  A third of adults in America use the Internet to diagnose medical concerns,
and online symptom checkers are increasingly part of this process. These tools
are powered by diagnosis models similar to clinical decision support systems,
with the primary difference being the coverage of symptoms and diagnoses. To be
useful to patients and physicians, these models must have high accuracy while
covering a meaningful space of symptoms and diagnoses. To the best of our
knowledge, this paper is the first in studying the trade-off between the
coverage of the model and its performance for diagnosis. To this end, we learn
diagnosis models with different coverage from EHR data. We find a 1\% drop in
top-3 accuracy for every 10 diseases added to the coverage. We also observe
that complexity for these models does not affect performance, with linear
models performing as well as neural networks.
