This technique is also referred to as double sampling.
Double sampling is particularly useful in situations in which the
enumeration of the character under study (main character) involves
much cost or labour whereas an auxiliary character correlated with
the main character can be easily observed.
For example, in estimating the volume of a forest stand, we may use
diameter or girth of trees and height as auxiliary variables.
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Probability Proportional to Size Sampling
In many instances, the sampling units vary considerably in size and
simple random sampling may not be effective in such cases as it does
not take into account the possible importance of the larger units in
the population.
In such cases, it has been found that extra information about the size
of the units can be gainfully utilised in selecting the sample so as to
get a more efficient estimator of the population parameters.
One such method is to assign unequal probabilities for selection to
different units of the population.
For example, villages with larger geographical area are likely to have
larger area under food crops and in estimating the production, it
would be desirable to adopt a sampling scheme in which villages are
selected with probability proportional to geographical area.
Lecture Number 4
Introduction to Sampling Methods
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Probability Proportional to Size Sampling
When units vary in their size and the variable under study is directly
related with the size of the unit, the probabilities may be assigned
proportional to the size of the unit.
This type of sampling where the probability of selection is proportion
to the size of the unit is known as ‘PPS Sampling’.
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Convenient Sampling
A type of nonprobability sampling which involves the sample being drawn
from that part of the population which is close to hand or readily available
and convenient. It is sometimes known as grab or opportunity sampling or
accidental or haphazard sampling.
A research team may determine that the most convenient and
economical method is to set up an interviewing booth from which to
intercept consumers at a shopping center.
The researcher using such a sample cannot scientifically make
generalizations about the total population from this sample because it
would not be representative enough.
Convenience samples are best used for pilot testing or exploratory
research when additional research will subsequently be conducted with
a probability sample.
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Introduction to Sampling Methods
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Judgement Sampling
A nonprobability sampling technique in which an experienced individual
selects the sample based on his or her judgment about some appropriate
characteristics required of the sample member. The researcher chooses the
sample based on who they think would be appropriate for the study.
Researchers select samples that satisfy their specific purposes, even if
they are not fully representative.

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