  Evaluating generative adversarial networks (GANs) is inherently challenging.
In this paper, we revisit several representative sample-based evaluation
metrics for GANs, and address the problem of how to evaluate the evaluation
metrics. We start with a few necessary conditions for metrics to produce
meaningful scores, such as distinguishing real from generated samples,
identifying mode dropping and mode collapsing, and detecting overfitting. With
a series of carefully designed experiments, we comprehensively investigate
existing sample-based metrics and identify their strengths and limitations in
practical settings. Based on these results, we observe that kernel Maximum Mean
Discrepancy (MMD) and the 1-Nearest-Neighbor (1-NN) two-sample test seem to
satisfy most of the desirable properties, provided that the distances between
samples are computed in a suitable feature space. Our experiments also unveil
interesting properties about the behavior of several popular GAN models, such
as whether they are memorizing training samples, and how far they are from
learning the target distribution.
