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Analysis of Variance Example: Friend Or Pet to • Compare Means for More Than 2 Groups

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Analysis of Variance Example: Friend Or Pet to • Compare Means for More Than 2 Groups Announcements: • Special office hours for final exam: Friday, Jason will have 1-3 (instead of 1-2) Monday (just before exam) I will have 10-noon. Exam is Monday, 1:30 – 3:30. Analysis • Homework assigned today and Wed not due, but Chapter 16 solutions on website. of • Final exam review sheets posted on web (in list of lectures). Variance • Form and instructions for disputing points will be sent by email. Watch for it if you plan to do that. Fill out and bring to final exam to hand in. Homework: Chapter 16: 1, 7, 8, 16, 17 (not due) 1 ANOVA = Analysis of variance Example: Friend or Pet to • Compare means for more than 2 groups. Relieve Stress? • We have k independent samples and measure a quantitative variable on all units in all k • Randomized experiment using 45 women samples. volunteers who said they love dogs. • We want to know if the population means are • Each woman assigned to do a stressful task: all equal (null hypothesis) or if at least one is – 15 did it alone different (alternative hypothesis). – 15 did it with a good friend present • This is called one-way ANOVA because we – 15 did it with their dog present analyze variability in order to compare • Response variable = heart rate means. 3 4 16.1 Comparing Means Step 1 for the example (defining hypotheses) with an ANOVA F-Test H0: µ1 = µ2 = µ3 H0: µ1 = µ2 = … = µk Ha: The means are not all equal. Ha: The means are not all equal. Example: Parameters of interest are the population Step 2 (In general): mean heart rates for all such women if they were to do The test statistic is called an F-statistic. In words, it the task under the 3 conditions: is defined as: µ1: if doing the task alone. Variation among sample means µ if doing the task with good friend present. F = 2: Natural variation within groups µ3: if doing the task with dog present. 5 6 1 Variation among sample means Assumptions for the F-Test F = Natural variation within groups • Samples are independent random samples. • Distribution of response variable is a normal curve Variation among sample means is 0 if all within each population (but ok as long as large n). k sample means are equal and gets larger • Different populations may have different means. the more spread out they are. • All populations have same standard deviation, σ. If large enough => evidence at least one e.g. How k = 3 populations might look … population mean is different from others => reject null hypothesis. p-value found using an F-distribution (more later) 7 8 Conditions for Using the F-Test Notation for Summary Statistics k = number of groups • F-statistic can be used if data are not extremely xi , si, and ni are the mean, standard deviation, skewed, there are no extreme outliers, and group and sample size for the ith sample group standard deviations are not markedly different. N = total sample size (N = n1 + n2 + … + nk) • Tests based on F-statistic are valid for data with skewness or outliers if sample sizes are large. Example: Friends, Pets and Stress • A rough criterion for standard deviations is that Three different conditions => k = 3 the largest of the sample standard deviations n1 = n2 = n3 = 15; so N = 45 should not be more than twice as large as the xxx= 82.52, ==91.33, 73.48 smallest of the sample standard deviations. 123 sss123===9.24, 8.34, 9.97 9 10 Example, continued Conditions for Using the F-Test: Do friends, pets, or neither reduce stress more Does the example qualify? than when alone? Boxplots of sample data: • F-statistic can be used if data are not extremely Possible outlier 100 skewed, there are no extreme outliers. 9In the example there is one possible outlier 90 but with a sample of only 15 it is hard to tell. 80 • A rough criterion for standard deviations is that Heart rate Heart the largest of the sample standard deviations 70 should not be more than twice as large as the 60 smallest of the sample standard deviations. Alone Friend Pet 9That condition is clearly met. The sample Treatment groups 11 standard deviations s are very similar. 12 2 16.2 Details of the F Statistic Measuring variation between groups: for Analysis of Variance How far apart are the means? Fundamental concept: the variation among the data values in the overall sample can be separated into: Sum of squares for groups = SS Groups (1) differences between group means 2 (2) natural variation among observations within a group SS Groups = ∑ groups ni ()xi − x Total variation = Numerator of F-statistic = mean square for groups Variation between groups + Variation within groups SS Groups MS Groups = “ANOVA Table” displays this information in k −1 summary form, and gives F statistic and p-value. 13 14 Measuring variation within groups: Measuring Total Variation How variable are the individuals? Total sum of squares = SS Total = SSTO Sum of squared errors = SS Error SS Errors = ()()n −1 s 2 2 ∑groups i i SS Total = x − x ∑values ()ij Denominator of F-statistic = mean square error SS Error MSE = N − k SS Total = SS Groups + SS Error Pooled standard deviation: s p = MSE Measures internal variability within each group. 15 16 General Format of a Example: Stress, Friends and Pets One-Way ANOVA Table H0: µ1 = µ2 = µ3 Ha: The means are not all equal. One-way ANOVA: Heart rate versus treatment group Source DF SS MS F P Group 2 2387.7 1193.8 14.08 0.000 Error 42 3561.3 84.8 Total 44 5949.0 The F-statistic is 14.08 and p-value is 0.000... p-value so small => reject H0 and accept Ha. Conclude there are differences among population means. MORE LATER ABOUT FINDING P-VALUE. 17 18 3 Conclusion in Context: 95% Confidence Intervals •The population mean heart rates would differ if we for the Population Means subjected all people similar to our volunteers to the 3 conditions (alone, good friend, pet dog). In one-way analysis of variance, a confidence •Now we want to know which one(s) differ! interval for a population mean µι is Individual 95% confidence intervals (next slide for s p formula): x ± t* Individual 95% CIs For Mean Based on i Pooled StDev ni Group Mean --+---------+---------+---------+------- Alone 82.524 (------*------) Friend 91.325 (-----*------) where s p = MSE and t* is from Table A.2: Pet 73.483 (------*------) --+---------+---------+---------+------- t* is such that the confidence level is the probability 70.0 77.0 84.0 91.0 between -t* and t* in a t-distribution with df = N – k. 19 20 Multiple Comparisons Multiple Comparisons, continued Multiple comparisons: Problem is that each C.I. has Many statistical tests or C.I.s => increased risk 95% confidence, but we want overall 95% confidence. of making at least one type I error (erroneously Can do multiple C.I.s and/or tests at once. rejecting a null hypothesis). Several procedures Most common: all pairwise comparisons of means. to control the overall family type I error rate or overall family confidence level. Ways to make inferences about each pair of means: • Family error rate for set of significance tests is • Significance test to assess if two means probability of making one or more type I errors when significantly differ. more than one significance test is done. • Confidence interval for difference computed and • Family confidence level for procedure used to create a if 0 is not in the interval, there is a statistically set of confidence intervals is the proportion of times all significant difference. intervals in set capture their true parameter values. 21 22 Tukey method: Family confidence level of 0.95 The Family of F-Distributions • Skewed distributions with minimum value of 0. Confidence intervals for µi − µj (differences in means) • Specific F-distribution indicated by two parameters Difference is Alone subtracted from: called degrees of freedom: numerator degrees of Lower Center Upper freedom and denominator degrees of freedom. Friend 2.015 8.801 15.587 (Friend – Alone) Pet -15.827 -9.041 -2.255 (Pet – Alone) • In one-way ANOVA, Difference is Friend subtracted from: numerator df = k – 1, Lower Center Upper and Pet -24.628 -17.842 -11.056 (Pet – Friend) denominator df = N – k None of the confidence intervals cover 0! That • Looks similar to chi- indicates that all 3 population means differ. square distributions Ordering is Pet < Alone < Friend 23 24 4 Determining the p-Value Example: Stress, Friends and Pets Statistical Software reports the p-value in output. Reported F-statistic was F = 14.08 and p-value < 0.000 Table A.4 provides critical values (to find rejection region) for 1% and 5% significance levels. N = 15 women: num df = k – 1 = 3 – 1 = 2 • If the F-statistic is > than the 5% critical value, den df = N – k = 45 – 3 = 42 the p-value < 0.05. • If the F-statistic is > than the 1% critical value, the p-value < 0.01 . Table A.4 with df of (2, 42), closest available is (2, 40): • If the F-statistic is between the 1% and 5% critical The 5% critical value is 3.23. values, the p-value is between 0.01 and 0.05. The 1% critical value is 5.18 and The F-statistic was much larger, so p-value < 0.01. 25 26 Example 16.1 Seat Location and GPA Example 16.1 Seat Location and GPA (cont) Q: Do best students sit in the front of a classroom? • The boxplot showed two outliers in the group Data on seat location and GPA for n = 384 students; of students who typically sit in the middle of 88 sit in front, 218 in middle, 78 in back a classroom, but there are 218 students in that group so these outliers don’t have much influence on the results.
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