When two variables have no relationship, there is no straight-line relationship or non-linear relationship. For example, as age increases height increases up to a point then levels off after reaching a maximum height. Non-linear relationships have an apparent pattern, just not linear. Scatterplot of temperature versus wind speed. For example, as wind speed increases, wind chill temperature decreases. Negative relationships have points that decline downward to the right. For example, when studying plants, height typically increases as diameter increases. Positive relationships have points that incline upwards to the right.
Linear relationships can be either positive or negative. This is the relationship that we will examine.
A relationship is linear when the points on a scatterplot follow a somewhat straight line pattern. A relationship is non-linear when the points on a scatterplot follow a pattern but not a straight line. A relationship has no correlation when the points on a scatterplot do not show any pattern. We can see an upward slope and a straight-line pattern in the plotted data points.Ī scatterplot can identify several different types of relationships between two variables. In this example, we see that the value for chest girth does tend to increase as the value of length increases. When examining a scatterplot, we should study the overall pattern of the plotted points. In this example, we plot bear chest girth (y) against bear length (x). Scatterplot of chest girth versus length. Each individual (x, y) pair is plotted as a single point. A scatterplot (or scatter diagram) is a graph of the paired (x, y) sample data with a horizontal x-axis and a vertical y-axis. A scatterplot is the best place to start. We begin by considering the concept of correlation.Ĭorrelation is defined as the statistical association between two variables.Ī correlation exists between two variables when one of them is related to the other in some way. We can describe the relationship between these two variables graphically and numerically. As the values of one variable change, do we see corresponding changes in the other variable? Given such data, we begin by determining if there is a relationship between these two variables. We collect pairs of data and instead of examining each variable separately (univariate data), we want to find ways to describe bivariate data, in which two variables are measured on each subject in our sample. 
For example, we measure precipitation and plant growth, or number of young with nesting habitat, or soil erosion and volume of water. In many studies, we measure more than one variable for each individual.
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