Without the data, the affidavit was conclusory

The Foundation of Expert Testimony: Why Data Matters

In legal proceedings, expert witnesses play a crucial role in helping courts understand complex technical matters. However, their opinions must be grounded in reliable methodology and supporting data to have any legal weight. A recent Fourth Department case illustrates what happens when an expert fails to provide the foundational data underlying their conclusions.

The case of Costanzo v County of Chautauqua serves as an important reminder that expert affidavits must do more than simply state conclusions. Courts require experts to demonstrate the factual basis for their opinions, particularly in technical fields like accident reconstruction. This requirement ensures that expert testimony meets the standards of reliability and scientific validity that our legal system demands.

When experts fail to provide supporting data, their testimony becomes speculative and loses its probative value. This principle applies across various areas of litigation, from causation determinations to complex insurance fraud cases involving medical opinions.

Jason Tenenbaum’s Analysis:

Costanzo v County of Chautauqua, 2013 NY Slip Op 06475 (4th Dept. 2013)

“The County’s cross motion was based in part on the affidavit of a transportation engineer who offered his opinion as an accident reconstruction expert. We conclude that the affidavit was speculative and conclusory inasmuch as the expert failed to submit the data upon which he based his opinions, and thus the affidavit had no probative value”

The issue of substantiating an opinion with the data that the opinion is predicated upon is an interesting topic.

Key Takeaway

This decision underscores a fundamental principle in litigation: expert opinions must be supported by underlying data and methodology to have legal significance. Courts will not accept conclusory statements, even from qualified experts, when the factual foundation is absent. Attorneys must ensure their experts provide comprehensive documentation supporting their conclusions to avoid having critical testimony excluded or deemed valueless.

Legal Update (February 2026): Since this post’s publication in 2013, New York courts have continued to develop standards for expert testimony admissibility, particularly regarding data foundation requirements and methodological reliability in no-fault insurance litigation. Practitioners should verify current case law developments and any updated evidentiary standards that may affect expert witness requirements in personal injury and insurance disputes.