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- [Narrator] The path from cause 
to effect is dark and dangerous.

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But the weapons 
of Econometrics are strong.

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[Attack] with fierce 
and flexible instrumental variables

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when nature blesses you 
with fortuitous random assignment.

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[gong rings]

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Randomized trials are the surest 
path to ceteris parabus comparisons.

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Alas, this powerful tool 
is often unavailable.

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But sometimes, randomization 
happens by accident.

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That's when we turn 
to instrumental variables,

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IV for short.

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- [Voice whispers] Instrumental 
variables.

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

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Our first IV lesson begins 
with a story of schools.

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- [Josh] Charter schools 
are public schools

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freed from daily district oversight
and teacher union contracts.

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The question of whether charters 
boost achievement

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is one of the most important

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in the history 
of American education reform.