  GPT series models, such as GPT-3, CodeX, InstructGPT, ChatGPT, and so on,
have gained considerable attention due to their exceptional natural language
processing capabilities. However, despite the abundance of research on the
difference in capabilities between GPT series models and fine-tuned models,
there has been limited attention given to the evolution of GPT series models'
capabilities over time. To conduct a comprehensive analysis of the capabilities
of GPT series models, we select six representative models, comprising two GPT-3
series models (i.e., davinci and text-davinci-001) and four GPT-3.5 series
models (i.e., code-davinci-002, text-davinci-002, text-davinci-003, and
gpt-3.5-turbo). We evaluate their performance on nine natural language
understanding (NLU) tasks using 21 datasets. In particular, we compare the
performance and robustness of different models for each task under zero-shot
and few-shot scenarios. Our extensive experiments reveal that the overall
ability of GPT series models on NLU tasks does not increase gradually as the
models evolve, especially with the introduction of the RLHF training strategy.
While this strategy enhances the models' ability to generate human-like
responses, it also compromises their ability to solve some tasks. Furthermore,
our findings indicate that there is still room for improvement in areas such as
model robustness.
