Unit 3 - collecting data

sample survey

a study that collects data from a sample to learn about the population from which the sample was selected

random sampling

chance process → determines which members of a population re included in the sample

observational study

• observes individuals and measures variables

• does NOT influence the responses (no treatments)

retrospective observational study

uses existing data while observing individuals

prospective observational study

tracks individuals into the future while observing them

experimental study

• deliberately imposes treatments (conditions) on individuals to measure their responses

• can provide convincing evidence for cause and effect

simple random sampling (SRS)

• equal chance to be selected

• how to choose an SRS:

  • label the individuals

  • use technology or “names in hat”

  • choose the individuals that correspond to the #s chosen

sampling without replacement

an individual from a population can be selected only once

sampling with replacement

an individual from a population can be selected more than once

stratified random sampling

selects a sample by choosing an SRS from each stratum and combining them into one overall sample

cluster

a group of individuals in the population that are located near each other

cluster random sampling

• randomly chooses clusters

• includes all members of the selected cluster

• Ex. cluster by high school → randomly select 2 high schools → CS + WB → survey all seniors at the two schools

systematic random sampling

• selects a sample from an ordered arrangement of the population

• randomly selects one of the first k individuals + choosing every kth individual after

• ex - names listed alphabetically → choose the 10th name → then choose every 10 names after