In the last paragraph of p. 55, the
expectations of $f_{i}(s−4)$ is taken and the expectation of
$f_{i}(s−1)$ is not. The text reads:

Conditional on $X_{i}$, the average causal effect of one-year increase
in schooling is $E[f_{i}(s)−f_{i}(s−1)|X_{i}]$, while the average
causal effect of a four-year increase in schooling is
$E[f_{i}(s)−E[f_{i}(s−4)]|X_{i}]$

In the second equation there is an expectation inside the expectation.

## whoops

Eagle-eyed Robson Santos notes:

In the last paragraph of p. 55, the

expectations of $f_{i}(s−4)$ is taken and the expectation of

$f_{i}(s−1)$ is not. The text reads:

Conditional on $X_{i}$, the average causal effect of one-year increase

in schooling is $E[f_{i}(s)−f_{i}(s−1)|X_{i}]$, while the average

causal effect of a four-year increase in schooling is

$E[f_{i}(s)−E[f_{i}(s−4)]|X_{i}]$

In the second equation there is an expectation inside the expectation.

Indeed, Robson, that inner E is a typo!