statistics Y1 Flashcards
Population
the whole set of items that are of interest
Census
observes or measures every member of a population
Sample
a selection of observations taken from a subset of the population which is used to find out about the population as a whole
Advantages and disadvantages of a census
Advantages:
- gives a completely accurate result
Disadvantages:
- time consuming and expensive
- cannot be used when testing process destroys the item
- hard to process (as a large sample)
Advantages and disadvantages of using a sample
Advantages:
- less time consuming and expensive
- fewer people have to respond
- less data to process
Disadvantages:
- data may not be as accurate
- sample may not be large enough to give information about subgroups
Sampling units
Individual units of a population
Sampling frame
The list of sampling units named or numbered individually
Random sampling advantages
Every sample has an equal chance of being selected, meaning it is representative and removes bias
3 methods of random sampling
- simple random sampling
- systematic sampling
- stratified sampling
Simple random sampling
- need a sampling frame
- either generate random numbers corresponding to sampling frame or lottery sampling
- every sample has an equal chance of being selected
Systematic sampling
- required elements are chosen at regular intervals from an ordered list
e.g. if a sample of size 20 was needed from a population of 100, you would take every fifth person, starting at a random number from 1 to 5
Stratified sampling
The population is divided into mutually exclusive strata (e.g. male and female)
and a random sample is takes for each
number sampled in a stratum =
number in stratum/number of population * overall sample size
Adv and disadv of simple random sampling
adv:
-free of bias
-cheap and easy to implement for small populations and samples
- each sampling unit has a known and equal chance of selection
disadv:
- not suitable when population size or sample size is large as potentially time consuming, disruptive and expensive
Adv and disadv of systematic sampling
adv:
- simple and quick to use
- suitable for large samples and large populations
Disadv:
- sampling frame is needed
- can introduce bias if sampling frame is not random
Adv and disadv of stratified sampling
adv:
- sample accurately reflects the population structure
- guarantees proportional representation of groups within a population
Disadv:
- population must be clearly classified into distinct strata
- selection within each stratum suffers from the same disadvantages as simple random sampling
Two types of non-random sampling
- quota sampling
- opportunity sampling
Quota sampling
an interviewer or researcher selects a sample that reflects the characteristics of the whole population
- the population is divided into groups according to a given characteristic. The size of each group determines the proportion of the sample that should have the characteristic
- an interviewer would meet people, assess their group and allocate them to the appropriate quota
- continues until all the quotas have been filled
Opportunity sampling
taking the sample from people who are available at the time the study is carried out and who fit the criteria you are looking for
Quota sampling adv and disadv
Adv:
- allows small sample to still be representative of population
- no sampling frame requires
- quick, easy, cheap
- allows for easy comparison between different groups
Disadv:
- can introduce bias
- division into groups can be costly or inaccurate
- increasing scope of study increases number of groups, increasing time and expense
- non-responses are not recorded as such
Opportunity sampling adv and disadv
Adv:
- easy to carry out
- inexpensive
Disadv:
- unlikely to produce a representative sample
- highly dependent on individual researcher
Discrete variable
A variable that can only take specific values in a given range (e.g. you cannot have 2.3 children)
Continuous variable
A variable that can take any value in a given range
