Scatter Graphs As the name implies, points are scattered about between the axes of the graph.

 Scatter graphs are usually used where there is some uncertainty if there is a relationship between the two sets of data being looked at.  Usually there is no exact line that will fit and join all the points.  But often we can notice that the points show a trend.  They sometimes appear to slope in the positive or negative direction. (Remember from your y = mx + c graphs)

 When trying to plot a scatter graph, it is essential to look at the points to be plotted and make sure that the axes are extended so that all the points will fit!  Plot each linked data carefully and double check that they are correct.  Unlike a straight line graph it is easy to plot an incorrect coordinate and not realise it!!!

Looking again at the first two graphs. An upward slopeA downward slope

Scatter Graphs  The two graphs show a trend!!  The maths/science shows that as the science test score increases, there is a general increase in the maths test score.  The second graph shows there is a trend that indicates that increased absences are likely to be reflected in a poor test score.

The upward slope of the first graph is called a Positive Correlation. Whereas the downward slope of the second graph is called a Negative Correlation. We will now look at each graph in turn.

We can put in a trend line. Place ruler on the points so that the points are roughly equal on each side of the ruler. Now draw your line.

If required, we can work out the equation to the line. y = mx +c Once we have the line in place we can also extract other information from the graph. For example the possible maths score if a score of 60 was obtained in science.

Scatter Graphs We can draw a similar line on this graph. Again we can work out the equation for the line and extract information. Maths score if 5 periods missed? 57%

REMEMBER You can work out the equation to the line for the “line of best fit”. y = mx + c Where: “c” is where the line crosses the “y” axis And “m” is the slope or gradient of the line.

 The closer the points are to the line of best fit, the stronger the correlation.  The more spread, the weaker the correlation.  Perfect correlation occurs when all the points fit exactly on the line!!

Sometimes there is a graph that does not appear to have a line of best fit. Such a graph has No Apparent Correlation