Math 50: 2.2: Systematic, Stratified, and Cluster Sampling

Stratified Sampling

divides the population into groups called strata. - CHARACTERISTIC, NOT geographically. -EXAMPLE: the population might be separated into males and females. -A sample is taken from each of these strata using either random, systematic, or convenience sampling. EXAMPLE: As part of a study at an American community college, 25 students are randomly selected from African American freshmen​ 38 students are randomly selected from Asian American freshmen​, 29 students are randomly selected from Latino freshmen​, 110 students are randomly selected from Caucasian freshmen​, 2 students are randomly selected from Native American freshmen​, and 4 students are randomly selected from Pacific Islander freshmen. -ethnicity is a stratum from which individuals were randomly selected to form a sample.

Cluster Sampling

dividing the POPULATION in groups and selecting ALL the individuals in RANDOMLY selected groups. -dividing the population into groups -- usually geographically. -These groups are called clusters or blocks. -The clusters are randomly selected, and each element in the selected clusters are used.

Simple Random Sampling

putting everyone's name into a hat and drawing out several names. -Each element in the population has an equal chance of occuring. EXAMPLE: A software company wants to administer a satisfaction survey to its current customers. Using their customer​ database, the company randomly selects 60 customers and asks them about their level of satisfaction with the company. -used because the people surveyed were RANDOMLY selected from the ENTIRE POPULATION of customers.

Convenience Sample

selecting individuals who are easily contacted or those who voluntarily respond. - very easy to do, but it's probably the worst technique to use. -Readily available data is used. -That is, the first people the surveyor runs into. -EXAMPLE: A newspapernewspaper asks its readers to write in their opinion regarding the number of books they have read this month. - the easiest method of collecting data is to ask readers to submit the data themselves by writing in.

Systematic Sample

RANDOMLY selecting an individual and then selecting EVERY KTH individual from the POPULATION after the first one. -the list of elements is "counted off". -Every kth element is taken. -Similar to lining everyone up and numbering off "1,2,3,4; 1,2,3,4; etc". -When done numbering, all people numbered 4 would be used. -EXAMPLE: To estimate the percentage of defects in a recent manufacturing​ batch, a quality control manager at Sony selects every 17th music CD that comes off the assembly line starting with the ninth until she obtains a sample of 70 music music CDS .

To perform systematic​ sampling,

we randomly select an individual out of the first K individuals and also select every KTH individual AFTER the FIRST selected individual.