The data shown here is from the State of Oregon and is provided by the Oregon Health Authority.
One week ago, the state announced its highest number of daily new cases in the pandemic, so far. Omitted from the press release was that the State had received results on more tests in the prior 24 hours than on any other day during the pandemic – in fact, they received about 2.5 times more tests than done on average during recent weeks.
That prompted the question – what is the correlation between tests and new cases?
Here is a chart drawn from their official data showing total tests by week (this is not showing daily values).
The correlation coefficient is approximately 95%. Consequently, the number of new cases reported does track closely to the total number of tests given. During recent weeks, some college campuses are testing all arriving students which seems to have led to an increase in the total number of tests being run during this period and a similar rise in new cases.
It is not clear why there is a variation in number of tests given – early on, tests were only done for those with “classic” symptoms, and typically only if in an elder care center or hospital. Later, tests were given to those with even one symptom, and after that, tests were given to persons who may have had contact with someone with confirmed Covid-19.
Today, tests are given to asymptomatic people with no known contacts or risk factors – such as screening thousands of college students. Due to the changing criteria, I would expect to see a larger difference between the number of tests and the new cases – however, the two track each other quite closely over many months. That leads to more questions.
Note their official data table lags by about 2-3 weeks. Hence, the right most weeks are not yet showing full data.