1.1 Data collection Flashcards
What is a population
The whole set of items that are of interest.
What is a census
Observes or measures every member of a population.
What are advantages of a census
It should give completely accurate results.
What are some disadvantages of a census
- Time consuming and expensive
- Cannot be used when the testing process destroys the item (e.g. census of the ripeness of avocados you would have to cut them all open)
- Hard to process large quantities of data
What is a sample
A selection of observations taken from a subset of the population which is used to find out information about the population as a whole.
What are some advantages of a sample
- Less time consuming and expensive than a census
- Fewer people have to respond
- Less data to process than a census
What are some disadvantages of a sample
- The data may not be accurate
- The sample may not be large enough to give information about small sub-groups of the population.
What is a sampling unit
Individual units of a population.
What is a sampling frame
The list of sampling units which are either numbered or named.
What is random sampling
Every member of the population has an equal chance of being selected, so the sample should be representative of the population as a whole.
What are the 3 types of random sampling
- Simple random sampling
- Systematic sampling
- Stratified sampling
What is a simple random sample
A simple random sample of size n is one where every sample of size n has an equal chance of being selected.
E.g. 100 members of a yacht club are listed alphabetically in the club’s membership book. Explain how you would take a simple random sample of 12 members
- Allocate a number from 1 to 100 to each member of the yacht club.
- Use a random number generator to generate 12 random numbers between 1 and 100.
- Select the people corresponding to these numbers. This is your sample.
What is systematic sampling
- The required elements are chosen at regular intervals from an ordered list.
- E.g. If a sample of size 20 was required from a population of 100, you would take every 5th person since 100/20 = 5.
- The first person is to be chosen at random.
- E.g. of the first person chosen is number 2, the next would be 7, 12, 17, etc.
What is stratified sampling
- The population is divided into mutually exclusive strata (e.g. males and females) and a random sample is taken from each.
- Formula: number sampled in stratum = (number in stratum/ number in population) x sample size.
What are some advantages of simple random sampling
- Free of bias
- Easy and cheap to implement for small populations and sample sizes
- Each sampling unit has a known and equal chance of selection
What are some disadvantages of simple random sampling
- Not suitable when the population size or sample size is large as it is potentially time consuming, disruptive and expensive.
- A sampling frame is needed (not suitable for sampling trees in a forest).
What are some advantages of systematic sampling
- Simple and quick to use
- Suitable for large samples and populations
What are some disadvantages of systematic sampling
- A sampling frame is needed
- Can introduce bias if the sampling frame is not random
What are some advantages of stratified sampling
- Sample accurately reflects the population structure
- Guarantees proportional representation of groups within a population
What are some disadvantages of stratified sampling
- Population must be clearly classified into distinct strata
- Selection within each stratum suffers from the same disadvantages of simple random sampling
What are the 2 types of non-random sampling
- Quota sampling
- Opportunity sampling
What is quota sampling
An interviewer or researcher selects a sample that reflects the characteristics of the whole population.
What is opportunity sampling
- Taking a sample from people who are available at the time.
- The study is carried out on people who fit the criteria you are looking for.
