Ms. Albarico

Often, when real-world data is plotted, the result is a linear pattern. The general direction of the data can be seen, but the data points do not all fall on a line. This type of graph is a scatter plot. A scatter plot is often used to investigate the relationship (if one exists) between two sets of data. The data is plotted on a graph such that one quantity is plotted on the x-axis and one quantity is plotted on the y-axis. If the relationship does exist between the two sets of data, it will be visible when the data is plotted.

Example 1: The following graph represents the relationship between the price of lobster per pound and the number of lobsters sold.

From the graph, the connection between the cost of the lobster and the number sold is obvious: as the price decreased the number of lobsters sold increased.

From the graph, it is clearly seen that there is no relationship between the number of lottery tickets sold and the temperature of the surrounding environment.

Correlation refers to the relationship or connection between two sets of data. The correlation between two sets of data can be weak, strong, negative, or positive, or in some cases there can be no correlation. The characteristics of the correlation between two sets of data can be readily seen from the scatter plot. (Make sure you run the Diagnostic ON first or the r won’t be displayed.)

Correlation coefficient is a number that describes how well a line models the data. The closer to 1 or -1 the coefficient, the stronger the the correlation. A correlation that is positive means that as one variable increases, the other variable increases, resulting in a positive slope. A correlation that is negative means that as one variable increases, the other decreases, resulting in a negative slope.

A line of best fit is drawn on a scatter plot so that it joins as many points as possible and shows the general direction of the data. Also called as trend line.

Summary Point – one point that best “represents” all the points in a group of data. Its x-coordinate is the median of the x-values and its y- coordinate is the median of the y-values.

CYU # 8, 9, & 10 0n page , 18, 19, 20, and 21 on pages Submit Inv. 5 next Friday!