Correlation

Correlation describes how two quantities are related to each other
Positive correlation is when one quantity increases and the other quantity increases
- For example, as temperature increases, sales of cold drinks increase
Negative correlation is when one quantity decreases while the other quantity increases
- For example, the value of a car decreases as its age increases
No (zero) correlation is where there is no apparent relationship
- For example, the masses of snails and scores in an exam

If two quantities correlate, it does not mean that the first causes the second
For example, each day you record the height of a sunflower and the weight of a puppy
- As the height of the sunflower increases, the weight of the puppy increases
  - This is a positive correlation
- But you cannot claim that:
  - If you want your puppy to weigh more, make your sunflower taller!
  - Sunflowers grow better when puppies are heavier!
- Both quantities may be increasing due to another reason
  - In this case, time

Scatter graphs

Scattergraphs (or scatter diagrams) are used to plot pairs of data
- For example, students’ Maths grades against their Physics grades
The vertical and horizontal axes represent the two quantities being measured
Points are plotted as crosses, ×
- They are not joined up
The general shape formed by the points shows the type of correlation
- Positive correlation goes from bottom left to top right
  - A positive gradient
- Negative correlation goes from top left to bottom right
  - A negative gradient
- No (zero) correlation looks like a cloud of points
Correlations can be weak or strong
- The stronger the correlation, the closer to a straight line the data points lie