If there is a correlation between two sets of data, it means they are connected in some way.
We have seen that as the temperature increases, the number of ice-creams sold increases. The results are approximately in a straight line, with a positive gradient. We therefore say that there is positive correlation.
Look at the following scatter diagram. It shows the connection between the number of weeks a song has been in the Top 40 and sales of the single for that week.
There is a definite a connection between the two sets of data, as the results are approximately in a straight line. As the number of weeks increases, sales decrease. The line therefore has a negative gradient, and we say there is negative correlation.
The following scatter diagram shows the connection between a person's house number and their IQ (one measure of intelligence).
It is obvious that there is no connection between these values, and this is shown by the scatter diagram. We say there is no correlation.
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