Measuring once describes a moment. Measuring twice describes a change.
That difference is the foundation of the I AM 900 method: MEASURE → CALIBRATE → RETEST.
What a Baseline Actually Is
A baseline is a reference — nothing more and nothing less.
Before deciding what should change, the baseline establishes what is happening now: the physical response recorded during a defined task, alongside what the person reports. It is not a verdict, not a diagnosis and not a judgement about the person or athlete.
The most important property of a baseline is not the numbers it records. It is that the person becomes their own comparison. We are not comparing one athlete against a population average or one patient against a chart — we are comparing the individual against their own starting reference.
The Task Has to Be Repeatable
A comparison is only as good as the task it is built on.
Baseline and retest must use a repeatable, defined movement task: same movement, same measurement set, same instrument configuration, comparable conditions. If the task itself changes between measurements, any difference could belong to the task — not to the individual's response.
Repeatability is what makes the comparison legitimate. That is why repeatability is one of the measured variables, not just an assumption.
Measurement reliability is an established concern in sports-medicine measurement: how reliably an instrument reproduces a reading directly affects what a comparison can legitimately claim (Atkinson G, Nevill AM. Statistical methods for assessing measurement error (reliability) in variables relevant to sports medicine. Sports Medicine, 1998).
Change the Input — Then Retest
Between baseline and retest sits calibration: a defined input or condition changes — the loading condition, the movement task, the calibrated reference the individual works with.
The change is deliberate and specific. And the retest repeats the original task, not a different one.
The Comparison: What the Delta Tells Us
The delta — the difference between baseline and retest — is measurable information:
- which recorded variables changed, and by how much
- what the person reported before, and after
- whether the change repeated consistently across trials
A difference between baseline and retest is real information. But it is important to be precise about what kind of information it is.
What the Comparison Does Not Tell You
A baseline-to-retest difference describes the response. It does not, by itself, establish why the difference occurred.
- it does not prove that the changed input caused the change
- it does not rule out other explanations — learning, fatigue, conditions, natural variation
- it does not diagnose anything
- it does not guarantee that a second change will produce the same result
Repeating the measurement cycle — measure, change input, retest again — is how the description becomes more trustworthy over time. One comparison is an observation. Consistent comparisons across repeated cycles build a record.
Why Measuring Twice Changes the Question
Before the baseline: "What is happening?"
After the retest: "What changed, for this person, under this task?"
That second question is the one the I AM 900 method is built to answer — pathway by pathway, and individual by individual.