# Perplexity of BlenderBot

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**Category:** 🤗Transformers\
**Created:** [May 13, 2021, 9:16am UTC](/static-proxy?url=https%3A%2F%2Fdiscuss.huggingface.co%2Ft%2Fperplexity-of-blenderbot%2F6160 "2021-05-13T09:16:44Z")\
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**Author:** ![Alokin](https://sea2.discourse-cdn.com/hellohellohello/user_avatar/discuss.huggingface.co/alokin/32/1532_2.png) [@Alokin](/static-proxy?url=https%3A%2F%2Fdiscuss.huggingface.co%2Fu%2FAlokin)%5C
**Post date:** [May 13, 2021, 9:16am UTC](/static-proxy?url=https%3A%2F%2Fdiscuss.huggingface.co%2Ft%2Fperplexity-of-blenderbot%2F6160%2F1 "2021-05-13T09:16:44Z")

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Hello!

I have implemented Longformer’s self-attention in BlenderBot small and fine-tuned it. My question is can apply this guide about perplexity of fixed-lenght models: [Perplexity of fixed-length models — transformers 4.5.0.dev0 documentation](https://huggingface.co/transformers/perplexity.html) to BlenderBot, even though its a different Transformer than GPT2?

In addition, I am building a chatbot, thus, I have used the: BlenderbotSmallForConditionalGeneration and not the CausalLM one. Would it make a difference which one I use for evaluating perplexity(both have a language modelling heads)?

Thank you so much! 🤗

My question is: what should I modify, in order