Sampling methods Flashcards
Population
the whole set of items that are of interest
sample
some subset of the population intended to represent the whole population
sampling unit
each individual thing in the population that can be sampled
sampling frame
sampling units of a population which are individually named or numbered to form a list
census
data collected from the entire population
adv of census
should give completely accurate result
adv of sample
cheaper
quicker
less data to process
disadv of census
time consuming
expensive
cannot be used when testing involves destruction
large volume of data to process
disadv of sample
data may not be accurate
data may not be large enough to represent small sub-groups
what is simple random sampling
when every sample has an equal chance of being selected
method for simple random sampling
in the sampling frame, each item has an identifying number. a random number generator is used.
adv of simple random sampling
bias free
easy and cheap to implement
each number has a known equal chance of being selected
disadv of simple random sampling
not suitable when population size is big
sampling frame needed
methods for selecting a random number
calc
computer
rando number tables
how to use calc for simple random samp
use RAN# and multiply given number by population size.
ignore the decimal
what is systematic sampling
required elements are chosen at regular intervals in a ordered list
formula for systematic sampling
pop size / samp size = K
start at a random item between 1 and K
adv of systematic sampling
simple
quick to use
suitable for large populations/samples
disadv of systematic sampling
sampling frame again needed
can introduce bias if sampling frame not needed
what is stratified sampling
population divided in groups(strata) and a simple random sample carried out in each group
samp/pop size sampled from each strata
when is stratified sampling used
when sample is large and population naturally divides into groups
adv of stratified samp
reflects population structure
guarantees proportional representation of groups within population
disadv of stratified samp
population must be clearly classified into distinct data
Selection within each stratum suffers from same disadvantages as simple random sampling.
Quota sampling
The population is divided into groups (gender,age, etc)
A given number (quota) is surveyed in each group
This type of sampling is not random and is used in market research
Adv of quota
Cheap
Easy and quick to carry out
Convenience sampling
Choosing a sample that is given/seen first. For an example, a sample of 60 could be the first 60 people in a lecture to attend.
Adv of convenience
Quick/time efficient
Instant results
Disadvantage of convenience
Highly likely to be biased and unrepresentative
Can lead to errors
Unreliable
