Operational Definitions

There is no such thing as the speed of light.

What there is: is a number produced by a particular series of operations. For example, in 1879, using rotating mirrors, Michelson measured a speed-of-light of 299,910 kps. In 1882, using the same method, he found it to be 299,853 kps. OTOH, Mittelstaedt & Birge measured speed of light in 1928 using a Kerr cell and obtained 299,785 kps; while in 1950, using a geodimeter, Bergstrand obtained 299,793 kps.

Some of that may be due to better measurements; but some is due to the fact that different methods =define= different terms. Speed-o-light-measured-with-rotating-mirrors is not the same thing as speed-o-light-measured-with-geodimeters.

Thus, when the method of measurement is changed, such as a new definition of what-is-to-be-counted, we can expect a change in the reported measurements. This need not be nefarious. The new definition may actually be better at getting at the quantity desired. But it does have to be kept in mind because the new measurements don't mean the same thing as the olf ones. When the Census does this, or the BLS does, they always back-apply the new definition, or run both definitions in parallel to compare how the new and old data compares.

Example: when an official temperature for Ilulissaat in Greenland was redesignated from Jakobshavn to one in nearby Egedesminde, both numbers were noted for several years, until confident that Egedesminde = Jakobshavn minus 2 deg. 

Regarding the current SARS-CoV outbreak, we have to keep in mind that "reported cases" and "actual cases" may be quite different for a variety of reasons, including variations in the way reports are processed. Recently:

“WHO has developed the following definition for reporting COVID deaths: a COVID-19 death is defined for surveillance purposes as a death resulting from a clinically compatible illness in a probable or confirmed COVID-19 case, unless there is a clear alternative cause of death that cannot be related to COVID disease (e.g., trauma). There should be no period of complete recovery between the illness and death.”

-- Coronavirus disease 2019 (COVID-19) Situation Report – 82

This is not unusual. Consider H1N1. CDC shifted from reporting confirmed cases of 2009 H1N1 to reporting both confirmed and probable cases of 2009 H1N1.

Errors crop up when laymen such as news reporters and other "instant epidemiologists" interpret a sudden increase in a reported value as an actual increase in the number of occurrences. For example, that the speed of light is actually decreasing. A recent spike in COVID-19 deaths in NY caused some alarm, but it was only some earlier cases "catching up" after delays in autopsy reporting and which had not yet been allocated back to the earlier days on which they had actually occurred.

Measurement systems have the following basic qualities.

1. Sensitivity. The least amount of a quantity that can be detected.
2. Accuracy. The variation of a measurement from its true value. Requires an agreed-upon standard to evaluate.
3. Precision (or Repeatability). The variation between replicate measures of the same thing.
4. Reproducibility. The variation between different technicians performing the same test or using the same instrument.
5. Linearity. The variation in accuracy across the range of possible measurements.
6. Stability. The variation in accuracy over time.

When TOF has more time, he may revisit these points.

Further comment on measurement systems.