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

But the weapons 
of Econometrics are strong.

[Attack] with fierce 
and flexible instrumental variables

when nature blesses you 
with fortuitous random assignment.

[gong rings]

Randomized trials are the surest 
path to ceteris parabus comparisons.

Alas, this powerful tool 
is often unavailable.

But sometimes, randomization 
happens by accident.

That's when we turn 
to instrumental variables,

IV for short.

- [Voice whispers] Instrumental 
variables.

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

Our first IV lesson begins 
with a story of schools.

- [Josh] Charter schools 
are public schools

freed from daily district oversight
and teacher union contracts.

The question of whether charters 
boost achievement

is one of the most important

in the history 
of American education reform.