What Counts as a Meaningful Outcome?

Metric versus outcome

A metric is a number a device or test gives you, such as a heart-rate-variability score, a glucose reading or a sleep-stage estimate. An outcome is something that matters to you or your health: how you feel, how you function, or whether you avoid illness. A metric is only useful when it is linked, with evidence, to an outcome.

The FDA makes this distinction in drug development. It defines a biomarker as a characteristic measured as an indicator of normal biological processes, disease processes or responses to an exposure or intervention, and it states that a biomarker is not an assessment of how an individual feels, functions or survives. [1] Molecular, imaging and physiologic characteristics are examples of biomarkers. [1] Most consumer wearable readings are of this physiologic kind, and many are estimates.

So a change in a number is not automatically a benefit. This is the rule behind the labels in How We Label Evidence in Biohacking.

Why chasing a metric can backfire

Better numbers, worse results. In a randomized trial of 471 adults in a weight-loss program, people given a wearable device lost less weight over 24 months than those who monitored on a website (about 3.5 kg versus 5.9 kg). [2] Collecting more data did not produce the better result in that trial.

Numbers can create the problem. Clinicians have reported patients who seek treatment for self-diagnosed sleep problems based on tracker data, sometimes pursuing "ideal" sleep in a perfectionistic way. [3] The meaningful outcome there is daytime function and how rested someone feels, not a tracker's sleep chart. See Orthosomnia and Sleep-Tracking Anxiety.

Accurate is not the same as useful. A device may measure a number well, yet the number may not guide a useful decision. See Consumer Data vs Clinical Measurements.

Questions that help pick an outcome

• Does it matter in daily life? Energy, pain, mood, ability to do a task, and time off work are outcomes. A score on a screen is not.
• Do I notice it, or does only the device? Noticeable change is easier to judge than a small shift in a number.
• Is there evidence the metric predicts the outcome? If not, treat the metric as interesting, not decisive.
• Could chasing it cause harm? Targets for sleep, food or training can push some people toward worry, restriction or overexertion. People with a history of disordered eating should be especially careful and talk with a clinician. See Overexertion and Chasing Device Targets.

Picking one before you start

If you plan any careful personal comparison, choose the outcome first, then the measure. See Selecting a Meaningful Outcome and How to Evaluate a Biohack. If symptoms are involved, speak with a clinician rather than tracking alone. See Talking With a Health Professional.