Back to
GCSE Revision Guide homepage
Scatter Graphs
If you are given a
table of ‘paired’ data, you can construct a scatter graph Test scores of 9 people
We plot a point
for each pair of data which corresponds. In this case, each point corresponds
to a different person We can also draw
line of best fit through the points. The line of BEST
FIT DOES NOT HAVE TO GO THROUGH THE ORIGIN 



a) If you were asked
to estimate the English score of someone who achieved 40% in Maths, we could
use the line of best fit. From above, you can see the English score is about
53%. This is called INTERPOLATION because our estimation lies WITHIN the data
dots. This has been indicated on the diagram b) If you were asked
to estimate the English score of someone who achieved 10% in Maths then THIS
WOULD NOT BE VALID. This would be called EXTRAPOLATION. This is
because we ONLY KNOW THE LINE IS VALID within the data which we have been
given 

TERMINOLOGY POSITIVE CORRELATION: the dots go diagonally upwards The graph above is
positively correlated. It suggests that is you are better at Maths then you
are better at English. Other examples
would be: ·
Weight and height ·
Temperature and icecream sales ·
Time spend revising and exam score ·
Hand size and foot size NEGATIVE CORRELATION: the dots go diagonally downwards Examples would
include ·
Age and eyesight ·
Amount of alcohol drunk and
coordination STRENGTH OF CORRELATION The closer the
dots are to a straight line THE STRONGER THE CORRELATION. A perfectly straight
line indicates PERFECT CORRELATION 