Sampling

Sample

a subgroup of the population (in the study)

a ____ serves as the reference group to estimate characteristics of & to draw conclusions about the population

sample

target population

overall group to which findings will be generalized (theoretical population)

accessible population

persons who have an actual chance to be selected, who are available

why use sampling methods?

• practical

• cost effective

• convenient

• manageable

• more accurate (potentially)

sampling error

difference btwn sample values (statistics) and population values (parameters)

error in statistics doesn’t mean a mistake but refers to ____

unexplained variance

sampling bias

occurs when individuals selected over or under-represent certain population attributes that are related to the phenomenon under study

conscious sampling bias

sample selected purposefully

unconscious sampling bias

geographic or time of recruitment excludes persons not in area or perhaps working

a ___ sample does not automatically control for bias or equal better population representation

larger

develop a sampling plan

• define the population of interest (including inclusion & exclusion criteria)

• select a sampling method

• determine a sample size

• invest full on effort early to execute sampling plan

inclusion criteria

the primary traits of the target & accessible populations that will make someone eligible to be a participant

exclusion criteria

factors that would preclude someone from being a subject

probability sampling

every member of the population has an equal opportunity (probability) of being selected

probability sampling is best used to…

• limit bias

• minimize sampling errors

nonprobability sampling

chosen on some bias other than random selection

simple random sampling

each member of the defined accessible population has an equal chance of being selected

systematic sampling

Persons are randomly chosen from unordered lists using a fixed sampling interval. The interval is determined by the desired sample size & the number of persons on the list

stratified random sampling

a particular sample characteristic is used to partition members of a population into nonoverlapping subsets or strata, & random samples are drawn from each stratum based on the proportion of the population w/in each stratum

cluster sampling

w/ larger populations, samples are randomly chosen at multiple stages

disproportional sampling

a form of stratifies sampling, disproportional sampling is used when certain strata are underrepresented in the population, which leads to their having small representation w/in the total sample

types of nonprobability sampling

• convenience/accidental sampling

• quota sampling

• purposive sampling

• snowball sampling

convenience/accidental sampling

subjects are chosen on the bias of their availability

quota sampling

a form of stratified sampling that is not random, where subjects are recruited to represent various strata in proportion to their number in the population

purposive sampling

subjects are hand-picked & invited to participate bc of known characteristics

snowball sampling

small numbers of subjects are purposively recruited & they help to identify other potential participants thru networking

sample size

the number of participants who ultimately participate in the study

n

number

____ directly affects the statistical power of a study

sample size

power

probability that statistical test will show a relationship or difference when it actually exists