North Hunterdon-Voorhees Regional High School District



Chapter 3: Examining Relationships

Intro:

This section is going to focus on relationships among several variables for the same group of individuals. In these relationships, does one variable cause the other variable to change? In this relationship we can think of one variable as the explanatory variable and the other as the response variable. A response variable measures an outcome of a study. An explanatory variable attempts to explain the observed outcomes. Other names for the two variables are independent variables and dependent variables.

Principles that guide examination of data are the same for studying relationships among variables as they were for one-variable methods from chapters 1 and 2.

• First plot the data, then add numerical summaries.

• Look for overall patterns and deviation from those patterns.

• When the overall pattern is quite regular, use a compact mathematical model to describe it.

Section 3.1: Scatterplots

The most effective way to display the relationship between two quantitative variables is a scatterplot. A scatterplot shows the relationship between two quantitative variables measured on the same individuals. Each individual in the data appears as the point in the plot. Always plot the explanatory variable, if there is one, on the horizontal axis and the response variable on the vertical axis.

Examining scatterplots:

In any given graph of data, look for the overall pattern and for striking deviations from that pattern.

You can describe the overall pattern of a scatterplot by the form, direction, and strength of the relationship. Form – linear, quadratic, logarithmic, ect. Direction – positive or negative. Strength – weak, moderate, or strong.

An important kind of deviation is an outlier.

Two variables are positively associated if an increase in one variable is tied together with an increase in the other.

Two variables are negatively associated if an increase in one variable is tied together with a decrease in the other.

Tips for drawing scatterplots:

• Scale the vertical and horizontal axes. The intervals must be uniform; that is, the distance between tick marks must be the same. If the scale does not begin at zero at the origin, then use a symbol to indicate a break.

• Label both axes, and title the graph.

• If you are given a grid, try to adopt a scale that makes your plot use the whole grid.

Homework: #’s 3.15 – 3.23

Section 3.2: Correlation

The correlation measures the direction and strength of a linear relationship between two variables. Correlation is usually written as r.

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The formula for correlation is a little complex and most of the time we will use our calculators.

Facts about correlation:

• Correlation makes no distinction between explanatory and response variables. It makes no difference which variable you call x and which you call y in calculating the correlation.

• Correlation requires that both variables be quantitative, so that it makes sense to do the arithmetic indicated by the formula for r. We cannot calculate a correlation between the incomes of a group of people and what city they live in, because city is a categorical variable.

• Because r uses the standardized values of the observations, r does not change when we change the units of measurement of x, y, or both. The correlation r itself has no unit of measurement; it is just a number.

• Positive r indicates positive association between the variables, and negative r indicates negative association.

• The correlation r is always a number between -1 and 1. Values of r near 0 indicate a very weak linear relationship. The strength of the linear relationship increases as r moves away from 0 toward either -1 or 1.

• Correlation measures the strength of only a linear relationship between two variables. Correlation does not describe curved relationships between variables, no matter how strong they are.

• Like the mean and standard deviation, the correlation is not resistant: r is strongly affected by a few outlying observations. Use r with caution when outliers appear in the scatterplot.

• Correlation is not a complete description of two-variable data, even when the relationship between the variables is linear. You need to use the means and standard deviations of both x and y along with the correlation when describing the data.

Here are some charts to look at.

[pic]

Homework: #’s 3.29 – 3.37

Section 3.3: Least-Squares Regression

Correlation measures the strength and direction of the linear relationship between any two quantitative variables. If a scatterplot shows a linear relationship, we would like to summarize this overall pattern by drawing a line through the scatterplot. Least-squares regression is a method for finding a line that summarizes the relationship between two variables, but only in a specific setting.

A regression line is a straight line that describes how a response variable y changes as an explanatory variable x changes. We often use a regression line to predict the value of y for a given value of x. Regression unlike correlation, requires that we have an explanatory and response variable.

The least-squares regression line (LSRL).

The equation is pretty simple.

[pic]

[pic] is used instead of y since this equation is that of a prediction. y is still the response variable. The slope of the equation is b and the intercept is a.

[pic] [pic]

It can be shown that every least-squares regression line passes through the point [pic].

Interpretation of the slope and intercept are important. The slope of a regression line is the rate of change, the amount of change in [pic] when x is increased by 1. The intercept of the regression line is the value of [pic] when x = 0. It is only meaningful when x can actually take values close to zero.

The role of r2 in regression.

Your calculator will compute a quantity called r2. This r2 value is referred to as the coefficient of determination. The coefficient of determination, r2, is the fraction of the variation in the values of y that is explained by least-squares regression of y on x.

Facts about least-squares regression.

Fact 1. The distinction between explanatory and response variables is essential in regression. LSR looks at the distances of the data points from the line only in the y direction. If we reversed the roles of the two variables, we get a different LSRL.

Fact 2. There is a close connection between correlation and the slope of the regression line. As the correlation gets weaker, the prediction [pic] moves less in response to changes in x.

Fact 3. The LSRL always passes through the point [pic].

Fact 4. The correlation r describes the strength of a straight line relationship.

Residuals:

A regression line is a mathematical model for the overall pattern of a linear relationship. Deviations from the overall pattern are also important. These deviations are vertical distances measured between the observed points and the predicted points on [pic]. A residual is the difference between an observed value of the response variable and the value predicted by the regression line.

residual = observed y – predicted y

= y - [pic]

The residuals from the LSRL have a special property: the mean of the least-squares residuals is always zero.

You can make a residual plot with all the residuals from the LSRL. A residual plot is a scatterplot of the regression residuals against the explanatory variable. Residual plots help to assess the fit of a regression line.

Influential observations:

An outlier is an observation that lies outside the overall pattern of the other observations.

An observation is influential for statistical calculations if removing it would markedly change the result of the calculation. Points that are outliers in the x direction of a scatterplot are often influential for the LSRL. Influential points often have small residuals, because they pull the regression line toward themselves. If you just look at residuals, you will miss influential points. Influential observations can greatly change the interpretation of data.

Homework: #’s 3.50 – 3.59

Chapter Review

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Homework: #’s 3.62 – 3.64, 3.65 Data set A only, 3.66, 3.68 – 3.70

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