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Hi, everyone, and welcome to module nine.

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This module starts our discussion
of effect modification,

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which you may have heard called
interaction or moderation.

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You'll hear those terms
used interchangeably

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throughout this class and in
the field of public health.

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We've actually been talking about

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threats to internal validity
way back since module five.

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We started out with bias,

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then we spent three modules
talking about confounding,

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and now we're going to
spend the next three modules

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talking about effect modification
using the same structure.

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We'll start here in module
nine going over the theoretical

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and conceptual framework
of effect modification.

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In the next module, we'll
extend that into SPSS

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and the statistical techniques

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that you use to evaluate
effect modification.

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Then in module 11, we'll look
at interpreting the results

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of effect modification in
the public health literature.

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Effect modification and
confounding are similar in concept,

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but they're actually very
different in practice.

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It's a very good idea to read the section

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on interaction from your Epi 1 textbook,

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"Gordis Epidemiology," before you get into

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your current text.

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As always, please reach out
if you have any questions.

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We'll see you in the
next module, take care.