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SAMPLING Obtaining a Sample From a Population
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SAMPLING
Obtaining a Sample From a Population
A population is all the people or objects ofinterest in a study.
The population for a study about UNI studentsis all the students at UNI.
If a study utilized everyone in a population,the sample would be called a
Census.
A CENSUS is a sample of an entire Population….
They are seldom done because…..
A CENSUS is a sample of an entire Population….
They are seldom done because…..
1. Impossible
A CENSUS is a sample of an entire Population….
They are seldom done because…..
1. Impossible2. Unprofitable
A CENSUS is a sample of an entire Population….
They are seldom done because…..
1. Impossible2. Unprofitable3. Less accurate
So… instead of a census,a sample is taken.
Sampling is the process of discovering what a populationis like….. but
You can never look directly at the population!
The population is like a large container… you can pull things outbut you are NEVER allowed to look at all its contents.
Sampling must be done properly orsampling itself can invalidate your research!
Sampling is a five step process:
Sampling is a five step process
Each step is important
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Sampling is a five step processEach step is important
Or the sample will not givean accurate estimate of the
population.
1.Before a sample can be taken from a population: that population must be defined.
1. Before a sample can be taken from a population that population must be defined.
A population: The total member of objects orpersons that are of interest for a study.
Male Female Male Female
France India
1. Before a sample can be taken from a population that population must be defined.
A population: The total member of objects orpersons that are of interest for a study.
The characteristics of a population are called:
Parameters.
2. Identify the sampling frame
2. Identify the sampling frame
A sampling frame is a list of the units of apopulation:
telephone numbersaddressesemail addresses
3. Select a sampling procedure
3. Select a sampling procedure
Two kinds:1. Nonprobability samples2. Probability samples
3. Select a sampling procedure
1. Nonprobability samples
3. Select a sampling procedure
1. Nonprobability samplesConvenience Sample (Accidental Sample)
3. Select a sampling procedure
1. Nonprobability samplesConvenience Sample (Accidental Sample)
Judgmental Sample
3. Select a sampling procedure
1. Nonprobability samplesConvenience Sample (Accidental Sample)
Judgmental SampleQuota sample
3. Select a sampling procedure
2. Probability samples
3. Select a sampling procedure
2. Probability samplesSimple Random Sample (SRS)
3. Select a sampling procedure
2. Probability samplesSimple Random Sample (SRS)
Special case of the convenience sample…..
3. Select a sampling procedure
2. Probability samplesSimple Random Sample (SRS)
Stratified Sample
3. Select a sampling procedure
2. Probability samplesSimple Random Sample (SRS)
Stratified SampleCluster Sample
Cluster Sample
3. Select a sampling procedure
2. Probability samplesSimple Random Sample (SRS)
Stratified SampleCluster SampleSystematic Sample
Note: Important!
The human mind will not randomize!
4. Determine a sample size
4. Determine a sample size
What is the correct sample size?
The answer is always the same.
IT DEPENDS!
4. Determine a sample size
And what does it depend upon?
4. Determine a sample size
And what does it depend upon? 1. Sampling error
http://www.rasmussenreports.com/public_content/politics/obama_administration/daily_presidential_tracking_poll
4. Determine a sample size
And what does it depend upon? 1. Sampling error
National Survey of 1,000 Likely VotersConducted September 20-21, 2010By Rasmussen Reports 1* Is the current structure of the Social Security system a Ponzi scheme? 27% Yes36% No37% Not sure
NOTE: Margin of Sampling Error, +/- 3 percentage points with a 95% level of confidence
4. Determine a sample size
And what does it depend upon? 1. Sampling error2. Precision
4. Determine a sample size
And what does it depend upon? 1. Sampling error2. Precision3. Degree of confidence
4. Determine a sample size
And what does it depend upon? 1. Sampling error2. Precision3. Degree of confidence4. Other factors
a. timeb. costc. type of scalesd. number of cross-classifications
4. Determine a sample size
And what does it depend upon? 1. Sampling error2. Precision3. Degree of confidence4. Other factors
f. historical and secondary datag. statistical considerationsh. political considerations
4. Determine a sample size
Since any given interval on the normal curve can be calculated by:
HZ
n
4. Determine a sample size
Since any given interval on the normal curve can be calculated by:
HZ
n
x
xxZ
HZ
n
H = Z SE
SEn
nZ
H
2 2
2
Since any given interval on the normal curve can be calculated by:
HZ
n
The sample size can then be seen as:
nZ
H
2 2
2
The sample size for nominal and ordinal datacan then be seen as:
nZ pq
H
2
2
H Zpq
n
H = ZSE
SEpq
n
nZ pq
H
2
2
What size sample would be needed if you wished to make a decision to within 2 IQ point with 95% accuracy?
IQ has a standard deviation of 15…… therefore:n
Z
H
2 2
2
With Z = 2, H = 2; therefore….
nZ
H
2 2
2
2 15
2225
2 2
2
It is assumed that 55% of all students are women, what size sample would beneeded to test this assumption to a 95% accuracy at 2%?
nZ pq
H
2
2
p = .55, q = .45 (p + q = 1); H = .02; Z = 2
n 2 55 45
022475
2
2
(. )( . )
(. )
The largest sample size that will ever be needed for any given H is when nominal data is beingused and p = q = 0.5. Therefore the largest sampleEVER needed is:
nH
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What size sample would be needed to:
1. Use a GPA scale with a SD = 15 to an accuracy of 95% at 3 IQ points?
What size sample would be needed to:
1. Use a GPA scale with a SD = 15 to an accuracy of 95% at 3 IQ points?
2. A Likert scale to a 95% accuracy of 0.4?
What size sample would be needed to:
1. Use a GPA scale with a SD = 15 to an accuracy of 95% at 3 IQ points?
2. A Likert scale to a 95% accuracy of 0.4?
3. Income scale with a SD = $15,000 to within a 95% accuracy of $5,000?
What size sample would be needed to:
1. Use a GPA scale with a SD = 15 to an accuracy of 95% at 3 IQ points?
2. A Likert scale to a 95% accuracy of 0.4?
3. Income scale with a SD = $15,000 to within a 95% accuracy of $5,000?
4. The largest sample that would ever be needed to do an over-night politicalpoll with a 95% accuracy of 2%?
What size sample would be needed to:
1. Use a GPA scale with a SD = 15 to an accuracy of 95% at 3 IQ points?
2. A Likert scale to a 95% accuracy of 0.4?
3. Income scale with a SD = $15,000 to within a 95% accuracy of $5,000?
4. The largest sample that would ever be needed to do an over-night politicalpoll with a 95% accuracy of 2%?
5. To test a hypothesis that 80% of all customers will buy a product again to within 95% accuracy at 3%?
National Survey of 1,000 Likely VotersConducted September 20-21, 2010By Rasmussen Reports 1* Is the current structure of the Social Security system a Ponzi scheme? 27% Yes36% No37% Not sure
NOTE: Margin of Sampling Error, +/- 3 percentage points with a 95% level of confidence
Why is it +/- 3 percent at 95% confidence?
What is the sample size?
4. Determine a sample size
What about the size of the population?
4. Determine a sample size
What about the size of the population?
Does that change the size of the sampleneeded?
4. Determine a sample size
Surprisingly so,
Usually not!!
4. Determine a sample size
Pragmatically, the sample size is independentof the population size!
Except for very small populations, or very, very large samples.
5. Collect data (go do it!!)
An example of the problems in samplingand the value of demographics….
http://www.dailymail.co.uk/news/article-1315078/Race-maps-America.html
http://www.prb.org/Publications/Articles/2008/workingaroundtheclock.aspx
