S3 Ch2 Flashcards
Advantages of a census?(1)
It should give a completely accurate result
Disadvantages of a census?(3)
- time consuming and expensive
- cannot be used when the testing process is to destruction
- info is difficult to process because there is so much of it
Advantages of sample survey?(4)
- cost is low
- results are obtained quickly
- less people have to respond
- less data to deal with
Disadvantages of sample survey?(2)
- data not as accurate as a census
- sample may not be large enough to give info about small sub groups of population
What is a simple random sample?
Equal chance of being selected, set size of sample.
Sampling frame?
List identifying every single sampling unit that could be included in the sample
When is systematic sampling used?
When the population is too large for simple random number sampling.
Advantages of systematic sampling?(2)
Simple
Suitable for large samples
Define census?
A census observes or measures every member of a population
Define population?
A whole set of items that are of interest
Advantages of random number sampling? (3) and a disadvantage? (1)
- easy to use
- each number has a known equal chance of selection
- the numbers are truly random and free from bias
- not suitable where the population size is large
3 advantages and 2 disadvantages of lottery sampling?
- tickets drawn at random
- easy to use
- each ticket has a known chance of selection
- not suitable for large population
- sampling frame is needed
What is lottery sampling?
Each member can be identified by a name or number and this is put on a ticket
Tickets (all same size and shape) are put into a container and drawn one at a time without replacement
Learn random number sampling
Now
What is systematic sampling?
The required elements are chosen at regular intervals from an ordered list
Systematic sampling interval equation?
K = (population size)/(sample size)
Advantages of systematic sampling? (2)
- simple to use
- suitable for large samples
Disadvantages of systematic sampling? (2)
- only random if ordered list is truly random
- can introduce bias
What is stratified sampling?
- population is divided into mutually exclusive strata and a random sample is taken from each
Advantages of stratified sampling? (2)
- reflects population structure
- can give more accurate estimates than simple random sampling where there are clear strata present
When is stratified sampling used?
Sample is large
Population divides naturally into mutually exclusive groups
Disadvantages of stratified sampling? (2)
- within strata problems are the same as for any simple random sample
- if strata are not clearly defined they may overlap
When is systematic sampling used?
When the population is too large for simple random number sampling
What is quota sampling?
Population divided into groups in terms of gender, age etc.
Number of people in each group I set to try and reflect the groups proportion in the whole population
Interview selects sampling units
Advantages of quota sampling? (3)
- enables fieldwork to be done quickly because a representative sample can be achieved with a small sample size
- costs are low
- easy to do
Disadvantages of quota sampling? (2)
- interviewer bias in who is included
- not possible to estimate the sampling errors (process not random)
What is primary data?
Data that is collected by the person who is going to use the data
What is secondary data?
Data that is neither collected by, nor on behalf of, the person who to use the data
Advantages of primary data? (3)
Collection method is known
Accuracy is known
Exact data needed are collected
Disadvantage of primary data? (1)
Costly in time and effort
Advantages of secondary data? (3)
Cheap to obtain
Large quantity of data is available (eg. Internet)
Much of the data has been collected for years and can be used to plot trends
Disadvantages of secondary data? (2)
Bias not always recognised
Can be in a form that is difficult to deal with
What is a sample?
A selection of observations taken from a sub set of a the population.
