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Random Sampling
July 21, 2015
1
2
What is Random Sampling?
“A method of selecting a sample from a statistical
population in such a way that every possible
sample that could be selected has a predetermined
probability of being selected.”
3
What is Random Sampling?
• Every item has an equal chance of being
selected into the sample
• Avoids potential bias
• Will therefore be a representation of the
population
– A population being a complete set of items (e.g.
people) that share at least one property in common
that can then be statistically analyzed
4
Advantages
• The sample is highly representative of the
population being studied
– Assuming that there is limited missing data
• Since units selected for the sample are chosen
using random sampling, it allows us to make
generalizations (statistical inferences) from the
sample to the population
5
How do you Conduct a Random
Sample?
1. Define the population
2. Choose your sample size
3. List the population
4. Assign numbers to the units
5. Select your sample
6
Example
You are going into a school to analyze the progress
of the students academically and so you want to
take a random sample. You don’t have the time to
test all the students so you want a ‘representative
sample.’
7
1. Define the Population
• Your population in this case are the students
– Which grades?
• You then decide to test grades 2, 4 and 6
8
2. Choose Your Sample Size
• Finding the right sample size is critical
• It needs to be large enough to balance out
outliers and be representative of the population
• But also not too large or else it will take too long
and too many resources
• You decide to test 20 children per grade
9
3. List the Population
• In this case this may already be done for you as
the students are listed in a register
• But if it were not all students from the grades
being tested the students would need to be put
into a list
10
4. Assign Numbers to the Units
• Once the list is compiled all the units, in this
case students, can be assigned numbers
• So in a class of 40 students, the students would
be listed from 1 to 40 alphabetically
11
5. Select Your Sample
• If your sample size is 20 and you have 40
students in a class then mathematically you
would have to test every second student to end
up with 20
• And so in your list of 40 numbered students you
would test every second child
– Child 2, 4, 6, 8 and so on

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Random Sampling

  • 2. 2 What is Random Sampling? “A method of selecting a sample from a statistical population in such a way that every possible sample that could be selected has a predetermined probability of being selected.”
  • 3. 3 What is Random Sampling? • Every item has an equal chance of being selected into the sample • Avoids potential bias • Will therefore be a representation of the population – A population being a complete set of items (e.g. people) that share at least one property in common that can then be statistically analyzed
  • 4. 4 Advantages • The sample is highly representative of the population being studied – Assuming that there is limited missing data • Since units selected for the sample are chosen using random sampling, it allows us to make generalizations (statistical inferences) from the sample to the population
  • 5. 5 How do you Conduct a Random Sample? 1. Define the population 2. Choose your sample size 3. List the population 4. Assign numbers to the units 5. Select your sample
  • 6. 6 Example You are going into a school to analyze the progress of the students academically and so you want to take a random sample. You don’t have the time to test all the students so you want a ‘representative sample.’
  • 7. 7 1. Define the Population • Your population in this case are the students – Which grades? • You then decide to test grades 2, 4 and 6
  • 8. 8 2. Choose Your Sample Size • Finding the right sample size is critical • It needs to be large enough to balance out outliers and be representative of the population • But also not too large or else it will take too long and too many resources • You decide to test 20 children per grade
  • 9. 9 3. List the Population • In this case this may already be done for you as the students are listed in a register • But if it were not all students from the grades being tested the students would need to be put into a list
  • 10. 10 4. Assign Numbers to the Units • Once the list is compiled all the units, in this case students, can be assigned numbers • So in a class of 40 students, the students would be listed from 1 to 40 alphabetically
  • 11. 11 5. Select Your Sample • If your sample size is 20 and you have 40 students in a class then mathematically you would have to test every second student to end up with 20 • And so in your list of 40 numbered students you would test every second child – Child 2, 4, 6, 8 and so on