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Early termination bias in forced vital capacity occurs when a recording misses late expired gas but its incomplete total is treated as FVC. If the first second was validly measured, FEV1 can remain unchanged while the denominator of the FEV1 to FVC ratio becomes too small. This can make the ratio appear larger without any change in early emptying.

An original algebraic model writes true total expired volume as $B$, measured first-second volume as $A$, and lost tail volume as $L$, with $0\le L<B$. ThenFor positive $A$, the relative ratio inflation is $\epsilon /(1-\epsilon )$. Losing 10% of the total volume raises the ratio by about 11.1%; losing 25% raises it by one third. For stipulated $A=2.4$ L and $B=4$ L, losing 1 L changes the ratio from 0.60 to 0.80.

Tail-volume loss and ratio inflation in truncated expiration can also be studied as a function of time. In a constructed exponential model $E(t)=B(1-e^{-t/\tau })$, a stop at $T\ge 1$ s loses fraction $\epsilon =e^{-T/\tau }$. Keeping the ratio inflation below a chosen positive fraction $\delta$ requiresAt stipulated $\tau =2$ s and $\delta =0.01$, this model needs about 9.23 s. This is a mathematical illustration of tail capture, not a prescribed clinical expiration duration. Different time constants give different uncollected tails at the same clock time.

The primary technical statement assesses end of forced expiration and distinguishes FEV1 from FVC acceptability rather than treating one fixed duration as universally sufficient.^[ref] If the stop also invalidates the first second, the assumption that $A$ is unchanged no longer holds, and the ratio's error cannot be inferred from denominator loss alone.

Repeatability does not remove this bias. Repeated equally incomplete traces can agree closely while missing the same late gas. Timing quality, maneuver completeness and reproducibility are separate observations.