ST 210 Lecture 2: Ch. 1b Notes

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Published on 10 Mar 2017
School
USA
Department
Statistics
Course
ST 210
Professor
Sampling Methods
Simple Random Sample
Random Sampling - process of using chance to select individuals from a population to be
included in the sample
o The key to obtaining a sample representative of a population is to let chance or
randomness play a role in dictating which individuals are in the sample, rather than
convenience.
Frame - complete list of the population that is being studied; pool from which the sample
is chosen
For the results of a survey to be reliable, the characteristics of the individuals in the
sample must be representative of the characteristics of the individuals in the population.
If convenience is used to obtain a sample, the results of the survey are meaningless.
o Convenience Sampling - individuals for the sample are chosen based on how easily
they are obtained, and NOT based on randomness.
o The most popular convenience sample are those in which the individuals are self-
selected, also called voluntary response sample
Stratified Sample - sample is obtained by dividing the population into non-overlapping
groups called strata and then obtaining a simple random sample from each stratum
o The individuals within each stratum should be homogenous in some way
Systematic Sample - obtained by selecting every Kth individual from the population
o The first individual selected corresponds to a number between 1 and K
o If K is too small: it may not be representative of the population
o If K is too large: the entire population may not give enough people to survey
o Example: Manager of a grocery store wants to measure satisfaction of 40 random
customers
Pik a uer, K …… 5
Randomly choose a number between 1-5……. 
Now after surveying the 3rd customer, every 5th customer will be surveyed
Steps to Obtain a Systematic Sample
o Step 1 - If possible, approximate the population size (N)
o Step 2 - Determine the sample size (n)
o Step 3 - Divide the population size by the sample size (N/n) to get the (K) value
o Step 4 - Randomly select a number between 1 and k. Call this number p.
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