The California Department of Public Health publishes cumulative figures for number of Covid-19 tests performed in addition to their confirmed case numbers. I’ve been looking at that data, and much of it has me scratching my head. Take a look at this:
The blue dots represent tests in progress, and the orange ones refer to completed tests. It’s obvious that something unusual happened on the 4th of April.
If we look at the percentage of completed tests that are positive, we get this:
Pretty strange. The proportion of tests that were positive climbed steadily until April 4, where it fell to a bit more than 10% and stayed there.
Looking at the daily, instead of the cumulative, numbers:
On April 4, the number of completed tests skyrocketed, and then the number of tests started running about 10,000 per day, which was about three times what it was before the 4th.
The daily proportion of positive tests show some else strange going on:
Note that the vast majority of the tests completed on the 4th were negative, and that ever since the 4th — with the exception of the 15th — the proportion of positives has been much lower than it was before.
I have some guesses:
- Completed tests before the 4th of April increasingly prioritized patients who were likely to be positive.
- On the 4th, tests that were likely to be negative were completed.
- Since then, a different set of priorities has obtained. From the lower level of positives, it may be that health workers are being prioritized.