Scattergraphs 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 unjoined points on either side.

Rollover the scattergraphs 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

Example of a scattergraph 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