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Evidence pyramid......

Report from the heart of the storm..... Santa Clara County data

Dear Yolo County folks... the numbers for CV/Covid in Yolo County as so small it is hard to make statistical inferences from them.

But not so for Santa Clara County, Northern Caifornia's worst zone for the corona virus.  Here is the link to the Santa Clara County Public Health Department Corona Virus Dashboard:

Please note the large sample size from this pool of symptomatic patients who were referred by their provider for the CV test:  11607 tests.  Results:  10.55% were positive and 89.45% were negative.  The average turnaround time for the test was 2.88 days.

Please note:  Santa Clara folks tell me that those positive results are "tight".  Their testing standards are such that they avoided false positives as much as possible.

As you can imagine these results raise a lot of questions:

  1. Did you expect the percentage of "positives" to be higher?
  2. If 89% of these symptomatic patients were negative, what ailment do they actually have?
  3. What next?  What course do we take with these results.

This is National Public Health Week folks.... let's appreciate what public health people do including their unusual skills in analyzing data.  This is not a clinical skill, this is a PUBLIC HEALTH skill.

Enjoy the dashboard viewing!

John J. Troidl, MBA, PhD

(I have a PhD in public health and have also taught various courses in public health for a number of years).




Nancy Price

Yes, I am very surprised by the low figure for "positives." And so, indeed, what
disease do the "negatives" have?

John, I welcome further interpretation.

John Troidl

Hi Nancy, in order to provide more interpretation it would be necessary to have more data.

In that regard, I asked Santa Clara folks if they were receiving BOTH flu and CV tests for people who had been tested for the CV and their answer was "some times".

Seems to me that it would be helpful if people who present with symptoms and referred by physicians should be tested broadly, not just for CV. Then we could find out if this was actually a flu epidemic ......


Bill Wagman

I think the dashboard URL should be

Eileen Samitz


Thanks for this interesting info. Your questions are well-founded and as you say, I would hope that they are doing routine "flu" testing as well which hopefully is routine for testing anyone.

The other question that comes to mind, is while they may have confidence in the positive results being "tight" how "tight" are the negative results. In other words, could there be false negatives involved?

While it is true that if someone is tested too soon, the viruses may not be in numbers high enough to detect, but this would be a smaller percentage than what the Santa Clara results of 88.45% of the test being negative.

There are multiple companies doing their best quickly produce a COVID-19 test and they are not all the same methodology. I doubt that the 15-minute rapid test is not a PCR test which is usually the gold standard but take a few hours to run at minimum. So some additional questions are:

1) Are more than one the of testing kit being used?

2) What testing kit method(s) are being used, and what is the sensitivity and specificity of the testing kits being used?

John  Troidl


These are good questions!

At this time, I do not know where there is more than one testing kit being used in Santa Clara County (or in Yolo County for that matter). The test inputs in Santa Clara County come from hospitals, medical groups and clinics and the drive through testing programs set up by the Fire Department (Hayward) and others.

Regarding sensitivity and specificity of the testing kits, I do not know that either although apparently that was an important consideration in Santa Clara County from what I have been told.

On a related note, we sure are having an opportunity to learn and share new public health science vocabulary, aren't we? Here is a straightforward definition of the two words you introduced into the discussion:

SENSITIVITY: Sensitivity (also called the true positive rate, the recall, or probability of detection[1] in some fields) measures the proportion of actual positives that are correctly identified as such (e.g., the percentage of sick people who are correctly identified as having the condition).

Another way of saying this is "How solid is that 11% positive test result shown on the Santa Clara County's Corona Virus Dashboard?"

SPECIFICITY: "Specificity (also called the true negative rate) measures the proportion of actual negatives that are correctly identified as such (e.g., the percentage of healthy people who are correctly identified as not having the condition)."

Another way of saying this is "How solid is that 89% negative tests result from Santa Clara County's Department of Public Health's Dashboard?"

Definition source:

Potentially, one of the most important outcomes of this Corona Virus scare is an enhanced understanding of the functions and activities of the field of Public Health. Ironically, this is National Public Health Week (



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