The Metrology Atlas: describing recorded physiological data as a measurement system, with VitalDB as a first application

Abstract

Every value in a recorded physiological dataset is a pair: a recorded time and a recorded value. Neither is the truth. The time comes from a clock that drifts and is corrected in jumps, and often describes a window that ended earlier. The value was quantised, rounded, sometimes heaped by hand, and measured by an instrument with its own error. We present the Metrology Atlas, a dataset-independent way of describing a recorded dataset as a measurement system, and its first application, to VitalDB, an open dataset of 6,388 surgical patients, as a proof of principle. For every variable, the atlas states what was measured, the measurement model linking it to the recorded pair, and what is known about the true time and true value given a record. Each part of the model carries its evidence: 69 studies, 49 parameters and 54 observations, each marked as measured, derived, estimated or assumed. The atlas maps 11 clocks and 14 devices into one timing network, and closes loops in that network to test its own model. Synchronisation reduces the median timing error between the EEG and ECG from 794 ms to 42 ms, and between ventilator and monitor waveforms from 378 ms to 55 ms. The atlas also documents data-quality issues, several of which correct the dataset's own documentation. It is published as a website and a single machine-readable file, with open-source code and a scheme of time-aware labels. Everything is open under CC BY 4.0 and MIT licences. The engine, schema and timing tools carry over unchanged, and the aim is to describe many other datasets the same way.