By:Shirin Maghsoud SAMPLING

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 to generalize to? Groups in Sampling

How can you get access to them? Groups in Sampling The Theoretical Population The Study Population

Groups in Sampling The Theoretical Population The Study Population The Sampling Frame

Who is in your study? The Theoretical Population The Study Population The Sampling Frame

The Theoretical Population The Study Population The Sampling Frame The 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 practical Beeing practical small population of interest: sample about % 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 population exp;phone survey :phone book two basic approaches to sampling: probabilistic and nonprobabilistic exp: randomization, or random selection : how you draw the sample each 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 Sampling List of Residents Random Subsamples of n/N Strata surgicalNon-clinicalmedical

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