Perplexity is a crucial metric for evaluating language models, as it measures how well a model predicts a sequence of text by assessing its uncertainty about the next token. Defined mathematically as the inverse of the geometric mean of the token probabilities, perplexity provides insight into a model's predictive accuracy, with lower values indicating better performance. The metric is sensitive to vocabulary size, meaning it can vary significantly between models with different architectures. Using the HellaSwag dataset, which includes context and multiple possible endings for each sample, models like GPT-2 and Llama can be evaluated based on their ability to select the correct ending with the lowest perplexity. Larger models generally achieve higher accuracy, as demonstrated by the comparison between the smallest GPT-2 model and Llama 3.2 1B. This matters because understanding perplexity helps in developing more accurate language models that can better mimic human language use.
