Moderation Paul Jose PSYC 325 Thought for the day: “When you go tramping, stay on the trail!”

What are they? You’ve already been introduced to moderation. –“A moderating variable is a third variable that affects the relationship between two others, so that the nature of the impact of the predictor on the criterion varies according to the level or value of the moderator” (Holmbeck, 1997, p. 599). Hmm. Nice definition, but what does it mean in the real world with real variables? I think that learning about moderation is helped by learning what mediation is at the same time. So what’s mediation?

Mediation “A mediating variable is one which specifies how (or the mechanism by which) a given effect occurs” between an independent variable (IV) and a dependent variable (DV). (Holmbeck, 1997, p. 599). Okay, is that clear? Well, maybe not perfectly. Let’s delve into these two phenomena more deeply, and hopefully at the end of my explanation it will be clearer.

My definitions Moderation: this tells you the circumstances under which the main effect occurs. If you don’t obtain a significant moderation effect, then you know that the main effect applies about equally at all levels of the IV and moderator. If you do obtain a significant moderation, then you need to look at the figure to see under what joint conditions of the moderator and IV the main effect occurs. Mediation: this tells you about the “mechanism” of how the IV affects the DV. Does the effect of the IV “go through” the mediator, or does it merely have a simple direct effect? Let’s look at some diagrams.

StressCopingOutcome Stress Coping Outcome Mediation Moderation

Statistically, how are they different or similar? Let’s begin with moderation (a review). You focus on an IV (e.g., stress), and a potential moderating variable (e.g., coping), as predictors of a DV (e.g., depression). You should centre these two main effects, i.e., subtract the mean from the variables. Reduces multicollinearity. Not a z-score. Then you create a product term by multiplying the two main effects. (What do you do when you have a categorical moderator like gender? I’ll show you later.) Then you regress your DV on your IVs in a hierarchical fashion.

I’ve always been dubious about this centering thing—does it matter? Uncentered variables

Okay, this is with centering Alright, it does make a difference! So don’t forget to center first.

The centering controversy I said last week that one should centre one’s variables before creating the product term. Why? I have read that centering them prevents multicolliearity (and my correlations support this contention). I have never actually tested this hypothesis, so I decided to do so. I used the centered and uncentered stress and rumination scores in separate regressions, and I found the following:

Results No differences in beta weights, significances, or R 2 s for the main effects. No difference in significances or R 2 s on the third step, But the beta weights were different on the third step. Why? It has to do with this issue of correlations among the three predictors. Is this a reason to be concerned? Only if you report and use the beta weights. Most important question: do these two methods yield different figures? Answer: no. See the following pages.

So what to do? I don’t think that it’s a major issue. You can choose to do it or not as you wish, and it won’t affect the important outcomes: significances, R 2 s, or the resulting figures. Purists will require it, but this is because they hand-computed means for figures. We don’t have to do that anymore. So don’t sweat it.

Moderation in hierarchical regression Stress Coping Stress X Coping Depression Step 1 Step 2 Step 3

What are you looking for? You must have a significant main effect for the IV, in this case, stress. Without this you have nothing to moderate. (must have this) Second, you may or may not have a main effect for the moderating variable, coping. If you have a significant main effect, then you know that it has a direct effect on the DV. (usually have this) Third, you check to see whether you have significant moderation by determining whether the interaction term is a statistically significant predictor. (rare to find)

Let’s look at the SPSS print-out

Interpretation please... Stress (emuch: how much stress is caused by everyday life events) is a main effect predictor, R 2 =.29, p <.001. More stress, worse negative adjustment. Rumination (amount of thinking about one’s negative affect) is a positive and significant predictor, R 2 change =.10, p <.001. More rumination, worse negative adjustment. Stress by rumination (product term) is a negative and significant predictor, R 2 change =.005, p <.001. What does this mean? Notice the small unique variance. Huge sample (N = 2,500).

The magic of ModGraph TM Marc introduced you to ModGraph, and told you what it can do, but since I’m inordinately proud of it, I can’t help but show it off again. The point I’d like to make from the previous page is: “one cannot interpret the interaction from a simple beta weight”. One must graph the interaction to learn what it means. How does one graph these? Well, one could laboriously hand-compute 9 algebraic formulae, and then either draw a figure or import the means for PowerPoint, or... The enlightened researcher could employ ModGraph to do this in a fraction of the time.

The interpretation of moderation There are a multitude of patterns that will yield a statistically significant interaction. (I am in the process of trying to catalogue all of them.) Generally one looks for the “spread” or “fan” effect. The means for the three rumination groups are most different under the condition of low stress. Thus, the most easily understandable reading is something like, –“rumination worsens the impact of stress on negative adjustment under conditions of low stress”. Notice that the spread is not dramatic; huge sample size allowed us to find this subtle effect.

Okay, smartypants, what about categorical moderators? I think Marc or Ron discussed categorical moderators, but here’s a bit more on this. The previous example (i.e., rumination) featured a continuous moderator, but what about categorical variables like ethnic group, gender, religion, etc.? Well, what do you know, ModGraph can handle them too! At this time, only two-level variables can be probed. In the future, we will be able to handle n-level cases.

Moderation of social support by gender We know that females report using more social support than males. In our data, it’s: –Males: 18.9 –Females: 22.4 But moderation can tell you how they actually use it. For example, does social support reduce depression equally for the two genders, or is there a difference between them? Three terms: social support; gender (0 = males; 1 = females); and social support X gender (product term).

By the way... You can perform an ANOVA that is similar to this regression: –Neg adj as the DV; and –Gender and Social Support are the two IVs. Difference? All IVs in ANOVAs must be categorical (which is fine for gender), so you must dichotomize (high vs. low) or trichotomize (high, medium, and low) social support. I trichotomized in this case, and see what the print-out shows.

Two main effects and an interaction: ANOVA results

Same basic findings in the regression format

The resulting figure

Simple slopes Now there is another great feature of ModGraph that we can talk about! It has the ability to compute simple slopes, and tell you whether a particular slope is significantly different from zero. You need to obtain four more bits of information: –Variance of social support (SS); –Variance of the interaction (gender by SS) –Covariance of interaction by SS; and –N of sample. Let me show you.

Simple slopes The computation of the simple slopes indicates that the females evidenced a steeper slope compared to the males. What does that mean? –Males: slope = -.02, t = -2.61, p <.01; r = -.10*; and –Females: slope = -.04, t = -7.20, p <.00001; r = -.20**. This result indicates that females benefit from social support more than males.

Okay, so what did we learn here? (Well, besides the fact that ModGraph is just about the coolest programme this side of Asteroids.. ?) We learned: –That moderation tells you under what circumstances something has an effect, i.e., it qualifies the main effects. –That one cannot explain the moderation effect until you have looked at the graph. –That interpreting interactions is difficult. Practice, practice, practice.