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[Instructor] El camino de causa
a efecto es oscuro y peligroso,

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pero las armas
de la econometría son robustas.

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Atacan con variables instrumentales
feroces y flexibles

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caundo la naturaleza te bendice
con deberes fortuitos y casuales

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(gong sonando)

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Los ensayos aleatorizados
son la vía más segura

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para hacer
comparaciones ceteris paribus.

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Pero ¡qué pena!, esta poderosa herramienta
no siempre está disponible.

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Sin embargo, la aleatorización
a veces pasa por casualidad

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y es cuando utilizamos
las variables instrumentales,

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las cuales se abrevian VI.

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(susurros)
Variables instrumentales.

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[Instructor] La lección de hoy
es la primera de dos lecciones sobre VI.

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La primera lección de VI comienza
con una historia sobre las escuelas.

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(timbre de escuela sonando)

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[Josh] Las escuelas chárter
son escuelas públicas libres

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de la supervisión diaria
del distrito y de los contratos

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de la unión de maestros.
La pregunta es si las escuelas chárter

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fomentan el logro es
una de las más importantes

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en la historia de la reforma
de la educación estadounidense.

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[Josh] Las escuelas chárter más populares
tienen más aspirantes que cupos;

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por lo tanto, el azar en un sorteo decide
a quién se le ofrece un cupo.

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Es mucho lo que está en juego
para los estudiantes disputandose

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esa opción y esperando
que los resultados de la lotería

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les devuelva muchas emociones, que fue
lo que se plasmó en el documental premiado

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"Esperando a Superman".

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[Mother] No llores.
Vas hacer que mamá llore, por favor.

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[Josh] ¿Las escuelas chárter
dan mejor educación?

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Los críticos definitivamente dirán que no

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argumentando que esas escuelas matriculan,
ante todo, a los mejores estudiantes,

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los más inteligentes o más motivados,
así que las diferencias resultantes

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reflejan un sesgo de selección.

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[Kamal] Un momento, parece fácil.

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En una lotería, los ganadores
se seleccionan al azar,

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así solo se comparan
ganadores y perdedores.

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[Student] Obviamente.

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[Instructor] Vas por buen camino, Kamal,

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pero las loterías de las escuelas chárter
no hace que los niños

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entren o queden afuera
de una escuela particular,

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lo que hace es aleatorizar las ofertas
de cupos para una escuela chárter.

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Algunos niños corren con suerte,

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otros no.

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Si lo único que queremos
era saber el efecto

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de las ofertas de las escuelas chárters,

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podríamos tratar esto
como un ensayo aleatorizado,

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pero si

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