For example, we are interested in the average test score in a large
Biometry class of size, say, 80. A sample of size 10 scores from this
resulted in an average test score of 75. If the average test score for
the entire 80 students (the population) is 72, then the sampling error
is: 75

72 = 3.
(9)
Sampling Variance
 a measure of the divergence of the estimate
from its expected value.
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Introduction to Sampling Methods
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Steps in Conducting a Sample Survey
The main steps that are involved in a sample survey are:
(1)
Statement of objectives;
(2)
Definition of the population to be studied;
(3)
Determination of the sampling frame and sampling units;
(4)
Selection of proper sampling design;
(5)
Determining the sample size;
(6)
Organisation of field work;
(7)
Collection of data (i.e., conducting the survey);
(8)
Summary and analysis of data.
There are many sampling designs/schemes/methods available.
The choice between sampling methods depends on:
(1)
the nature of the problem or investigation;
(2)
the availability of good sampling frames;
(3)
the budget or available financial resources;
(4)
the desired level of accuracy; and
(5)
the method by which data will be collected, such as questionnaires or
interviews.
Lecture Number 4
Introduction to Sampling Methods
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Types of Sampling Methods
A
sampling method
is a specific technique used to select sampling
units that constitute a sample.
Sampling methods are generally grouped into two categories;
probability and nonprobability sampling methods.
(1)
Probability Sampling
is the method of selecting samples according to
certain laws of probability in which each unit in the population has
some definite probability of being selected in the sample.
(2)
Nonprobability Sampling
is a method of selecting samples in which
the choice of selecting of sampling units depends entirely on the
discretion or judgement of the sampler. The method is mainly used for
opinion surveys but cannot be recommended for scientific use as it is
subject to the drawbacks of prejudice and bias of the sampler. It is also
not possible to compute the degree of precision of the estimates from
the sample values.
Lecture Number 4
Introduction to Sampling Methods
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Types of Sampling Methods
There are four main types of
probability sampling methods
that are
standard across disciplines:
(1)
Simple Random Sampling;
(2)
Systematic Sampling;
(3)
Stratified Sampling; and
(4)
Cluster Sampling.
In addition to the four, we also have;
(5)
MultiStage Sampling;
(6)
MultiPhase Sampling; and
(7)
Probability Proportional to Size (PPS) Sampling
NonProbability Sampling
methods include:
(1)
Convenience Sampling;
(2)
Judgement (purposive) Sampling;
(3)
Snowball Sampling; and
(4)
Quota Sampling.
Lecture Number 4
Introduction to Sampling Methods
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Simple Random Sampling
This is a method of selecting
n
units out of a population of size
N
by
giving equal probability (chance) to all units, or a sampling procedure in
which all possible combinations of
n
units that may be formed from a
population of
N
units have the same probability of being selected.
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 Fall '18
 F. TAILOKA
 representative, Basic Concepts of Sampling