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1 CLG Handout Problem #1 (Examining interaction plots)

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1 1 CLG Handout Problem #1 (Examining interaction plots)

2 2 Interpreting interaction plots Be aware that you will never get “perfect” results from looking at a plot. In most cases we are identifying what we think is the case and certainly we need to follow up what we think we see with some hard numbers to verify it. So as we interpret the next 6 graphs, although we can comment on what we think we see, ultimately the F tests will tell us significance or not.

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4 4 (A) Interaction  None. Lines are parallel. Drug effect  None. Gender effect  Yes, probably—looks like it would be significant. (Men higher than women) General interpretation  Only a Gender difference!

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6 6 (B) Interaction  None. Lines are parallel. Drug effect  Yes, response goes down with Drug level 1. F test would determine if its significant. Gender effect  No. Men and women are nearly the same. General interpretation  Only a Drug effect!

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8 8 (C) Interaction  Yes. Lines are NOT parallel. Drug effect  Not sure (probably though). Either way it’s tied to the interaction anyway. Gender effect  Yes, probably, but also tied to the interaction. General interpretation  “Typical” interaction. Drug seems to work for the only the women but not for the men. Can’t discuss main effects by themselves.

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10 10 (D) Interaction  Yes. Lines are NOT parallel. Drug effect  No, shouldn’t be significant but it’s tied to the interaction anyway. Gender effect  Maybe, women seem a little higher than men but not much. Still tied to interaction though. General interpretation  Almost “reverse interaction”. Drug seems to work for the women, increasing their response. But at the same time it works for men lowering their response.

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12 12 (E) Interaction  Yes. Lines are NOT parallel. Drug effect  No, shouldn’t be significant but it’s tied to the interaction anyway. Gender effect  No, shouldn’t be significant but it’s tied to the interaction anyway. General interpretation  “Reverse interaction”. Drug seems to work for the women, increasing their response. But at the same time it works for men lowering their response. Additionally, Men started higher but are lower after taking the drug, women start lower but are higher after taking the drug. Hence the reverse!

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14 14 (F) Interaction  None. Very mild, probably not significant. Drug effect  None. Gender effect  Probably not, men seem slightly higher than women but probably not significant so. General interpretation  Nothing here at all. There are slight effects for all 3 terms but they are all mild enough that it probably is due to just randomness. We can’t expect all groups to be exactly the same so while we see mild differences, we want significant differences!

15 15 Problem #2 Exploring the relationships

16 16 ANOVA Table SOURCE DF SS MS F State Location State*Loc Error Total

17 17 Tests Interaction is marginally significant; Both main effects test quite significant.

18 18 Effect Sizes State Effect: ( )/3 – ( )/3 = 1.80 Rural vs. Suburb: ( )/2 – ( )/2 = Rural vs. Urban: ( )/2 – ( )/2 = Suburb vs. Urban: ( )/2 – ( )/2 = RuralSuburbUrban Arkansas Indiana

19 19 Effect Sizes (2) Arkansas has longer wait times than Indiana Urban > Suburban > Rural wait times Interaction? Very marginal. Description from plot: State effect size is slightly less for Rural areas than it is for Urban or Suburban areas.

20 20 Question #3 A SAS Example

21 21 Part (a) – Design Chart

22 22 Part (b) Assumptions are ok (you expect to see this type of QQ plot) since there are numerous identical responses

23 23 Part (b) Ad is clearly the most important effect Age and the Ad*Age interaction are marginally significant (small sample size so there’s a good chance they are important) Interaction plot tells some of the story

24 24 Interaction Plot

25 25 Interpreting the interactions Feature ads don’t appear to be effective. (Dump them entirely – we can draw this conclusion based on the picture and the fact that the main effect for ad is so significant). Visual ads may be more effective for older people; Budget ads almost surely the most effective for the younger age groups.

26 26 Tukey Groupings

27 27 Questions?


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