ANOVA ANOVA. TwoWay ANOVA. OneWay ANOVA. When to use ANOVA ANOVA. Analysis of Variance. Chapter 16. A procedure for comparing more than two groups


 Roy Pierce Wilcox
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1 ANOVA ANOVA Analysis of Variance Chapter 6 A procedure for comparing more than two groups independent variable: smoking status nonsmoking one pack a day > two packs a day dependent variable: number of coughs per day k number of conditions (in this case, 3) OneWay ANOVA OneWay ANOVA has one independent variable ( factor) with > conditions conditions levels treatments e.g., for a brand of cola factor, the levels are: Coke, Pepsi, RC Cola Independent variables factors TwoWay ANOVA TwoWay ANOVA has independent variables (factors) each can have multiple conditions Example Two Independent Variables (IV s) IV: Brand; and IV: Calories Three levels of Brand: Coke, Pepsi, RC Cola Two levels of Calories: Regular, Diet When to use ANOVA Oneway ANOVA: you have more than two levels (conditions) of a single IV EXAMPLE: studying effectiveness of three types of pain reliever aspirin vs. tylenol vs. ibuprofen Twoway ANOVA: you have more than one IV (factor) EXAMPLE: studying pain relief based on pain reliever and type of pain Factor A: Pain reliever (aspirin vs. tylenol) Factor B: type of pain (headache vs. back pain) ANOVA When a factor uses independent samples in all conditions, it is called a betweensubjects factor betweensubjects ANOVA When a factor uses related samples in all conditions, it is called a withinsubjects factor withinsubjects ANOVA PASW: referred to as repeated measures
2 ANOVA & PASW Why bother with ANOVA? 5 Independent Samples Related Samples samples Independent Samples ttest Paired Samples ttest or more samples Between Subjects ANOVA Repeated Measures ANOVA Mean Pain level 4 3 Tylenol Aspirin Ibuprofen Gin Pain Reliever Would require six ttests, each with an associated Type I (false alarm) rate. Familywise rate Overall probability of making a Type I (false alarm) somewhere in an experiment One ttest, familywise rate is equal to α (e.g.,.05) Multiple ttests result in a familywise rate much larger than the α we selected ANOVA keeps the familywise rate equal to α Posthoc Tests If the ANOVA is significant at least one significant difference between conditions In that case, we follow the ANOVA with posthoc tests that compare two conditions at a time posthoc comparisons identify the specific significant differences between each pair Mean Pain level Tylenol Aspirin Ibuprofen Gin Pain Reliever ANOVA Assumptions Partitioning Variance Homogeneity of variance σ σ σ 3 σ 4 σ 5 Normality scores in each population are normally distributed Mean Pain level Tylenol Aspirin Ibuprofen Gin Pain Reliever
3 Partitioning Variance Total Variance in Scores Partitioning Variance Total Variance in Scores Variance Within Groups () Variance Between Groups group (chance variance) (systematic variance: treatment effect) Mean Square; short for mean squared deviation Similar to variance (s x ) t obt F obt X X s X X difference between sample means difference expected by chance (standard ) variance between sample means variance expected by chance () systematic variance chance variance is an estimate of the variability as measured by differences within the conditions sometimes called the within group variance or the term chance variance (random + individual differences) Tylenol Aspirin Ibuprofen 5 Gin systematic variance.9.7 chance variance 3.3. Mean:.4.8. Pain level variance (within groups) Tylenol Aspirin Ibuprofen Gin Pain Reliever group is an estimate of the differences in scores that occurs between the levels in a factor also called between Treatment effect (systematic variance) Total Variance (variability among all the scores in the data set) X: Tylenol Aspirin Ibuprofen Gin Overall X. Between Groups Variance. Treatment effect (systematic). Chance (random + individual differences) Error Variance (within groups). Chance (random + individual differences) group variance between groups 3
4 FRatio When H 0 is TRUE (there is no treatment effect): F between group variance variance (within groups) Treatment effect + Chance FRatio Chance 0 + Chance Chance In ANOVA, variance Mean Square () between group variance FRatio variance (within groups) group When H 0 is FALSE (there is a treatment effect): F Treatment effect + Chance Chance > Signalto Noise Ratio ANOVA is about looking at the signal relative to noise group is the signal is the noise We want to see if the betweengroup variance (signal), is comparable to the withingroup variance (noise) Logic Behind ANOVA If there is no true difference between groups at the population level: the only differences we get between groups in the sample should be due to. if that s the case, differences between groups should be about the same as differences among individual scores within groups (). group and will be about the same. Logic Behind ANOVA If there are true differences between groups: variance between groups will exceed variance (within groups) F obt will be much greater than F obt F obt can also deviate from by chance alone we need a sampling distribution to tell how high F obt needs to be before we reject the H 0 compare F obt to critical value (e.g., F.05 ) group Logic Behind ANOVA The critical value (F.05 ) depends on degrees of freedom between groups: df group k  (within): df k ( n  ) Total: df total N  alpha (α) e.g.,.05,.0 4
5 ANOVA Example: Cell phones Research Question: Is your reaction time when driving slowed by a cell phone? Does it matter if it s a handsfree phone? Twelve participants went into a driving simulator.. A random subset of 4 drove while listening to the radio (control group).. Another 4 drove while talking on a cell phone. 3. Remaining 4 drove while talking on a handsfree cell phone. Every so often, participants would approach a traffic light that was turning red. The time it took for participants to hit the breaks was measured. A 6 Step Program for Hypothesis Testing. State your research question. Choose a statistical test 3. Select alpha which determines the critical value (F.05 ) 4. State your statistical hypotheses (as equations) 5. Collect data and calculate test statistic (F obt ) 6. Interpret results in terms of hypothesis Report results Explain in plain language A 6 Step Program for Hypothesis Testing. State your research question Is your reaction time when driving influenced by cellphone usage?. Choose a statistical test three levels of a single independent variable (cell; handsfree; control) OneWay ANOVA, between subjects 3. Select α, which determines the critical value α.05 in this case See Ftables (page 543 in the Appendix) df group k 3 (numerator) df k (n  ) 3(4) 9 (denominator) F.05? 4.6 F Distribution critical values (alpha.05) df group k 3 (numerator) 4. State Hypotheses df k (n  ) 3(4) 9 (denominator) H 0 : μ μ μ 3 H : not all μ s are equal referred to as the omnibus null hypothesis When rejecting the Null in ANOVA, we can only conclude that there is at least one significant difference among conditions. If ANOVA significant pinpoint the actual difference(s), with posthoc comparisons 5
6 Examine Data and Calculate F obt DV: Response time (seconds) X Control Is there at least one significant difference between conditions? X Normal Cell X 3 Handsfree Cell A Reminder about Variance Variance: the average squared deviation from the mean X, 4, 6 X 4 Sample Variance Definitional Formula s X (X  X) N Sum of the squared deviation scores X XX (XX) (X  X) 8 s X 8/ 4 ANOVA Summary Table Source Sum of df Mean F Squares Squares Group SS group df group group F obt Error SS df Total SS total df total SS Sum of squared deviations or Sum of Squares ANOVA Summary Table Source Sum of df Mean F Squares Squares Group.07 df group group F obt Error.050 df Total. df total SS total SS + SS group SS total ANOVA Summary Table Source Sum of df Mean F Squares Squares Group.07 group F obt Error Total. df between groups k  df (within groups ) k (n ) df total N (the sum of df group and df ) Examine Data and Calculate F obt Compute the mean squares SS group df SS df group group
7 Examine Data and Calculate F obt ANOVA Summary Table Compute F obt F obt group Source Sum of df Mean F Squares Squares Group Error Total. ANOVA Example: Cell phones Interpret results in terms of hypothesis 6.45 > 4.6; Reject H 0 and accept H Report results F(, 9) 6.45, p <.05 Explain in plain language Among those three groups, there is at least one significant difference X Interpret F obt Figure 6.3 Probability of a Type I as a function of the number of pairwise comparisons where α.05 for any one comparison Posthoc Comparisons Fisher s Least Signficant Difference Test (LSD) uses ttests to perform all pairwise comparisons between group means. good with three groups, risky with > 3 this is a liberal test; i.e., it gives you high power to detect a difference if there is one, but at an increased risk of a Type I 7
8 Posthoc Comparisons Bonferroni procedure uses ttests to perform pairwise comparisons between group means, but controls overall rate by setting the rate for each test to the familywise rate divided by the total number of tests. Hence, the observed significance level is adjusted for the fact that multiple comparisons are being made. e.g., if six comparisons are being made (all possibilities for four groups), then alpha.05/ Posthoc Comparisons Tukey HSD (Honestly Significant Difference) sets the familywise rate at the rate for the collection for all pairwise comparisons. very common test Other posthoc tests also seen: e.g., NewmanKeuls, Duncan, Scheffe Effect Size: Partial Eta Squared Partial Eta squared (η ) indicates the proportion of variance attributable to a factor 0.0 small effect 0.50 medium effect 0.80 large effect Calculation: PASW Effect Size: Omega Squared A less biased indicator of variance explained in the population by a predictor variable SSgroup ( k ) ω SS total +.07 (3 )(.0056) ω % of the variability in response times can be attributed to group membership (medium effect) PASW: OneWay ANOVA (Between Subjects) Setup a oneway between subjects ANOVA as you would an independent samples ttest: Create two variables one variable contains levels of your independent variable (here called group ). there are three groups in this case numbered 3. second variable contains the scores of your dependent variable (here called time ) PASW : OneWay ANOVA (Between Subjects) 8
9 Label the numbers you used to differentiate groups: Go to Variable View, then click on the Values box, then the gray box labeled Enter Value (in this case, or 3) and the Value Label (in this case: control, cell, hands) Click Add, and then add the next two variables. Performing Test Select from Menu: Analyze > General Linear Model > Univariate Select your dependent variable (here: time ) in the Dependent Variable box Select your independent variable (here: group ) in the Fixed Factor(s) box Click Options button, checkdescriptives (this will print the means for each of your levels) check Estimates of effect size for Partial Eta Squared Click the Post Hoc button for post hoc comparisons; move factor to Post Hoc Tests for box; then check LSD, Bonferroni, or Tukey Click OK Dependent Variable: rt Tukey HSD control and cell groups are significantly different if p <.05, then significant effect (I) condition control cell hands (J) condition cell hands control hands control cell *. The mean difference is significant at the.05 level. Mean Difference 95% Confidence Interval (IJ) Std. Error Sig. Lower Bound Upper Bound * * * * hands and cell groups are NOT significantly different Complete explanation Any kind of cell phone conversation can cause a longer reaction time compared to listening to the radio. There is no significant difference between reaction times in the normal cell phone and handsfree conditions. PASW and Effect Size Click Options menu; then check Estimates of effect size box This option produces partial eta squared Partial Eta Squared 9
10 PASW Data Example ANOVA Summary Three groups with three in each group (N 9) Fast Medium Slow X if p <.05, then significant effect Effect size Post Hoc Comparisons slow and medium groups are not significantly different slow and fast groups are significantly different ShamayTsoory SG, Tomer R, AharonPeretz J. (005) The neuroanatomical basis of understanding sarcasm and its relationship to social cognition. Neuropsychology. 9(3), A Sarcastic Version Item Joe came to work, and instead of beginning to work, he sat down to rest. His boss noticed his behavior and said, Joe, don t work too hard! A Neutral Version Item Joe came to work and immediately began to work. His boss noticed his behavior and said, Joe, don t work too hard! Following each story, participants were asked: Did the manager believe Joe was working hard? 0
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