  Methods and Materials: We investigated transferability of neural
network-based de-identification sys-tems with and without domain
generalization. We used two domain generalization approaches: a novel approach
Joint-Domain Learning (JDL) as developed in this paper, and a state-of-the-art
domain general-ization approach Common-Specific Decomposition (CSD) from the
literature. First, we measured trans-ferability from a single external source.
Second, we used two external sources and evaluated whether domain
generalization can improve transferability of de-identification models across
domains which rep-resent different note types from the same institution. Third,
using two external sources with in-domain training data, we studied whether
external source data are useful even in cases where sufficient in-domain
training data are available. Finally, we investigated transferability of the
de-identification mod-els across institutions. Results and Conclusions: We
found transferability from a single external source gave inconsistent re-sults.
Using additional external sources consistently yielded an F1-score of
approximately 80%, but domain generalization was not always helpful to improve
transferability. We also found that external sources were useful even in cases
where in-domain training data were available by reducing the amount of needed
in-domain training data or by improving performance. Transferability across
institutions was differed by note type and annotation label. External sources
from a different institution were also useful to further improve performance.
