By:Shirin Maghsoud SAMPLING. Sampling is the procedure a researcher uses to gather people, places,...

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 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 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

By:Shirin Maghsoud SAMPLING. Sampling is the procedure a researcher uses to gather people, places,...

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