This document discusses random sampling and its advantages. Random sampling is defined as selecting a sample from a population in a way that every possible sample has an equal probability of being chosen. It avoids potential bias and provides a representative sample of the population. The key steps to conducting a random sample are: 1) defining the population, 2) choosing a sample size, 3) listing the population, 4) assigning numbers to each unit, and 5) randomly selecting the sample. An example of randomly sampling students at a school is provided to illustrate the process.

1. Random Sampling
July 21, 2015
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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.”

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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

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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

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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

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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.’

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1. Define the Population
• Your population in this case are the students
– Which grades?
• You then decide to test grades 2, 4 and 6

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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

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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

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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

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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