What does a scatterplot tell you about a relationship?
The most important information that a scatterplot shows is the strength of the relationship between the variables.
If higher values of one variable tend to be associated with higher values of the other variable, the crosses on the scatterplot will be in a band with positive slope. The relationship is then said to be positive.
If high values of one variable tend to be associated with low values of the other variable, we say that there is a negative relationship.
Weight gain of children in a year
The example below is an artificial one and is only intended to help explain the differences between strong and weak relationships, and between positive and negative ones.
Imagine 200 children who keep records of exercise and food consumption over a year. We are interested in how these variables affect weight gain in the year.
The diagram initially shows the relationship between weight gain and fatty food consumption. The more fatty food consumed, the higher the childrens' weight gain — the cloud of crosses extends from bottom left to top right. This is therefore a positive relationship.
Use the slider to change the strength of the relationship. If the relationship is weak, there is little difference between the types of weight gain of children who consume little fatty food and of those who consume a lot. (Click in the left of the scatterplot to see the range of weight gains of children whose fatty food consumption is low, then drag to the right to see the weight gains of those with high fatty food consumption.) If the relationship is strong, you will be able to predict weight gain fairly accurately from fatty food consumption.
Now select Amount of exercise in year from the pop-up menu. The relationship between weight gain and exercise is negative — the more exercise, the lower the weight gain. The slider can again be used to adjust the strength of the relationship.