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The remaining-life validation cutoff is the information boundary used when reconstructing a forecast of remaining useful life at a particular issue time. Observations available by that time can enter the forecast; subsequent observations can establish the eventual end-of-life criterion crossing for scoring it. Keeping these roles separate prevents data leakage from turning a retrospective calculation into an apparently successful prediction.

For an original example, suppose a test reaches its specified endpoint at cycle100 and a forecast is evaluated as if issued at cycle60. The reference remaining life is40 cycles, known only after observing the later endpoint. Constructing a predictor that returns $100-t$ because the complete test record already reveals100 produces zero remaining-life validation error by construction. That is not evidence that the endpoint was predictable from the first60 cycles.

Preprocessing has the same information constraint. A trailing three-sample average $z_t=(x_{t-2}+x_{t-1}+x_t)/3$ uses observations available at $t$. A centered average $z_t^{\prime }=(x_{t-1}+x_t+x_{t+1})/3$ needs a future observation. The centered average can be legitimate with a declared one-sample reporting delay, but then its forecast issue time is $t+1$, not $t$. Parameter fitting, normalization and selection among candidate models also belong inside the information boundary.

NASA's battery-aging records provide a physical example of the required distinction: they contain measured discharge current, terminal potential and temperature, together with a capacity-loss aging endpoint and run-to-failure histories.^[ref] A validation experiment can reveal only the prefix through its chosen cutoff to a prediction method, save the forecast, and subsequently compare it with the endpoint in the withheld suffix.

A fixed model trained on other complete histories can be evaluated on a new component's prefix. A method that updates online must instead document which new observations have become available before each forecast. In either case, preserving the cutoff, endpoint definition and assumptions about future operation makes the resulting remaining-life validation error interpretable. Pooling all prediction times into one score can conceal a violation at a particular cutoff.

A remaining-life validation error compares a forecast of remaining useful life made at a specified time with the subsequently observed time to the same end-of-life criterion. If the criterion is reached at $t_f$ and a prediction is issued at $t<t_f$, the reference life is $R(t)=t_f-t$. Define the signed error asWith this convention, positive error means predicting more useful life than was observed. A forecast must retain its issue time and endpoint definition for this difference to have a physical meaning.

For an original cycle-count example, suppose the specified endpoint is observed at cycle100. At cycle60, a forecast of50 remaining cycles has reference life40 and error $+10$. At cycle90, a forecast of20 has reference life10 and the same error $+10$. Their absolute relative errors are25% and100%, respectively. Equal cycle errors therefore need not have equal significance near the endpoint. A relative error divided by the reference life is undefined at the endpoint itself.

Errors should also be distinguished from aggregate scores. Two forecasts with errors $+10$ and $-10$ have zero mean signed error, but mean absolute error10 and mean squared error100 in squared cycle units. Cancellation can hide large individual discrepancies. Reporting errors by prediction time preserves information that a single average discards.

NASA's controlled battery tests supply a concrete physical setting: measured current, terminal potential and temperature accompany charge/discharge records, with a defined capacity-loss aging endpoint. The records can support testing forecasts against later observations while retaining the endpoint definition.^[ref] Their different operating histories also make indiscriminate pooling at equal cycle count inappropriate.

The remaining-life validation cutoff separates information available when issuing the forecast from information revealed later for scoring it. Using the eventual crossing time to choose the forecast, or tuning a model on the same future record used to claim validation, changes the experiment. For a prediction interval, one can additionally count how often the subsequently observed life lies inside the interval, while retaining its stated probability level and assumptions about future operation.