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Probabilistic Equivalence in Experimental Design

Probabilistic Equivalence in Experimental Design. What Is Probabilistic Equivalence?. It means that we know perfectly the odds that we will find a pretest difference between the two groups. It doesn’t mean that the two groups will have identical pretest means.

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Probabilistic Equivalence in Experimental Design

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  1. Probabilistic Equivalence in Experimental Design

  2. What Is Probabilistic Equivalence? • It means that we know perfectly the odds that we will find a pretest difference between the two groups. • It doesn’t mean that the two groups will have identical pretest means.

  3. What Is Probabilistic Equivalence? Group 1 49

  4. What Is Probabilistic Equivalence? Group 1 Group 2 51 49

  5. What Is Probabilistic Equivalence? Group 1 Group 2 51 49 With  = .05, we expect that we will observe a pretest difference 5 times out of 100.

  6. What Is Probabilistic Equivalence? • Even if we observe a pretest difference, it must be due to chance because we assigned by chance (so it’s one of the 5 out of 100 cases).

  7. What Is Probabilistic Equivalence? • If we observe a pretest difference, what does this mean for the posttest? • NOTHING! • The odds we will observe a posttest difference by chance are still 5 out of a 100 (with =.05).

  8. What Is Probabilistic Equivalence? • Because we assigned randomly, we could observe differences on either the pretest or posttest, but these must be due to chance (or the luck of the draw). • We know perfectly the odds of observing a chance difference. • We can control this (through ).

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