Chapter 6 Sampling

Population Group of individuals defined along some characteristic or set of characteristics –Superintendents –Music teachers in Quincy public schools –4 th grade students 4 th grade students in Quincy –4 th grade students in Quincy parochial schools »4 th grade students in Quincy St. Peter’s school »4 th grade student in Ms. Jones’ class at St. Peter’s

Population Target population – desired population Accessible population – feasible/practical population Target population: All 4 th graders Accessible population : 4 th graders at St Peter’s

Sample Subset of a Population Individuals in the Sample are the participants in the study Population (in the green) A person Sample (in the small circle)

Choosing the sample Random –Every person in the population has an equal chance of being selected –Best way to achieve “representative sample” –Difficulty is truly achieving a random sample

Simple Random Sample Sampling or selection is done by simply randomly selecting one member of the population, then another, then another, etc… Until the desired sample size is achieved Simple example: 100 people in the population, put their names in hat, and draw 20.

Simple Random Sample More likely methods: –Use computers –Use table of random numbers and a list

Sample Size The larger, the better - no magic number, a matter of confidence in the results But for simple random samples: 95% confidence interval +/-3%: will represent 2000 to 20, will represent 200 million 250 will be needed to represent 400

Stratified Random Sampling Every person still has an equal chance of being selected Select the sample based upon one or more characteristics – strata Determine the stratum or strata Determine their proportions Select persons from the strata to create a sample that is consistent with the strata proportions

Stratified Random Sampling Strata are gender and race –In Population, 60% women, 70% white –Thus you have four groups White women – 42% Minority Women - 18% White men – 28% Minority men – 12% –Want 200 sample size. Select: 84 White women from all the white women in the population 36 Minority women from all the minority women in the pop 56 White men from all the white men in the pop 24 Minority men from all the minority men in the pop Another example next slide:

Middle school students 7th, 8th, 9th –Population th graders 1500/3700 = 41% th graders 1200/3700 = 32% th graders 1000/3700 = 27% 100 students –41 are 7 th graders –32 are 8 th graders –27 are 9 th graders Helps creates representation, but is more work Stratified Random Sampling

Cluster Sample Still Random – every person in population has equal chance of being chosen But you are sample groups (clusters) of people rather than individuals Examples: 8 elementary schools in Quincy –Randomly select 2 3 sections of Ed Psych –Randomly select 1 Simpler than simple random sample but risk an non-representative sample

Two – Stage Random Sampling Combination of cluster and simple Conduct a cluster sampling Randomly select participants from the selected clusters 8 elementary schools, cluster sampling of 3 of the schools, then randomly select 30 students from school

Non-random sampling Systematic Sampling –Using a list, select a starting point at random and then choose every Nth person on list – determined by percent needed for sample –1000 names, sample size of 250 or one-fourth (25%). –Select a starting point at random, the select every 4 th name.

Systematic Sampling List must not be ordered in along any particular characteristic that might affect result If intelligence might be related to the dependent variable, then don’t use a list that ordered by GPA.

Representation Is the sample representative of the population? The terms for this are: –Generalization –External Validity Example: –Results from 4 th graders in Adams Generalize to all 4 th graders in Quincy? Generalize to all 4 th graders in Illinois? Ecological Generalizabilty – next slide

Ecological Generalizabilty Degree to which the results of the study can generalize to other situations or conditions. 4 th graders = 3 rd graders? Live skits = video? Urban = rural?

Random is best The use of random sampling is strongest method to provide external validity or generalization. True Random sampling is often not possible Replication becomes more important

End chapter 6