- Measure each thing the same way every time: the same device, the same time of day, the same conditions, the same arm, the same site.
- Change one thing at a time.
- When you change the device you sleep in, VT notices and, after seven nights in a row, rebuilds your sleep normal; expect a week of thin results.
- Read the trend, never the reading.
- Use the kit you have every day rather than kit you use sometimes.
Why consistency beats precision
A slightly wrong number measured the same way every day is a usable series; a perfect number measured differently each time is not. Most of VT compares you with you, against your own baseline, so a constant error cancels in the comparison and a varying one does not: a scale that reads body fat three points low every morning draws the right trend, and a scale read at different hours on different days draws no trend at all. The device studies say the same thing in their own terms. Consumer scales differed from reference methods by several points on average (Bosy-Westphal 2008), but three low-cost scales repeated their own lean-mass estimates between sessions with agreement of 0.92 to 0.99 (Vasold 2019); the error that matters for you is the second number, and it is small when the routine is the same.
Where the rule already lives
The scale in the morning, after the toilet, before food or drink, bare feet on a hard floor, and the bowel question answered. The waist at the iliac crest, in the morning, every two to four weeks. The cuff after five minutes seated, the same arm, a seven-day block then weekly. The same laboratory for every draw, at the same hour, fasting. One device for your nights, and seven nights before a switch counts. The fasting timer every day, not only on fasting days. The same name for a training, a protocol or a supplement every time you log it. Sleep regularity is the rule applied to your body rather than your instruments, and it is the sleep measure that carries the most weight.
How VT holds you to it
Each part of the app reads the one source you chose, and a change of sleep device rebuilds your sleep normal, so old and new readings are not mixed; a typed value is never lost; a missing day is empty, not zero; and when an HbA1c, waist or blood pressure has run past its window, VT asks for a fresh one. The places a value is read against a fixed scale rather than your own baseline are the safety notes, when a reading crosses a limit whatever your usual is, and the published scales the Vitality Score and the Sleep Health Index score against.
The exceptions
When a device dies, replace it cleanly: one switch and a week of thin results, rather than weeks of alternating. When something in your life changes, a new medicine, a new plan, a holiday, log the change and let the baseline carry on; an unusual period explains, it does not reset. And when a number is far from anything you have seen, measure again the same way before you read anything into it: a second reading an hour later is worth more than a search of the Knowledge Base.
Sources
- Bosy-Westphal A, et al. Accuracy of bioelectrical impedance consumer devices for measurement of body composition in comparison to whole body magnetic resonance imaging and dual X-ray absorptiometry. Obes Facts 2008;1:319–24. doi:10.1159/000176061
- Vasold KL, et al. Reliability and validity of commercially available low-cost bioelectrical impedance analysis. Int J Sport Nutr Exerc Metab 2019;29:406–10. doi:10.1123/ijsnem.2018-0283
- Vitality Tracker, How it works (switching devices, missed measurements, the safety message); the engine catalogue of 15 October 2026 (how VT keeps the record); Knowledge Base brief, how-to 15.