sampling methods

population

entire group of individuals we want information about

sample

a subset of individuals in the population form which we actually collect data

census

collects data from every individual in the population

sample survey

study that collects data from a sample that is chosen to represent a specific population

convenience sample

members of this sample are chosen in a way that is easy to the person doing the choosing — individuals are easily accessible

voluntary response sampling

allows people to choose to be in the sample by responding to general invitation

how does a study show bias

design of a study shows bias if it is very likely to underestimate or overestimate the value you want to know

random sampling

involves using a chance process to determine which members of a population are included in the sample

simple random sample

a group of n individuals in the population has an equal chance to be selected as the sample

how to select a simple random sample

1. label each person in population from 1 to n

2. use a random number generator to obtain n unique integers from 1 to N

3. choose the individuals that correspond to the randomly selected integers

stratified sample

chosen by dividing the population into homogeneous groups based on similar characteristics then randomly choosing the same number of individuals from each group

• take same proportion/percent

systematic sample

chosen by doing a methodical selection of subjects starting at a randomly selected subject

• every nth item

cluster

group of individuals in the population that are close to each other (locationally)

cluster sample

chosen by dividing the population into heterogenous groups based on location then randomly choosing one or more of those entire groups

undercoverage

occurs when some members of populations are less likely to be chosen or cannot be chosen in a sample

nonresponse

occurs when an individual chosen for the sample can’t be contacted or refuses to participate

response bias

occurs when there is a systematic pattern of inaccurate answers to a survey question

• wording of questions