Engzell, Per;    Frey, Arun;    Verhagen, Mark D;      (2021)    Learning loss due to school closures during the COVID-19 pandemic.                   Proceedings of the National Academy of Sciences , 118  (17)    , Article e2022376118.  10.1073/pnas.2022376118 <https://doi.org/10.1073/pnas.2022376118>.       Green open access