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I have found a significant interaction with two continuous variables. I proceeded to create simple slopes graphs as recommended by Preacher (i.e., - 1 SD, mean, and +1SD slopes). What exactly to these different lines mean? I understand the mean - but what do the -1 SD and +1SD slopes represent - have we created three different groups (low group, medium group, and high group)? Is the +1SD line representing an individual? Would like any clearing up on this! Thanks.

I understand the interpretation of an interaction etc. Just want some intuition on how to explain the graph (simple slopes) to someone who keeps thinking I took the data set and divided it into tertiles.

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The simple slope for -1SD is the model predicted slope for someone with a value of the moderator one standard deviation below the mean of the moderator. The simple slope for +1SD is the model predicted slope for someone with a value of the moderator one standard deviation above the mean of the moderator. You're not creating new groups; you're computing the predicted slope for someone with a given value of the moderator. This comes directly from the one regression model you run.

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  • $\begingroup$ Thanks. And what difference does it make that I pick one variable as the moderator over the other? Is this purely a theoretical choice. $\endgroup$
    – mindhabits
    Aug 21, 2019 at 19:40
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    $\begingroup$ Yes, this is a theoretical choice. If one variable is more like a manipulable treatment and the other is more like a background characteristic, the latter would be a good choice as the moderator. $\endgroup$
    – Noah
    Aug 21, 2019 at 20:23

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