Sampling methods Flashcards
define population
the whole set of items that are of interest
define census
observes/measures every member of a population
define sample
a selection of observations taken from a subset of the population => used to find out info abt the population as a whole
what are the advantages and disadvantages of a census?
advantage
- accurate results
disadvantages
- time consuming + expensive
- hard to process large quantity of data
what are the advantages and disadvantages of a sample?
advantages
- less time consuming + expensive than census
- less data to process than census
disadvantages
- data may not be accurate
- sample may not be large enough to give info abt small sub-groups of the population
define sampling units
individual units of a population
eg - each student
define sampling frame
sampling units of a population = individually named/numbered to form a list
what are the 5 sampling methods?
RANDOM SAMPLING
- simple random
- systematic
- stratified
NON RANDOM SAMPLING
- quota
- opportunity
define random sampling
every member of population has an equal chance of being selected
SAMPLING FRAME NEEDED
- representative of whole population
- unbiased
- define simple random sampling
- method?
- advantages and disadvantages?
- every member of population has an equal chance of being selected
- number students 1-300
use a calculator/computer to randomly generate 60 (sample size wanted) unique numbers between 1 and 300
people corresponding with these numbers are chosen - advantages
- bias free
- easy + cheap to implement for small populations
- each sampling unit has a known and equal chance of selection
disadvantages
- sampling frame needed
- not suitable for large populations
- may be missing values in the sample => data not available for particular day
- define systematic sampling
- method?
- advantages and disadvantages?
- required items = chosen @ regular intervals from an ordered list
- number students 1-300
300 / 60 = 5 (total population / sample size wanted)
use a calculator/computer to randomly generate a start point between 1 and 5 and take every 5th student from that point - advantages
- simple + quick to use
- suitable for large samples
disadvantages
- sampling frame needed
- can introduce bias if sampling frame = not random
- may be missing values in the sample => data not available for particular day
- define stratified sampling
- method?
- advantages and disadvantages?
- population = divided into mutually exclusive strata (male + female) + simple random sample = carried out in each strata
2. number boys 1-100 number girls 1-200 total population = 300 SAMPLE SIZE / TOTAL POPULATION x NO. IN STRATA
60/300 x 100 = 20 boys
60/300 x 200 = 40 girls
use simple random sampling to choose 20 unique no.s between 1 and 100 for the boys and 40 unique no.s between 1 and 200 for the girls
- advantages
- sample accurately reflects population structure
- guarantees proportional representation of groups within population
disadvantages
- sampling frame needed
- population must be clearly classified into distinct strata
define non random sampling
may be used if population = large
x sampling frame
may be biased
- define quota sampling
- method?
- advantages and disadvantages?
- interviewer/researcher selects a sample that reflects characteristics of whole population
2.
total population = 300
SAMPLE SIZE / TOTAL POPULATION x NO. IN STRATA
60/300 x 100 = 20 boys
60/300 x 200 = 40 girls
the interviewer/researcher chooses students to interview until these quotas (of 20 boys + 40 girls) = filled.
3.
advantages
- field work = done quickly as representative sample can be achieved w small sample size
- x sampling frame needed
- quick, easy + inexpensive
- easy comparison between diff groups in population
disadvantages
- non random => can introduce bias
- population must be divided into groups => costly/inaccurate
- non-responses = not recorded
- ↑ scope of study ↑ no. of groups => time + expense
- define opportunity sampling
- method?
- advantages and disadvantages?
- sample taken from ppl who = available @ time of study + who meet criteria
- interviewer/researcher chooses 60 students to interview
3. advantages - easy to carry out - inexpensive - x sampling frame
disadvantages
- unlikely to provide representative sample => likely to be biased
- highly dependant on individual researcher
