Presentation on theme: "Chapter 3: Examining relationships between Data"— Presentation transcript:
1 Chapter 3: Examining relationships between Data Explanatory Variable – attempts to explain the observed outcomes. Calling one variable explanatory and the other response does not imply a cause and effect relationship.Response Variable – Measure the outcome of a study.Explanatory VariableResponse Variable
2 Look for patterns or deviation from patterns. Analyzing DataStart with a graph.Look for patterns or deviation from patterns.Examine numerical descriptors of the data. (mean median, IQR, etc.)ScatterplotsShows the relationship between two quantitative variables.Always put explanatory variable on the x axis if one can be identified. (Chicken or egg?)
3 Interpreting scatterplots Look for pattern- direction; negative or positive association.- Form; clusters, linear, curved etc.- Strength; strong if points in any discernable line or curve. Weak if points are scattered.- Pay careful attention to any outliers or clusters. You may want to split the data into categories to reveal information.Activity - length of ring and first finger.
4 There is quite some variation in BAC for the same number of beers drunk. A person’s blood volume is a factor in the equation that we have overlooked.Now we change the number of beers to the number of beers/weight of a person in pounds.Note how much smaller the variation is. An individual’s weight was indeed influencing our response variable “blood alcohol content.”So let’s look at this scatterplot. Overall pattern: in general, the BAC increases with the number of beers you drink.
5 But which line best describes our data? Correlation tells us about strength (scatter) and direction of the linear relationship between two quantitative variables.But which line best describes our data?
6 If Xi and Yi agree most of the time then r will be positive; if they do not agree most of the time then r will be negative.Insert Definition 14.6 from page 724
9 Properties of rPos r positive association; Neg r negative associationr always lies between -1 and 1.strong neg. no association strong pos.3. Changing units will not change r; example converting lbs. to kgThe correlation r describes the strength and direction of a straight-line relationship.r is nonresistant; like the mean and std it is affected by outliers or extreme observationsCorrelation is not a complete description of two variable data. The means and std of both variables are important as well.
10 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.The average x value and the average y value are always a point on the regression lineLeast squares regression line app
12 The least-squares regression line is the unique line such that the sum of the squared vertical (y) distances between the data points and the line is the smallest possible.Distances between the points and line are squared so all are positive values. This is done so that distances can be properly added (Pythagoras).
13 The least-squares regression line can be shown to have this equation: is the predicted y value (y hat)b is the slopea is the y-intercept"a" is in units of y"b" is in units of y/units of x
14 Determine the regression line First we calculate the slope of the line, b, from statistics we already know:r is the correlationsy is the standard deviation of the response variable ysx is the the standard deviation of the explanatory variable xOnce we know b, the slope, we can calculate a, the y-intercept:where x and y are the sample means of the x and y variablesThis means that we don’t have to calculate a lot of squared distances to find the least-squares regression line for a data set. We can instead rely on the equation.But typically, we use a 2-var stats calculator or a stats software.
15 The equation completely describes the regression line. To plot the regression line, you only need to plug two x values into the equation, get y, and draw the line that goes through those two points.Hint: The regression line always passes through the mean of x and y.They are NOT points from your sample data (except by pure coincidence).The points you use for drawing the regression line are derived from the equation.
16 ResidualsThe distances from each point to the least-squares regression line give us potentially useful information about the contribution of individual data points to the overall pattern of scatter.These distances are called “residuals.”The sum of these residuals is always 0.Points above the line have a positive residual.Points below the line have a negative residual.So where does this line come from?^Predicted yObserved y
17 Residual plotsResiduals are the distances between y-observed and y-predicted. We plot them in a residual plot.If residuals are scattered randomly around 0, chances are your datafit a linear model, were normally distributed, and you didn’t have outliers.
18 Residuals are randomly scattered—good! A curved pattern—means the relationshipyou are looking at is not linear.A change in variability across plot is a warning sign. You need to find out why it is and remember that predictions made in areas of larger variability will not be as good.
19 The x-axis in a residual plot is the same as on the scatterplot. The line on both plots is the regression line.Only the y-axis is different.
20 Coefficient of determination, r2 r2, the coefficient of determination, is the square of the correlation coefficient.r2 represents the fraction of the variance in y (vertical scatter from the regression line) that can be explained by changes in x.Go over point that missing zero is OK - this is messy data, a prediction based on messy data.Residuals should be scattered randomly around the line, or there is something wrong with your data - not linear, outliers, etc.
21 Here the change in x only explains 76% of the change in y Here the change in x only explains 76% of the change in y. The rest of the change in y (the vertical scatter, shown as red arrows) must be explained by something other than x.r = 0.87r2 = 0.76r = −1r2 = 1Changes in x explain 100% of the variations in y.y can be entirely predicted for any given value of x.r = 0r2 = 0Changes in x explain 0% of the variations in y.The value(s) y takes is (are) entirely independent of what value x takes.
22 Grade performance If class attendance explains 16% of the variation in grades, what is the correlation between percent of classes attended and grade?1. We need to make an assumption: Attendance and grades are positively correlated. So r will be positive too.2. r2 = 0.16, so r = +√0.16 = A weak correlation.
23 There is quite some variation in BAC for the same number of beers drunk. A person’s blood volume is a factor in the equation that was overlooked here.r =0.7r2 =0.49We changed the number of beers to the number of beers/weight of a person in pounds.r =0.9r2 =0.81So let’s look at this scatterplot. Overall pattern: in general, the BAC increases with the number of beers you drink.In the first plot, number of beers only explains 49% of the variation in blood alcohol content.But number of beers/weight explains 81% of the variation in blood alcohol content.Additional factors contribute to variations in BAC among individuals (like maybe some genetic ability to process alcohol).
24 Outliers and influential points Outlier: An observation that lies outside the overall pattern of observations.“Influential individual”: An observation that markedly changes the regression if removed. This is often an outlier on the x-axis.Child 19 = outlier in y directionChild 18 = outlier in x directionChild 19 is an outlier of the relationship.Child 18 is only an outlier in the x direction and thus might be an influential point.
25 Are these points influential? All dataWithout child 18Without child 19Outlier in y-directionInfluentialAre these points influential?
26 boy may be well within the range of height for his age, but That’s why typically growth charts show a range of values (here from 5th to 95th percentiles).This is a more comprehensive way of displaying the same information.So, any individualboy may be well within the range of height for his age, butalso be within the distributions for boys year or two olderor younger as well. Two possible size distributions are shown.
27 (in 1000’s)There is a positive linear relationship between the number of powerboats registered and the number of manatee deaths.The least-squares regression line has for equation:Thus, if we were to limit the number of powerboat registrations to 500,000, what could we expect for the number of manatee deaths?Roughly 21 manatees.