  Large Language Models (LLMs) have recently been shown to be effective as
automatic evaluators with simple prompting and in-context learning. In this
work, we assemble 15 LLMs of four different size ranges and evaluate their
output responses by preference ranking from the other LLMs as evaluators, such
as System Star is better than System Square. We then evaluate the quality of
ranking outputs introducing the Cognitive Bias Benchmark for LLMs as Evaluators
(CoBBLEr), a benchmark to measure six different cognitive biases in LLM
evaluation outputs, such as the Egocentric bias where a model prefers to rank
its own outputs highly in evaluation. We find that LLMs are biased text quality
evaluators, exhibiting strong indications on our bias benchmark (average of 40%
of comparisons across all models) within each of their evaluations that
question their robustness as evaluators. Furthermore, we examine the
correlation between human and machine preferences and calculate the average
Rank-Biased Overlap (RBO) score to be 49.6%, indicating that machine
preferences are misaligned with humans. According to our findings, LLMs may
still be unable to be utilized for automatic annotation aligned with human
preferences. Our project page is at: https://minnesotanlp.github.io/cobbler.
