AS Statistics: 1 Data Collection Flashcards
What is a population?
The whole set of items that are of interest.
What is a census?
A measure of every member of a population.
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 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 a 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 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 sampling units?
Individual units of a population.
What is a sampling frame?
When sampling units are individually named or numbered to form a list.
What is simple random sampling?
Where every sample has an equal chance of being selected.
What are the advantages of simple random sampling?
- Free of bias.
- Easy and cheap to implement for small populations and small samples.
- Each sampling unit has a know 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 is systematic sampling?
When elements are chosen at regular intervals from an ordered list.
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?
- A sampling frame is needed.
- It can introduce bias if the sampling frame is not random.
What is stratified sampling?
When the population is divided into mutually exclusive strata (e.g. males and females) and a random sample is taken from each.
What are the advantages of stratified sampling?
- Sample accurately reflects the population structure.
- Guarantees proportional representation of groups within a population.
What are the disadvantages of stratified sampling?
- Population must be clearly classified into distinct strata
- Selection within each stratum suffers from the same disadvantages as simple random sampling.
What is quota sampling?
An interviewer or researcher selects a sample that reflects the characteristics of the whole population.
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 increase number of groups, which adds time and expense.
- Non-responses are not recorded as such.
What is 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.
Sometimes called convenience sampling.
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 are quantitative variables/data?
Variables/data associated with numerical observations.
What are qualitative variables/data?
Variables/data associated with non-numerical observations.
What are continuous variables?
Variables that can take any value in a given range.
What are discrete variables?
Variables that can take only specific values in a given range.
What can we tell from classes in frequency tables?
- Class boundaries tell you the max and min values that belong to each class.
- The midpoint is the average of the class boundaries.
- The class width is the difference between the upper and lower class boundaries.
What does the large data set show?
Consist of weather data samples provided by the Met Office for 5 UK weather stations and 3 overseas over 2 time periods.
What were the time periods of the large data set?
May - Oct 1987.
May - Oct 2015.
Where were the large data set samples taken?
- Leuchars.
- Leeming.
- Heathrow.
- Hurn.
- Camborne.
- Jacksonville.
- Perth.
- Bejing.
What variables were measured as part of the large data set in the UK?
- Daily mean temperature (ᵒC).
- Daily total rainfall (mm).
- Daily total sunshine.
- Daily mean wind direction and wind speed (in knots: 1kn = 1.15mph) (also categorized according to the Beaufort scale).
- Daily maximum gust (kn).
- Daily maximum relative humidity (%).
- Daily mean cloud cover (oktas).
- Daily mean visibility (Dm).
- Daily mean pressure (hPa).
What variables were measured as part of the large data set overseas?
- Daily mean temperature (ᵒC).
- Daily total rainfall (mm).
- Daily mean pressure (hPa).
- Daily mean windspeed.
