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Sampling
Continued from Measurement of Variables
A sample is a part of the whole, also called population, whose properties are to be studied to gain information about the entire lot. When dealing with people, it can be defined as a set of respondents or representatives selected from a larger population for the purpose of a study.
Researcher need not study the entire population for many reasons such as (i) the cost is too high and (ii) the population is dynamic or could change over time. In case of sample, there are many benefits like (i) lower cost, (ii) faster collection and (iii) accuracy.
In some cases, sampling is the only method when the entire population is inaccessible as wildlife in the forest. Moreover, due to study or experiment, there could be some wastage. So the entire population cannot be destroyed for the sake of study. Supposing, a researcher wants to find out average life of a light bulb. For this purpose, the researcher would switch on the light of a number of bulbs and keep them on till the bulbs burnout. By taking an average, life of a light bulb could be determined. But for this purpose, all the bulbs (the population) cannot be put to test else there would be nothing left to sell.
If the population is small, the researcher can study each and every member of the population. In this case, it would be called census. For example, if there are only 10 fertilizer plants in a country, it would be worthwhile to study all rather than a sample of them.
Representativeness of the sample
An appropriate and adequate sample is expected to reflect the properties or characteristics of the population. Depending upon the situation, the researcher may feel comfortable with a random sample. If population is not homogenous, the researchers can resort to other ways of taking samples such as stratified or cluster or even convenience sample. Moreover, the degree of representativeness of the sample may be limited by cost or convenience.
There are some other problems in sampling. First is called coverage error which depends upon how good is the population frame. If we want to study people of Karachi and we draw our sample from telephone directory, some percentage of population may not have telephones and hence no chance to be included in our survey. Under such a situation, a sample drawn from telephone directory may not be a representative one. Next is sampling error which pertains to sample size relative to the population.. Similarly, there are nonsampling errors which arise due to nonparticipation of some of the selected samples.
Sample versus Population Mean
STRATAFIED SAMPLE
TYPES OF SAMPLING
Basically, there are two types of sampling: NonProbability and Probability Sampling.
In Nonprobability, there is no known or predetermined chance of being selected. It is used when time is critical. The researcher resorts to convenience or judgement. It includes assigning a quota to certain areas and selecting the sample as conveniently as possible. Another technique is snowballing which is used when people are reluctant to speak or there is no way of locating them. It includes illegal immigrants, junkies, lefthanded, vegetarian or homeless people. In this case, the researcher selects one person as a sample and ask him or her to refer him to other people in the same category.
In probability sampling, elements have some known chance of being selected. If out of 100, a researcher decides to pick 10 by lottery, everyone has 10% chance of being selected. This method is used when a wider generalizability is needed or when selection bias is to be reduced to minimum possible.
Brief description of two type of samples is give below:
Techniques
 Strength
 Weaknesses


NONPROBABILITY SAMPLING
 
Convenience Sampling
 Least expensive, least time consuming, most convenient
 Selection bias, sample not representative

Judgement Sampling
 Low cost, convenient, not time consuming
 Subjective, does not allow generalization

Quta Sampling
 Sample can be controlled for certain characteristics
 Selection bias, no assurance of representativeness

Snowball Sampling
 Can estimate rare characteristics
 Time consuming

PROBABILITY SAMPLING
 
Simpling random Sampling
 Easily understood, results projectable
 Difficult to construct sampling frame, expensive

Systematic Sampling
 Can increase representativeness, easier to implement than simple random sampling, sampling frame no necessary
 Can decrease representativeness

Stratified Sampling
 Includes all important subpopulation, precision
 Difficult to select relevant stratification variables; not feasibile to stratify on many variables, expensive

Cluster Sampling
 Easy to implement, cost effective
 Imprecise, difficult to compute and interpret results

Issue of precision and confidence in determining sample size
Sampling is the process of selecting a sufficient number of elements for study and generalization. It may be noted that there is no direct relationship between population and sample size. But, of course, larger the sample, lesser the sampling error. Also, larger the sample, more the cost but it would not reduce the error by the same proportion. In other words, if we double size of our sample, the cost would also be doubled but the reduction in error would not correspondingly 50%.
In determining the sample size, we need three pieces of information:
 Confidence Interval or desired precision: ± 10 or ±1,000
 Confidence Level or in other words how sure you want to be: 68%, 95% or 99.9%
 How heterogeneous are the numbers as indicated by the standard deviation of the population.
A bank manager is interested to know weekly cash withdrawal from his bank. She wants to find out quantum of monthly withdrawal with 95% confidence that the results would be within a narrow range of ± 500. How large a sample, she should select?
First, she would pickup a small sample just to know standard deviation of the samples by a random selection. Suppose, she takes five samples of weekly withdrawal which come to 200,000, 202,000, 204,000, 206,000 and 209,100. The standard deviation of these amounts would be 3,527.
Z value of the 95% probability would be 1.96. Using the formula given in the side sheet, the sample size would work out to be 191,
There is a trade off between confidence and precision. The more precise you want to be, the more confidence level would be lowered. For example, you see a person and from his dress you want to make a guess of his income. You can be 90% sure that you have made a good guess if you say that his income is between Rs.5,000 and Rs.50,000. But that is a wide range. If you reduce it or be more precise by saying between Rs.40,000 and Rs.45,000, you cannot be 95% confident but maybe 50%.
Random sample of cars
SUMMARY
In any research, selection of an appropriate sample is very important. This involves sampling plan and sample size.
Probability samples are good for generalization as samples are selected on random basis. But if time and cost is the concern, there are many techniques of nonprobability samples which are cheapest and quickest.
Awareness of sample designs and sample size helps researcher to strengthen their research. It also helps assess cost implication of different designs and the trade off between the precision and confidence.
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Comments
A very good ,useful article with deep knowledge of the process. Sampling is an integral part of research work, where the population is large.This hub elaborately describes the nature of sample, ways of collecting it, and the advantages and limitations of each type of sample.A studious piece of writing.
dear sir i would like to request you that all these research topics i have studied but just make me understand trough one or single hub pages where you can use these all topics in one organizational problem it will be more good for us trough that page we would understand the interconnection of some topic i hope you will do this for us and i really enjoyed your all classes i learned allot of you and may Allah gives you a very long life i will be really miss your classes zohaib noor
the goodness of your hub pages is that every tom dick harry can easily understand your thought and can applied it in every profession of life keep imparting your knowledge in bahria university may Allah gives you a long life
Very informative hub! Thank you
thanks for the ideas of sampling.
dear sir, crisp and precise presentation...thoroughly explained....supported with finest example...thnx
Dear Hafeez malik, it is very nice information for me, and i hope it bring a lot of knowledge for others. it is true sampling is a necessary part of research and we cant by pass it.
thanks for your work.
This is a wonderful way to spend your time by sharing your knowledge. You made this very understandable. Your are a very effective teacher. Thank you so much!
Dear Sir,
superbly explained, basically the true essence of a good research is sampling, indeed informative.
THANKS FOR ADDING TO MY KNOWLEDGE BANK!
Regards,
RUFI SHAHZADA
Very informative hub! Thank you.
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