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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 fuera
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 quisiéremos
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, nosotros estamos interesados

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en los efectos de la asistencia

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y no el de las ofertas.

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No siempre al que se le ofrece
un cupo lo acepta.

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VI convierte el efecto del ofrecimiento
de un cupo chárter en el efecto

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de la asistencia real
a una escuela chárter.

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- [Estudiante] Genial.
- ¡Oh!, excelente.

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[Instructor] Veamos el ejemplo.
Una escuela chárter

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del programa el Conocimiento Es Poder,
que se abrevia PCEP.

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Esta escuela PCEP está en Lynn,

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un pueblo industrial olvidado,
en la costa de Massachusetts.

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La escuela tienen
más aspirantes que cupos

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y, por tanto, escoge
a los estudiantes utilizando la lotería.

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Entre 2005 a 2008 371 estudiantes
de cuarto y quinto grando

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pusieron sus nombres
en la lotería del PCEP de Lynn,

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235 estudiantes ganaron un cupo en el PCEP

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y 118 estudiantes perdieron.

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Un años después, los ganadores tenían
mayores puntajes en matemática

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que los perdedores de la lotería.

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Pero, recuerden,
no estamos tratando de determinara

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si ganar la lotería
te hace mejor en matemáticas.

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Queremos saber si asistir a una PCEP
te hace ser mejor en matemáticas.

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De los 253 ganadores de la lotería,

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únicamente 199 fureon
de verdad a una PCEP.

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Los otros escogieron
una escuela pública tradicional.

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