I benchmarked fixed-budget RAG selection on 250 QASPER questions. BM25 retained a complete evidence set in 60.4%—here are the 99 failures
I wanted an observability metric stricter than “how many tokens did we remove?”
So I froze a benchmark measuring whether a context selector preserves the human-annotated evidence needed to answer a question.
Protocol
- 250 answerable QASPER development questions
- 136 full scientific papers
- Mean input: 6,447 BPE tokens
- Fixed 2,048-token selection budget
- Cohort selected deterministically by SHA-256 of question ID
- Exact human-highlighted evidence spans
- Six extractive selection methods under the same allowance
- No LLM judge in the primary metric
The primary question was deliberately narrow: did at least one complete human evidence set survive selection?
Results
- BM25: 60.4% complete evidence retention
- Keyword selection: 54.0%
- Front truncation: 25.6%
- Tail/recency: 20.4%
- Seeded random: 21.6%
- Gold-evidence oracle: 99.6%
BM25 reduced the input by 74.4% on average and retained at least some annotated evidence in 72% of cases.
The more useful finding was inside the 99 incomplete cases:
- 29 partial hits: some evidence survived, but not a complete evidence set
- 70 total misses: none of the annotated evidence survived
Those are different observability failures. A partial hit suggests incomplete coverage or multi-passage ranking failure. A total miss suggests the retrieval vocabulary, segmentation, or ranking never reached the relevant material.
Document position was not the entire explanation. BM25 retained complete evidence in:
- 62.1% of front-position cases
- 62.4% of middle-position cases
- 54.8% of back-position cases
Front truncation, by comparison, retained 0% of complete evidence sets in both the middle and back buckets.
The oracle result is also important. It reached 99.6% under the same token allowance, suggesting that the budget could usually hold the required evidence. The remaining 39.2-point gap is mostly ranking and selection headroom—not proof that a larger context window is necessary.
Important limitation: this does not measure generated-answer correctness, factuality, or citation quality. It only measures whether exact independently annotated evidence remained available downstream. I excluded generative summarizers because exact-span scoring penalizes legitimate paraphrases, while an LLM judge would make the result model-dependent.
Benchmark, methodology, and downloads:
https://www.mahastrategies.com/benchmarks/context-retention
Raw case-method records:
https://www.mahastrategies.com/benchmarks/mcrb-1/cases.jsonl
I built both the compiler and the benchmark, so treat this as a reproducible first-party evaluation rather than independent validation.
For people running RAG systems in production: do you distinguish complete hit, partial hit, and total miss in your telemetry—or does everything collapse into one retrieval score?