Statistics 1 Flashcards
Population
The whole set of items that are of interest
Raw data
Unprocessed information
Census (purpose)
Observe or measure every member of a population
Sample
A selection of observations taken from a subset of the population, used to find out information about the population as a whole
Census (adv vs dis)
Advantages:
- should give a completely accurate result
Disadvantages:
- Time consuming
- Expensive
- Can’t be used when testing process destroys the item
- Hard to process large quantities of data
Sample (adv vs dis)
Advantages:
- Less time consuming and expensive
- Fewer people have to respond
- Less data to process
Disadvantages:
- Data may not be as accurate
- Sample may not be large enough to give information about small sub-groups of the population
Size of sample -> validity of conclusions
• Generally, the larger the sample, the more accurate it is, but you will need greater resources
• If population is very varied, larger sample needed than if the population were uniform
• Different samples can lead to different conclusions due to the natural variation in a population
Sampling units
Individual members of population
Random sampling
Every member of the population has an equal chance of being selected
It’s representative of the population
Helps remove bias from a sample
Simple random sampling
A SRS of size n is one where every sample of size n has an equal chance of being selected
Simple Random Sampling (adv vs dis)
Advantages:
- Free of bias
- Easy and cheap to implement for small samples and populations
- Each sampling unit has a known and equal chance of selection
Disadvantages:
- Not suitable for large population/sample sizes (time consuming/ disruptive/ expensive)
- A sampling frame is needed
Systematic sampling
The required elements are chosen at regular intervals from an ordered list
Systematic Sampling (adv vs dis)
Advantages:
- Simple and quick to use
- Suitable for large samples and populations
Disadvantages:
- Sampling frame needed
- Can introduce bias if sampling frame is not random
Stratified sampling
The population is divided into mutually exclusive strata (of the same size) and a random sample is taken from each
Numbered sampled in a stratum
Number in stratum
—————————— x overall sample size
Number in population
Stratified sampling (adv vs dis)
Advantages:
- Sample accurately reflects population structure
- Guarantees proportional representation of groups within a population
Disadvantages:
- Population must be clearly classified into distinct strata
Selection within each stratum
- Sampling frame needed
- Consuming, disruptive, expensive for large sizes
Random number generator (SRS)
- Number each member
- Use a random number generator to generate x random numbers within range
- Select members who correspond to numbers
Lottery sampling (SRS)
- Write names of members on identical cards
- Place them in a hat
- Draw x cards
- Select members
Quota sampling
Researcher selects a sample that reflects the characteristics of the whole population
Quota (adv vs dis)
Advantages:
- Allows a small sample to be representative of the whole population
- No sampling frame needed
- Quick, easy, inexpensive
- Easy comparison between groups within a population
Disadvantages:
- Can introduce bias (non-random)
- Costly or inaccurate to divide pop into groups
- Non responses not recorded
Increasing scope of study
Increases number of groups
- adds time and expenses
Opportunity or convenience sampling
Sample taken from people who are available at the time of study and who fits the criteria your are looking for
Opportunity/ convenience (adv vs dis)
Advantages:
- Easy and inexpensive
Disadvantages:
- Unlikely to be representative
- Highly bias (dependent on researcher)
Quantitative data/ variables
Variables or data associated with numerical observations
Qualitative data/ variables
Variables or data associated with non-numerical values
Continuous variable
Can take any value in a specific range
Discrete variable
Can only take specific values in a given range
UK weather stations
Camborne
Hurn
Heathrow
Leeming
Leuchars
International weather stations
Jacksonville
Beijing
Perth
Daily mean temperature
In C
The average of the hourly temperature readings during 24 hours
Daily total rainfall
Including solid precipitation (is melted before recording)
Trace: Less than 0.05 mm
Daily total sunshine
Recorded to nearest tenth of an hour
Daily mean wind direction
Bearings and cardinal (compass) directions
Daily mean windspeed
In knots, averaged over 24 hours from midnight to midnight
1 kn=1.15mph
Beaufort scale: calm/light/moderate/fresh
Daily maximum gust
In knots
The highest instantaneous wind speed recorded
Directions recorded too
Daily maximum relative humidity
Given as a % of air saturation with water vapour
Above 95%: misty/foggy
Daily mean cloud cover
Oktas
‘Eighths of sky covered by cloud’
Daily mean visibility
Measured in Decametres
The greatest horizontal distance at which an object can be seen in daylight
Daily mean pressure
hPa
Missing data values
N/a - Not available
Data recorded for overseas locations
Daily mean temperature
Daily total rainfall
Daily mean windspeed
Daily mean pressure
Sampling frame
A list of all sampling units
