SPSS Learning Objectives:

Transcription

1 Tasks Two lab reports Three homeworks Shared file of possible questions Three annotated bibliographies Interviews with stakeholders 1 SPSS Learning Objectives: Be able to get means, SDs, frequencies and histograms for any variable Be able to determine the Cronbach s alpha for any set of items. Be able to reverse score a single item and combine items into a single scale 1. Check for errors (range, distribution) 2. Reverse code any relevant items 3. Estimate reliability (Cronbach s alpha) 4. Combine items Be able to correlate any two variables (represented as single item or as scale). Be able to center a variable before entering it as a predictor in a multiple regression equation. Be able to distinguish between mediators and moderators. Be able to draw simple path diagrams. Be able to create product terms to test interaction questions. Be able enter predictors in multiple steps to test mediation questions. 1

2 Resources (Kristopher Preacher s website) (Andrew Hayes website) (David Kenny s website) Basic Research 2

3 The Mediational Model The Four Paths X Y: path c X M: path a M Y (controlling for X): path b X Y (controlling for M): path c 3

4 Mediation A tri variate unidirectional (thus causal) hypothesis: What are the mechanisms of effect underlying a preestablished causal statistical relationship? A pre established causal relationship between two variables (X, Y) is theorised to exist due to an intermediate third variable (Me) The additional (third) variable that is hypothesised to have this effect is known as a mediator though is also sometimes referred to as an: intermediate variable (Kraemer et al., 2001) explanatory link (Rose, Holmbeck, Coakley, & Franks, 2004) Mediators have mediating effects that are otherwise labelled: indirect effects surrogate effects intermediate effects intervening effects (MacKinnon, Lockwood, Hoffman, West, & Sheets, 2002; *Wu & Zumbo, 2007) 7 Why the Interest in Mediation? Understand the mechanism theoretical concerns cost and efficiency concerns Find more proximal endpoints Understand why the intervention did not work finding the missing link compensatory processes (suppressor effects) 8 4

5 The Four Step Approach Figure 1. Multiple Mediation Model for Mental Health.07 (.09+) Family Income Campus Climate -.36** -.15* -.23*.27*** Individual Relative Deprivation Social Support -.31***.23*** Mental Health.11* (.27**) Note. N=461. Numbers represent standardized regression coefficients from equations that include respondent gender, age and minority/majority ethnic background. Final model explained 29% of the adjusted variance. + p<.10 *p<05 **p<.01 ***p<

6 The Four Steps Step 1: X Y (test path c) Step 2: X M (test path a) Step 3: M (and X) Y (test path b) Step 4: X (and M) Y (test path c ) Note that Steps 3 and 4 use the same regression equation. Total and Partial Mediation Total Mediation Meet steps 1, 2, and 3 and find that c equal zero Partial Mediation Meet steps 1, 2, and 3 and find that c is smaller in absolute value than c Misplaced focus? Measurement error Could miss other sources of indirect effects Test under powered (too conservative) No single test of reliability 6

7 Indirect Effects Approach Figure 1. Multiple Mediation Model for Mental Health Family Income -.12 (.01)**.07 (.02)** Individual Relative Deprivation -.25 (.07)*** Mental Health -.31 (.06)*** Social Support.14 (.05)**.51 (.08)*** Campus Climate Note. N=461. Numbers represent standardized regression coefficients (and standard errors) from equations that include respondent gender, age and minority/majority ethnic background. Final model explained 29.0% of the adjusted variance. *p<05 **p<.01 ***p<

8 Decomposition of Effects Total Effect = Direct Effect + Indirect Effect c = c + ab (This equality exactly holds for multiple regression, but not necessarily for other estimation methods.) The Indirect Effect or ab provides one number that summarizes the amount of mediation. Note that ab = c c That is, the indirect effect exactly equals the amount of the reduction in the total effect (c) after the mediator is introduced. 15 Estimating the Total Effect (c) The total effect or c can be inferred from direct and indirect effect as c + ab. We need not perform the Step 1 regression to estimate c. This can be useful in situations when c does not exactly equally c + ab. 16 8

9 Inconsistent Mediation ab and c have a different sign X as a suppressor variable Consequences The Total Effect or c may not be significant Percent mediated greater than 100% Do we have mediation? Yes. There is an indirect effect (ab > 0). No. There is no effect that is mediated. 17 What to Present Decomposition of Effects Total Effect Indirect Effect(s) Direct Effects Less of an emphasis on complete versus partial mediation. 18 9