Making Inferences about Causality

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Making Inferences about Causality

• In general, children who watch violent television
  programs tend to behave more aggressively toward
  their peers and siblings.
• Question Can we assume a causal relationship
  between these two variables?

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Making Inferences about Causality

• Answer not necessarily
• Although causality generally implies correlation,
  correlation does not necessrily imply causality.
• There are at least three other ways to explain
  the correlation between TV viewing and aggressive
  behavior.

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Making Inferences about Causality

• (a) Acting aggressively makes you want to watch
  more violent TV

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Making Inferences about Causality

• (b) Acting violent makes you want to watch more
  TV and watching TV makes you act more violently

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Making Inferences about Causality

• (c) A third variable influences both variables,
  causing them to be correlated

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How can we tease apart these various
possibilities?

• One way to do so is to conduct an experiment
• In an experiment, at least one variable is
  manipulated (i.e., systematically varied) by a
  researcher in order to study its effects on
  another variable.

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Experimental Research

• Features of an experiment
• (a) At least one variable is manipulated or
  varied by the experimenter independent variable
  (IV)
• (b) The variable presumably affected by the
  manipulation is called the dependent variable
  (DV)
• (c) random assignment to conditions

IV
DV

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IV
DV

• Independent Variable Watching violent TV
• Levels
• view an episode of the Sopranos
• view an episode of the Sopranos in which the
  violent scenes have been edited

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Random Assignment

• Why is random assignment important?
• Consider what would happen if we assigned men to
  the violent level of the IV and women to the
  non-violent level of the IV.
• Sex would be associated with the IV.

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Random Assignment

• Confounding variable a variable that influences
  the dependent variable and is associated with the
  independent variable
• When confounding variables are present, we cannot
  make a strong inference that the independent
  variable causes the dependent variable.

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Random Assignment

• Random assignment to conditions helps to remove
  the problem of confounding variables.
• When people are randomly assigned to conditions,
  we should (in the long run) have equal numbers of
  men and women in our two conditions.
• As a result, the possible confound (e.g., sex) is
  uncorrelated with the independent variable.

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Random Assignment

• Previously, we had discussed the possibility that
  the violence of the family context is a third
  variable that might be causing both violent TV
  viewing and aggressive behavior.
• We could control for this possible confound by
  randomly assigning people to conditions.
• There should be, in theory, an equal number of
  people from violent families in each condition.

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Confounding Variables and Non-confounding
Variables

• A variable can exist that has a genuine effect on
  the dependent variable but that is uncorrelated
  with the independent variable.

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Confounding Variables and Non-confounding
Variables
Acting violent




Living in a violent family
Living in a violent family
Watching violent TV
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Experimental Research

• Between- and within-subjects designs
• between-subjects different people are exposed to
  each level of the IV
• within-subjects the same people exposed to each
  level of the IV

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Experimental Research

• Pros and cons of between and within designs
• between different people in each condition. They
  may differ in certain respects
• within same DV assessed many times. Problems
  with learning effects and awareness of the
  manipulation
• counter-balancing balancing the order of DVs

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Counter-balancing

• t1 t2 t3
• Person 1 T1 T2 T3
• Person 2 T1 T3 T2
• Person 3 T2 T1 T3
• Person 4 T2 T3 T2
• Person 5 T3 T1 T2
• Person 6 T3 T2 T1

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Experimental Research

• Multiple independent variables
• called factorial designs
• of levels ? of levels
• IV1 IV2
• Example a 2 ? 3 factorial design
• 2 levels of the first IV, violent TV viewing
  (e.g., watching violent TV and nonviolent TV) and
  3 levels of second IV, provocation (e.g., 0
  threats, 1 threat, 2 threats)

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Experimental Research

• Mixed design one factor is a between-subjects
  factor and the other is a within-subjects factor