Download Example 1 Sampling Schemes

Example 1
2. Collecting Data
continued
How do we select a small subset of
a population representative of the
whole population?
The sample should represent the
population well, so sample
statistics from a random sample
should provide reasonable
estimates of population
parameters
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Given that are 806
students in MA238,
determine the total
possible number of
samples of size 50.
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Sampling Schemes
1. Probability
Sampling
2. Non-Probability
Sampling
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SimpleRandom
Sampling.
‘a sample of n units
chosen from a
population of N units
in such a way that
every sample of n
units has the same
chance of selection’
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Stratified Random
Sampling.
(i) chose strata within
the population so that
units in each stratum
resemble each other.
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Cluster Sampling.
(i) Divide the population into
groups, or clusters and
label
(ii) take a SRS of these
clusters.
(ii) Take an SRS in
each stratum
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One consequence of natural
variation is that two samples drawn
by the same method from the same
population will give somewhat
different estimates of the population
parameters (sampling variation).
Sampling Errors
1.Random Sampling
Errors
Example 2 (Opinion poll)
46% Fianna Fail;
32% Fine Gael;
15% Labour;
4% PDs;
3% Others.
2.Non-Sampling
Errors
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Margin of Error
Key Point
When a sample is drawn using a
probability sampling method,
then we can use probability
theory to indicate the extent to
which the estimates obtained from
different samples are likely to
differ from one another, and from
the true value of the population
parameter.
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In order to quantify
random sampling
errors we must first
determine the
sampling
distribution
of the statistic.
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