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Basics of Sampling Techniques



Population 
             A population is a group of individuals(or)aggregate of objects under study.It is also known as universe.
The population is divided by (i)finite population  (ii)infinite population, (iii) hypothetical population,  subject to a statistical study. A population includes each element from the set of observations that can be made.
(i)Finite population: A population is called finite if it is possible to count its individuals. It may also be called a countable population. The number of vehicles crossing a bridge every day,
(ii)Infinite population: Sometimes it is not possible to count the units contained in the population. Such a population is called infinite or uncountable.
ex, The number of germs in the body of a patient of malaria is perhaps something which is uncountable
 (iii) Hypothetical population: Statistical population which has no real existence but is imagined to be generated by repetitions of events of a certain type.
Sample: A sample consists subset observations drawn from the population.
  Typically, the population is very large, making a census or a complete enumeration of all the values in the population impractical or impossible. The sample represents a subset of manageable size.
Parameter and  Statistics:
As a measurable characteristic of a population, such as a mean or standard deviation, is called a parameter; but a measurable characteristic of a sample is called a statistic.
Sampling :The act or process of selecting a sample for testing. the selection of a suitable sample for study. A set of elements drawn from and analyzed to estimate the characteristics of a population. Also calledsampling.
Merits:
Advantages and Limitation of Sampling:
1. Sampling saves time and labour.
2. It results in reduction of cost in terms of money and manhour.
3. Sampling ends up with greater accuracy of results.
4. It has greater scope.
5. It has greater adaptability.
6. If the population is too large, or hypothetical or destroyable sampling is the only method to be used.
The limitations of sampling are given below:
1. Sampling is to be done by qualified and experienced persons. Otherwise, the information will be unbelievable.
2. Sample method may give the extreme values sometimes instead of the mixed values.
3. There is the possibility of sampling errors.
Demerits:
Sample method also has a number of drawbacks. Some of the important drawbacks of this method are given below.
i) If it is a question of deliberate selection, the result may be very much biased. This shall mislead the enquiry.
ii) All characteristics of the population may not be found in the samples drawn from the population.
iii) Information from sampling method are relatively less accurate than that from census method.
iv) Sample survey needs proper planning and execution by trained personnel. Otherwise it may give wrong results.
v) The law of inertia of large numbers, accuracy and approximation are less accurate in this method as compared to the census type of enquiry.
In spite of the above demerits the sampling method is gaining popularity day by day. This is primarily because the method is theoretically more correct and practically more convenient.
Why we Prepare the sample?
Very often sampling method is preferred to census method of collecting data, because of the following reasons.
i) The sample method involves less cost than the census method. Because here only a part of the population is examined. So it is economical.
ii) Sample study saves time and provides quick result.
iii) Sampling method often provides more accurate information than the census method. Because here we survey only a few items of the population. Sampling is generally done by trained and experienced persons. It facilitates intensive study and getting detailed information about the population.
iv) To get approximate or aggregate results sampling is generally preferred to census method.
v) In case of large population, sample method is more suitable than census method for collecting information.
vi) Non-sampling errors can be better controlled in sample survey than in census method.
Therefore, sampling involves less time provides accurate informations in a scientific and economical manner in comparison to census method.

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