convenience_sampling.htm ,
Accessed on 11 September 2014