Scatter graphs are used to investigate the relationship between
two variables (or aspects) for a set of paired data. The pattern of the scatter
describes the relationship as shown in the examples below. Best-fit or trend
lines should:
Follow the trend of the data
Join as many points as possible
Leave an equal number of unconnected points on either side.
Rollover the scatter graphs below to see the lines of
best-fit.
No correlation
Weak positive correlation
Strong positive correlation
Strong negative correlation
Example: Price changes of a convenience item along an
environmental gradient in El Raval, Barcelona. The hypothesis tested is that
prices should decrease with distance from the key area of gentrification
surrounding the Contemporary Art Museum. The line followed is Transect 2 in the
map below, with continuous sampling of the price of a small bottle water at
every convenience store.
Map to show the location of environmental gradients for
transect lines in El Raval, Barcelona
Distance along transect from Contemporary Art Museum
Price of a small bottle of water (euros)
1
1.80
2
1.20
3
2.00
4
1.00
5
1.00
6
1.20
7
0.80
8
0.60
9
1.00
10
0.85
Example of a scatter graph for the above data, with the line
of best-fit to be drawn.
Spearman's Rank Correlation Coefficient is a further technique
for analysing this data set and is illustrated in the Statistical Techniques section
