![]() ![]() You can assign different colors or markers to the levels of these variables. You can use categorical or nominal variables to customize a scatter plot. Scatterplot The most useful graph for displaying the relationship between two quantitative variables is a scatterplot. Either way, you are simply naming the different groups of data. You can use the country abbreviation, or you can use numbers to code the country name. Country of residence is an example of a nominal variable. For example, in a survey where you are asked to give your opinion on a scale from “Strongly Disagree” to “Strongly Agree,” your responses are categorical.įor nominal data, the sample is also divided into groups but there is no particular order. With categorical data, the sample is divided into groups and the responses might have a defined order. Scatter plots are not a good option for categorical or nominal data, since these data are measured on a scale with specific values. Some examples of continuous data are:Ĭategorical or nominal data: use bar charts Scatter plots make sense for continuous data since these data are measured on a scale with many possible values. Scatter plots and types of data Continuous data: appropriate for scatter plots Annotations explaining the colors and markers could further enhance the matrix.įor your data, you can use a scatter plot matrix to explore many variables at the same time. The colors reveal that all these points are from cars made in the US, while the markers reveal that the cars are either sporty, medium, or large. ![]() There are several points outside the ellipse at the right side of the scatter plot. From the density ellipse for the Displacement by Horsepower scatter plot, the reason for the possible outliers appear in the histogram for Displacement. In the Displacement by Horsepower plot, this point is highlighted in the middle of the density ellipse.īy deselecting the point, all points will appear with the same brightness, as shown in Figure 17. This point is also an outlier in some of the other scatter plots but not all of them. In Figure 16, the single blue circle that is an outlier in the Weight by Turning Circle scatter plot has been selected. It's possible to explore the points outside the circles to see if they are multivariate outliers. The red circles contain about 95% of the data. Remember a correlation does not imply causation.The scatter plot matrix in Figure 16 shows density ellipses in each individual scatter plot. There are many other factors that could influence both, such as medical care and education. The fertility rate does not necessarily cause the life expectancy to change. Caution: just because there is a correlation between higher fertility rate and lower life expectancy, do not assume that having fewer children will mean that a person lives longer. ![]() It appears that there is a trend that the higher the fertility rate, the lower the life expectancy. This correlation would probably be considered moderate negative correlation. It looks a little stronger than the previous scatter plot and the trend looks more obvious. Graph 2.5.4: Scatter Plot of Life Expectancy versus Fertility Rate for All Countries in 2013Īgain, there is a downward trend. Let’s see what the scatter plot looks like with data from all countries in 2013 ("World health rankings," 2013). ![]() There are three types of correlation: positive, negative, and none (no correlation). The trend is not strong which could be due to not having enough data or this could represent the actual relationship between these two variables. With scatter plots we often talk about how the variables relate to each other. What this says is that as fertility rate increases, life expectancy decreases. Graph 2.5.3: Scatter Plot of Life Expectancy versus Fertility Rateįrom the graph, you can see that there is somewhat of a downward trend, but it is not prominent. Note: Always start the vertical axis at zero to avoid exaggeration of the data. The vertical axis needs to encompass the numbers 70.8 to 81.9, so have it range from zero to 90, and have tick marks every 10 units. The horizontal axis needs to encompass 1.1 to 3.4, so have it range from zero to four, with tick marks every one unit. In this case, it seems to make more sense to predict what the life expectancy is doing based on fertility rate, so choose life expectancy to be the dependent variable and fertility rate to be the independent variable. Sometimes it is obvious which variable is which, and in some case it does not seem to be obvious. To make the scatter plot, you have to decide which variable is the independent variable and which one is the dependent variable. \): Life Expectancy and Fertility Rate in 2013 Countryįertility Rate (number of children per mother) ![]()
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