Populations and Samples Flashcards
What is a census
A census observes or measures every member of a population
What is a sample
A sample is a selection of observations taken from a whole subset of the population which is used to take information about the population as a whole
What are the advantages of a census
-It should give a completely accurate result
What are the disadvantages of a census
- Time consuming and expensive
- Cannot be used when the testing process destroys the item
- Hard to process large quantity of data
What are the advantages of a sample
- Less time consuming and expensive than a census
- Fewer people have to respond
- Less data to process than in a census
What are the disadvantages of a sample
- The data may not be as accurate
- The sample may not be large enough to give information about small sub-groups of the population
What are three methods 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
What is a systematic sample
In a systematic sample, the required elements are chosen at regular intervals from an ordered list
What is a stratified sample
In stratified sampling, the populations is divided into mutually exclusive strata (men and women etc etc) and a random sample is taken from each
What is the equation for a systematic sample
Population/sample size = Intervals
What is the equation for a stratified sample
Number sampled in a stratum = (No in stratum/No in population) * overall sample size
What are the 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 the disadvantages of simple random sampling
- Not suitable when the population size or the sample size is large
- A sampling frame is needed
What are the advantages of systematic sampling
- Simple and quick to use
- Suitable for large samples and large populations
What are the disadvantages of systematic sampling
- Population must be clearly classified into distinct strata
- Selection with each stratum suffers from the same disadvantages as simple random sampling
What are the advantages of stratified sampling
- Sample accurately represents the population structure
- Guarantees proportional representation of groups within a population
What are the disadvantages of stratified sampling
- Population must be classified into distinct strata
- Selection with each stratum suffers from the same disadvantages as simple random sampling
What are the two types of non-random sampling
Quota sampling or opportunity sampling
What is quota sampling
In quota sampling, an interviewer or researcher selects a sample that reflects the characteristics of the whole population
What is opportunity sampling
Opportunity sampling consists of 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
What are the advantages of quota sampling
- Allows a small sample to still be representative of the population
- No sampling frame required
- Quick, easy and inexpensive
- Allows for easy comparison between different groups within a population
What are the disadvantages of quota sampling
- Non-random sampling can introduce bias
- Population must be divided into groups, which can be costly or inaccurate
- Increasing scope of study increases the number of groups, which adds time and expense
- Non-responses are not recorded as such
What are the advantages of opportunity sampling
- Easy to carry out
- Inexpensive
What are the disadvantages of opportunity sampling
Unlikely to provide a representative sample
-Highly dependent on individual researcher
What is quantitative data
Data associated with numerical observations e.g shoe sizes
What is qualitative data
Data associate with non-numerical observations e.g Hair colour
What is continuous data
A variable that can take any value within a given range e.g time 2 seconds, 2.1 seconds, 2.01 seconds
What is discrete data
A variable that can only take a few specific values in a given range e.g number of people in a family, can’t have 3.4 members