What gets selected

There are cases where the result doesn’t just pass.

Over time, certain efforts are favored.

The effort looks strong.
The selection feels intentional.
One number improves.

In some labs, when reviewing slow vital capacity (SVC) maneuvers, the focus is placed on the largest expiratory reserve volume (ERV).

The reasoning seems straightforward.

A larger ERV is assumed to produce a larger, more accurate residual volume (RV).

So those efforts are selected.

Over time, that becomes the standard.

The “best” effort is the one with the largest ERV.

But ERV isn’t used in isolation.

It exists in relation to inspiratory capacity (IC)—and both factor into the final result.

When you consistently select for larger ERV, you can reduce IC.

And when IC decreases, the downstream values shift.

Not dramatically.
Not enough to fail.

But enough to change what the result represents.

The maneuvers still meet criteria.
The selection is consistent.
The result passes every check.

But the system has been nudged in a specific direction.

Nothing fails.
But something changes.