1 Data Collection and LDS Flashcards
What is a population?
A collection of all of the items we are interested in.
What is a sample?
A subset of items chosen from a population.
What is a sampling unit?
Each individual item in a sample.
What is a sampling frame?
A list containing all sampling units, often numbered or individually named.
What is a census?
Observing and measuring information from every member of a population.
Advantages of census
Should give completely accurate result
Disadvantages of census
Time consuming and expensive, can’t be used when testing involves destruction, large volume of data that is hard to process
Advantages of sample (compared to census)
Less expensive, less time consuming and less data to process
Disadvantages of sample (compared to census)
Data may not be accurate, sample may not be large enough to represent small sub-groups of population
How is a simple random sample carried out?
Using X as a sampling frame. Assign each sampling unit a number from 1 to N. Use a random number generator to select ‘n’ unique numbers. Choose the items corresponding to the numbers to form your sample.
What is a simple random sample?
A sample where every item in the sample has an equal chance of being selected.
Advantages of simple random sampling
Bias free, easy and cheap for small populations and samples, each sampling unit has equal chance of selection.
Disadvantages of simple random sampling
not suitable for big populations, sample may not be accurate for whole population, sampling frame needed.
What is a systematic sample?
A sample where the required elements are chosen at regular intervals from an ordered list. (first item chosen at random)
How is a systematic sample carried out?
Using X as a sampling frame. Assign each item a number from 1 to N. Randomly select the first item using a random number generator to select the first item between 1 and K and select every kth item to form sample. (k = population size / sample size)
Advantages of systematic sampling
Simple and quick to use, suitable for large samples and populations.
Disadvantages of systematic sampling
Can introduce bias if sampling frame is small and not random as patterns can be picked up in the data, sampling frame needed
What is a stratified sample?
A sample which is proportional to the number of items in each stratum/group
How is a stratified sample carried out?
Calculate proportion of each group required in sample. Within each group, assign each sampling unit a number. Use a random number generator to select amount of ‘unique’ numbers required. Choose the items corresponding to the numbers to form your sample.
Advantages of stratified sample
Accurately reflects population structure, guarantees proportional representation of groups within population.
Disadvantages of stratified sample
Sampling frame needed and population must be clearly classified into distinct strata, selection within each stratum suffers from same disadvantages as simple random sampling.
What is quota sampling?
A sample chosen to reflect the proportion of characteristics of the whole population. Quotas in each group try to reflect the group’s proportion in whole population.
How is a quota sample carried out?
Population is divided into groups according to characteristic. Create each quota (same proportion of sample as proportion of population). Select the sampling units until quotas are full. Once quota is full, ignore subsequent sampling units that also match that characteristic.
Advantages of quota sample
Allows a small sample to still be representative of population, no sampling frame required, relatively quick, easy and inexpensive
Disadvantages of quota sample
Non-random sapling can introduce bias, population must be divided into groups which may be costly or inaccurate, can depend on knowledge of researcher
What is opportunity sampling?
Sample taken from people who are available at time of study that meet criteria.
How is opportunity sampling carried out?
Select first ‘n’ amount of people who fit criteria to form sample
Advantages of opportunity sampling
Easy to carry out, inexpensive
Disadvantages of opportunity sampling
Unlikely to provide a representative sample, highly dependent on knowledge of individual researcher
What is qualitative data?
Non-numerical data e.g. colour
What is quantitative data?
Numerical values
What is discrete data?
Data that only takes specific values e.g. shoe sizes, number of children
What is continuous data?
Data that can take any decimal value e.g. height, weight
Why may class intervals be inaccurate?
Use of the midpoint of class intervals assumes values are evenly distributed within the interval, which may not be accurate.
Name the 5 UK weather stations
Leuchars, Leeming, Heathrow, Hurn, Camborne
Name the 3 International weather stations
Jacksonville, Beijing, Perth
Leuchars general info
Most northern in UK, lowest average temperatures
Leeming general info
Second most northern in UK, sheltered location leads to dry, almost semi-arid, climate
Heathrow general info
Far from city so temperatures not raised by ‘urban heat island’ effect, below average rainfall for Britain, relatively hot summer temperatures due to southerly latitude and close proximity to continental Europe
Hurn general info
Close to Southern coast, rainfall well below national average
Camborne general info
Most southern in UK, mildest and sunniest UK climate due to southern location and warm water from Gulf stream. Sea moderates extreme temperatures in summer and winter but extreme rainfall is not uncommon
Beijing general info
Humid and continental climate. Lower latitude than UK, humid summers due to East Asian monsoon, cold and windy but dry winters due to Siberian anticyclone
Jacksonville general info
Humid and subtropical climate. Low lying so winters are mild and sunny. Summers are hot, very humid and prone to thunderstorms.
Perth general info
Hot summer and Mediterranean climate, winters cool and wet, summers are hot dry and sunny. Some summer rainfall.
What happened October 16 1987?
A large storm hit the UK.
What is wind direction?
The direction the wind is blowing FROM
What is the meaning of tr?
Trace meaning values of rainfall less than 0.05mm
What value should be used if tr is being used in a calculation?
0.025
What is total rainfall?
Total precipitation that falls in a 24 hour period measured in mm
What is maximum gust?
Highest instantaneous windspeed measured in knots.
What are the units for windspeed?
Knots and also a corresponding description given on Beaufort Scale.
What does n/a mean?
Data not available and so should be discounted from sample.
What is total sunshine measured in?
Nearest 1/10th of an hour
What is humidity?
The % of air saturation with water vapour.
What is mean pressure measured in?
Hectopascals
What is mean visibility?
How far can be seen into horizon during daylight hours measured in decametres. 1Dm = 10m
What is mean cloud cover measured in?
Oktas which means the number of 1/8ths of the sky is covered from 0 to 8. There are 9 possible options (important in probability qs)
How many possible options on a compass?
16
What happened October 2015 in Perth?
Perth had warmest October since records began
What happened October 1987 and 2015 in Beijing?
Beijing significantly colder in October relative to May-September
