# A dot plot shows the relationship between two variables

This scatter plot, or scatter diagram, shows a positive correlation, i.e. as x used to investigate the possible relationship between two variables that both relate to. relationship between two quantitative variables, it is always helpful to create a graphical A scatterplot shows the relationship between two quantitative variables Each dot on the scatterplot represents one individual from the data set. The. A scatter plot is a two-dimensional data visualization that uses dots to For example this scatter plot shows the height and weight of a fictitious set of Scatter plots are used when you want to show the relationship between two variables.

An example of a situation where you might find a perfect positive correlation, as we have in the graph on the left above, would be when you compare the total amount of money spent on tickets at the movie theater with the number of people who go.

This means that every time that "x" number of people go, "y" amount of money is spent on tickets without variation.

- Graphical Displays: Two Variables
- Scatter plot

An example of a situation where you might find a perfect negative correlation, as in the graph on the right above, would be if you were comparing the amount of time it takes to reach a destination with the distance of a car traveling at constant speed from that destination. On the other hand, a situation where you might find a strong but not perfect positive correlation would be if you examined the number of hours students spent studying for an exam versus the grade received.

This won't be a perfect correlation because two people could spend the same amount of time studying and get different grades. But in general the rule will hold true that as the amount of time studying increases so does the grade received.

Let's take a look at some examples. The graphs that were shown above each had a perfect correlation, so their values were 1 and Even if the plot is not used in the final presentation, it may highlight outliers and will help to indicate the appropriate form of analysis to use.

### Scatter Plot: Is there a relationship between two variables?

For example, the plot below comes from a study from the BMJ, looking at the association between age and ear length. They found a weak positive association, meaning higher values of 'age' are associated with higher values of 'ear length' the article was published in the Christmas edition, where more light-hearted articles are encouraged. Why do old men have big ears? Alternatively, the same variable may be measured in a matched pair of individuals. Any 'pairing' that is inherent because of the way in which the data was collected should be retained in both displaying and analysing the data.

## Scatter Plots

Line diagrams are often used whereas a scatterplot is generally more appropriate. Milliner et al, Results of long-term treatment with orthophosphate and pyridoxine in patients with primary hyperoxaluria, New England Journal of Medicine,;Vol ,No.

Measurements were made of calcium oxalate inhibition in 12 patients, pre and post treatment. The authors displayed the data as a line-plot. The values for each patient are shown pre and post treatment and are joined by lines to show the within person pairing of the measurements.

Inhibition of the formation of calcium oxalate crystals during treatment with orthophosphate and pyridoxine in 12 patients with primary hyperoxaluria. The line plot shows that all individuals have values that rise during treatment.

### Scatter plot - Wikipedia

One individual shows a very large increase from about 25 pre-treatment to about during treatment as illustrated by the steeply rising diagonal line. The same data is presented below as a scatterplot: The line of equality no change in values pre to during treatment is shown as a dashed line on the display. All points lie above the line of equality showing that values rose for each individual.

Whilst the same information is given by the two displays, the scatterplot uses only one point to represent each individual compared to 2 points and a line for the line diagram. The line diagram may be confusing to assess if there are changes in various directions, the scatterplot with the line of equality superimposed if necessary is easier to interpret.

No information is lost, the display clearly shows the relationship between the variables and also highlights possible outliers. We saw an example earlier of the times it takes for a scorpion to capture its prey presented as a dot plot. Optimal sting use in the feeding behavior of the scorpion Hadrurus spadix.

## What is a Scatter Plot and When to Use It

No universal best-fit procedure is guaranteed to generate a correct solution for arbitrary relationships. A scatter plot is also very useful when we wish to see how two comparable data sets agree to show nonlinear relationships between variables.

The scatter diagram is one of the seven basic tools of quality control. The researcher would then plot the data in a scatter plot, assigning "lung capacity" to the horizontal axis, and "time holding breath" to the vertical axis. The scatter plot of all the people in the study would enable the researcher to obtain a visual comparison of the two variables in the data set, and will help to determine what kind of relationship there might be between the two variables.

Scatterplot matrices[ edit ] For a set of data variables dimensions X1, X2, For k variables, the scatterplot matrix will contain k rows and k columns.