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By:Shirin MaghsoudBy:Shirin Maghsoud
SAMPLINGSAMPLING
Sampling is the procedure a researcher uses to gather people, places, or things to study
Research conclusions and generalizations are only as good as the sample they are based on
Who do you want Who do you want to generalize to?to generalize to?Who do you want Who do you want to generalize to?to generalize to?
Groups in SamplingGroups in Sampling
How can you get How can you get access to them?access to them?How can you get How can you get access to them?access to them?
Groups in SamplingGroups in SamplingThe Theoretical The Theoretical
PopulationPopulationThe Theoretical The Theoretical
PopulationPopulation
The Study The Study PopulationPopulationThe Study The Study PopulationPopulation
Groups in SamplingGroups in Sampling
The Theoretical The Theoretical PopulationPopulation
The Theoretical The Theoretical PopulationPopulation
The Study The Study PopulationPopulationThe Study The Study PopulationPopulation
The Sampling The Sampling FrameFrame
The Sampling The Sampling FrameFrame
Who is in your study?Who is in your study?Who is in your study?Who is in your study?
The Theoretical The Theoretical PopulationPopulation
The Theoretical The Theoretical PopulationPopulation
The Study The Study PopulationPopulationThe Study The Study PopulationPopulation
The Sampling The Sampling FrameFrame
The Sampling The Sampling FrameFrame
The Theoretical The Theoretical PopulationPopulation
The Theoretical The Theoretical PopulationPopulation
The Study The Study PopulationPopulationThe Study The Study PopulationPopulation
The Sampling The Sampling FrameFrame
The Sampling The Sampling FrameFrame
The SampleThe SampleThe SampleThe Sample
The greater the diversity and differences that exist in your population, the larger your sample size should be
Defining sampling frame : inhibit systematic error, or bias
Exp: phone survey ; phone book randomization, or random selection:
each unit in the population has an equal chance of being selected
quota sample :to sample everybody and
everything Population: refers to the larger group
from which the sample is taken
How to perform sampling: know some of the overall demographics; age, sex,
class, etc Beeing practicalBeeing practical small population of interest: sample about 10-
30% of that population large population of interest :(over 150,000), a
sample as low as 1%
sampling frame: procedure for listing all the accessible members of your populationexp;phone survey :phone booktwo basic approaches to sampling: probabilistic and nonprobabilistic exp:
randomization, or random selection : how you draw the sampleeach unit in the population has an equal
chance of being selected
Simple random sampling :
clearly defined population /obtain a list of all residents /using a sequence of numbers from a random numbers table (or draws of a hat, flips of a coin)
exp:
educational survey in primary school students
Stratified random sampling :
is appropriate when you're interested in correcting for gender, race, or age disparities in your population.
exp:
smoking prevalence in medical students
Stratified Random SamplingStratified Random SamplingList of ResidentsList of ResidentsList of ResidentsList of Residents
Random Subsamples of n/NRandom Subsamples of n/NRandom Subsamples of n/NRandom Subsamples of n/N
StrataStrataStrataStrata
surgicalsurgical Non-clinicalNon-clinicalmedicalmedical
Systematic random sampling : Suppose you had a huge list of people,
places, or things to select from, like 100,000 people or more
sampling interval /do it systematically and use the entire list
select every 10th, 20th, or 30th person from that list .
exp :system satisfaction amond patients in X hospital
Cluster (area) random sampling : you have a population that is dispersed
across a wide geographic region. This method allows you to divide this population into clusters (usually counties, census tracts, or other boundaries) and then randomly sample everyone in those clusters
exp: IQ survey in IRAN
multi-stage sampling : systematically sample within your
clusters
exp:depression prevalence in medical students in IRAN
refers generally to any mixing of sampling methods
You're not reducing bias or anything by increasing sample size, only coming closer to the total number in the population
LETS PRACTICE!!! 1: esophageal cancer prevalence in iran 2: MI prevalence in Tehran 3:height and weight in high schools in
IRAN