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58 Terms

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Parameter
Number that describes the whole population
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Sample
Specific number of points taken from the population
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Cencus
Collection of every data point within the population
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Simple random sampling

  • Each item allocated a unique number

  • Numbers chosen at random using random number generator

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Advantages of simple random sampling

- Bias free

- Fast, easy, cheap

- Each number has a known equal chance of being selected

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Disadvantages of simple random sampling

- More difficult as the population size gets larger

- Full sampling frame needed

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Systematic sampling

Elements are chosen at regular intervals from an ordered list. First member chosen randomly then goes up in e.g 5ths

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Advantages of systematic sampling

- Simple, fast, cheap

- Suitable for large samples and large populations

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Disadvantages of systematic sampling

- Full sampling frame is needed

- It can introduce bias if the sampling frame is not random

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Stratified sampling
Population is split into groups and a simple random sample is carried out in each group
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How to find how many people to put in each strata for stratified sampling

(actual number in group ÷ total population) × sample size

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Advantages of stratified sampling

- Sample accurately reflects the population structure

- Guarantees proportional representation of groups within a population

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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

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Quota sampling

  • Interviewer creates groups for population to be put into and decides the proportions

  • Meet each member and put them into correct group

  • Continues until all quotas (groups) are filled

  • If a person refuses to be interviewed or the quota is already full then ignore the answer

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Advantages of quota sampling

- No sampling frame required

- Small sample can represent whole population

- Fast, easy, cheap

- Easy comparison between groups in population

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Disadvantages of quota sampling

- Non-random sampling is unrepresentative, can introduce bias

- Population must be divided into groups, which can be costly or inaccurate

- Larger sample increases number of groups which adds time and expense

- Non-responses are not recorded

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Opportunity sampling
Sample taken from people who happen to be available at the time who meet the criteria
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Advantages of opportunity sampling

- No sampling frame required

- Fast, easy, cheap

- Inexpensive

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Disadvantages of opportunity sampling

- Non-random sampling is unrepresentative, can introduce bias

- Highly dependent on individual researcher

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What is cluster sampling

  • Population is split into clusters where each member of population can only be in one cluster

  • Sample is taken from each cluster

  • The sample taken can be using any sample technique

  • Often the clusters are geographic e.g taking clusters from different parts of the UK where a particular type of bird is common

  • Is usually two stage

  • Can be random or non-random

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Types of random sampling

- Simple random

- Systematic

- Stratified

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Types of non-random sampling

- Opportunity

- Quota

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Comparison of spread
IQR
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Comparison of central tendancy
Median
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Equation for median
(n+1)/2
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How to find IQR

Q3-Q1 = IQR

3(n+1)/4 - (n+1)/4 = IQR

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Upper quartile equation
3(n+1)/4
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Lower quartile equation
(n+1)/4
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How to find mean of data in a table

1) Find midpoint of range

2) Multiply by frequency

3) Find mean of new values

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Frequency density equation
Frequency density = frequency ÷ class width
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How much data 1, 2, and 3 standard deviations from the mean

1 SD = 68%

2 SD = 95%

3 SD = 99.8%

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For normal distribution, how many quartile is the data split into?
4: this means each quartile has probability of 0.25
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Equations to test independence

P(AandB) = P(A) × P(B)

P(A|B) = P(A|B’) = P(A)

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Area of bar on a histogram
Frequency
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Regression line
Line of best fit
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Box plot has negative skew when...

It has a left tail

Q3 - Q2 > Q2 - Q1

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Box plot has positive skew when...

It has a right tail

Q2 - Q1 > Q3 - Q2

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Extrapolation

Estimating a value outside the range of measured data

CAN'T DO THIS!

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Interpolation
An estimation of a value within the measured data
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Range of PMCC
-1 ≤ PMCC ≤ 1
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P Value
Probabibility of getting the critical value
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P(X = __ ) in normal distribution
P = 0, in normal distribution X can't equal a specific number
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Lower quartile equation
(n+1)/4
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Conditions for binomial distribution

1. A fixed number of trials, n

2. Each trial has two possible outcomes

3. The probability of success, p, is the same for each trial

4. Each observation is independent

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Mutually exclusive
Events that cannot occur at the same time - on Venn diagram the circles don't overlap
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Test for mutual exclusivity
P(AorB) = P(A) + P(B) if mutually exclusive
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P(A|B) =
P(A ∩ B) / P(B)
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Discrete data

Data that can only take certain values, e.g shoe size.

Shown using bar charts, tally charts, pie charts.

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Continuous data

Data that can take any value, e.g height.

Shown using line graph.

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When can you approximate binomial distribution with normal distribution

- Probability is close to 0.5

- There is a large number of trials, n > 50

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Key facts about the large data set

- 3827 cars in total

- Only five makes of car are included: Ford, BMW, Vauxhal, Toyota and VW. Ford is the most frequently registered.

- Only one electric vehicle in whole data set and only gas petrol hybrid vehicle in data set

- 5 door hatchback is the most common body type

- Data is only from a few days in summer, June, in 2002 and 2016: there is more data from 2016 than 2002

- Mass of vehicle includes 75kg driver

- Emissions data is only known for approx 80% of the whole data set: CO2, CO, NOX

- Particulate emissions are applicable to diesel cars

- Doesn’t include drivers of company cars / only shows name the car is registered to, not the driver

- Doesn’t include all regions in England, only NW, SW and London

- CO2 emissions are in 10s and 100s

- CO emissions are in decimals

- Only cars, not vans, buses etc

- Some of the categories are codes ie numbers represent different types, e.g the body type is represented by a number

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Which categories in LDS are codes?

  • Propulsion type: petrol, diesel, electric, gas+petrol, electric+petrol

  • Body type

  • Owner of car: male, female, not used, unknown, company

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Conditions for normal distribution

- Data must be continuous

- 95% of the data must be within 2 standard deviations of the mean

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Hypothesis for PMCC

H0: 𝑝 = 0

H1: 𝑝 > 0, 𝑝 < 0 or 𝑝 ≠ 0

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In normal distribution, P(X ≠ __)
1
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How to find critical region for a normal?

Use the inverse normal function with the probability equal to significance level

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For a normal Hypothesis test, what is the new value for standard deviation when you change it to the sample?

standard deviation² ÷ n

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How many vehicles in LDS?

3827