  This paper reexamines the research on out-of-distribution (OOD) robustness in
the field of NLP. We find that the distribution shift settings in previous
studies commonly lack adequate challenges, hindering the accurate evaluation of
OOD robustness. To address these issues, we propose a benchmark construction
protocol that ensures clear differentiation and challenging distribution
shifts. Then we introduce BOSS, a Benchmark suite for Out-of-distribution
robustneSS evaluation covering 5 tasks and 20 datasets. Based on BOSS, we
conduct a series of experiments on pre-trained language models for analysis and
evaluation of OOD robustness. First, for vanilla fine-tuning, we examine the
relationship between in-distribution (ID) and OOD performance. We identify
three typical types that unveil the inner learning mechanism, which could
potentially facilitate the forecasting of OOD robustness, correlating with the
advancements on ID datasets. Then, we evaluate 5 classic methods on BOSS and
find that, despite exhibiting some effectiveness in specific cases, they do not
offer significant improvement compared to vanilla fine-tuning. Further, we
evaluate 5 LLMs with various adaptation paradigms and find that when sufficient
ID data is available, fine-tuning domain-specific models outperform LLMs on ID
examples significantly. However, in the case of OOD instances, prioritizing
LLMs with in-context learning yields better results. We identify that both
fine-tuned small models and LLMs face challenges in effectively addressing
downstream tasks. The code is public at
\url{https://github.com/lifan-yuan/OOD_NLP}.
