23-08-2012, 04:40 PM
Sampling and Sampling Distribution
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Sampling : Introduction
A sample is a definite plan for obtaining a sample from a given population.
It refers to the technique or the procedure the researcher would adopt in selecting items for the sample.
Meaning:-
It is the aggregate of element about which we wish to make inferences. A member of the population is an element. It is the unit of study.
A part of population is known as a sample.
The process of drawing a sample from a large population is called sampling.
The list of sampling unit from which a sample is taken is called the sampling frame.
Probability Sampling Methods
It is based on the theory of probability. It is also known as random sampling or chance sampling.
In probability sampling every population has a chance of being selected. Such chance is known as probability.
E.g. If a sampling frame is a list of 100 students of a specific course of study, in a simple random sample, each student has 1/100th chance of being selected
Probability sampling yields a representative sample.
Simple Random Sampling
This technique gives each element an equal & independent chance of being selected.
Equal probability selection method is described as Epsem sampling.
An independent chance means that the draw of one element will not affect the chances of other element being selected.
Where some element are purposely excluded from the sample , the resulting sample is not a random one. Hence all the element should be included in the sample frame to draw a random sample.
Lottery Method:-
This is the simplest and most familiar procedure of random sampling.
E.g. If a sample of 10 students is to be drawn out of 50 in a section, Write the no’s 1-50 in a slips and pick 10 slips , the units bearing the numbers of slips drawn constitute the random sample.
They are two alternatives:-
After a number is selected by draw, it may be replaced and consequently it has a chance of being selected again. This is referred as unrestricted random sampling.
The Selected number is set aside, & in the subsequent draws, it does not get a chance of being selected again. This is referred as restricted random sampling.
The table of random numbers
To select a random sample out of a given frame, One should start selecting the number from a table of Random number at any randomly selected point & with in the range of frame.
The table of random numbers is ideal for obtaining a random sample from relatively small populations. When population are quite large say lakhs, drawing number from the table becomes tedious.
Steps for random sampling
Identify and define the population
Determine the desired sample size
Identify the variable & sub-group (strata) for which you want to guarantee appropriate representation.
Classify all members of the population as member of one of the identified sub-groups.
Randomly select an appropriate number of individuals from each of the sub-group.