Instructions: Given a text, write a compressed version of it in a single sentence.
Input: Reviewers have noted some possible sources of upward bias in the long-term studies. Some have noted that the less robust estimates based on the Six-Cities Study are significantly higher than those based on the more broadly distributed ACS data sets.
Output:
Reviewers have noted some possible sources of upward bias in the long-term studies. Some have noted that the less robust estimates.