9:59:59.000,9:59:59.000
- [Narrator] The path from cause [br]to effect is dark and dangerous.

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But the weapons [br]of Econometrics are strong.

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[Attack] with fierce [br]and flexible instrumental variables

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when nature blesses you [br]with fortuitous random assignment.

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[gong rings]

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Randomized trials are the surest [br]path to ceteris parabus comparisons.

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Alas, this powerful tool [br]is often unavailable.

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But sometimes, randomization [br]happens by accident.

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That's when we turn [br]to instrumental variables,

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IV for short.

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- [Voice whispers] Instrumental [br]variables.

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- [Narrator] Today's lesson [br]is the first of two on IV.

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Our first IV lesson begins [br]with a story of schools.

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- [Josh] Charter schools [br]are public schools

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freed from daily district oversight[br]and teacher union contracts.

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The question of whether charters [br]boost achievement

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is one of the most important

9:59:59.000,9:59:59.000
in the history [br]of American education reform.