The following is a paragraph from the paper, "The Effects of Air Pollution on Students' Cognitive Performance: Evidence from Brazilian University Entrance Tests," by Juliana Carneiro, Matthew A. Cole, and Eric Strobl (2021). After using student fixed effects and controlling for weather conditions, there may still remain concerns about the existence of classical measurement error. For example, the students' actual exposure to outdoor air pollution may differ from the monitoring stations' readings, which may bias our estimates downward due to attenuator bias (Moretti and Neidell 2011; Schlenker and Walker 2015; Chen et al. 2018). Furthermore, as explained above, PM,. is not randomly assigned, leading to the potential presence of unobserved time-varying effects that we cannot account for. Therefore, we also experiment with an instrumental variable approach to estimate our models. In this regard we follow previous works, as, for example, Deryugina et al. (2019), Anderson (2020), and Bondy et al. (2020), and adopt wind direction as our instrument for air pollution, which can be considered a natural experiment that results in exogenous shocks to local air pollution in a specific area due to wind shocks (Currie et al. 2014). The endogenous variable here is_. For wind direction to be a valid instrument, the authors must assume that.. O student test scores; wind direction is not directly related to the students' scores, except through its influence on air pollution air pollution (PM10); wind direction is directly related to the students' scores, and does not directly impact air pollution OOO student test scores; wind direction is directly related to the students' scores, and does not directly impact air pollution air pollution (PM10); wind direction is not directly related to the students' scores, except through its influence on air pollution