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By Douglas Moran

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About this blog: Real power doesn't reside with those who make the final decision, but with those who decide what qualifies as the viable choices. I stumbled across this insight as a teenager (in the 1960s). As a grad student, I belonged to an org...  (More)

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Obsolete Guesstimates Keeping Businesses Closed

Uploaded: May 28, 2020
If you had a mathematic model that produced the same result despite substantial changes in crucial parameters, would you use it in important policy decisions? Apparently the State government does. Case in point, counties cannot proceed to the next stage in opening up from the lockdown until they are daily testing 0.15% of its population (and other criteria). Santa Clara County doesn't qualify because its testing rate is only 0.10%. From what I can find, the 0.15% testing rate was chosen back when testing rates were far lower because of shortages of test kits, and this figure was chosen as what a county should be capable of. This number seemed to be nothing more than a guesstimate because there wasn't a proper statistical basis for setting it. What statistics were being gathered had inconsistent and conflicting definitions, shifting qualifications for being tested, and multiple changes in these. Thus, multiple apples-to-oranges problems were created. Furthermore, there were various incentives for misreporting.

In contrast to those days when there were enough tests available to test those who should have been tested, the reverse now exists: more tests are available than those seeking testing. Think about that. The combination of the doctors and patients doesn't produce enough people needed to be tested to meet the State's arbitrary threshold -- to their criteria, too few tests being needed is a deciding sign that COVID-19 is out-of-control in Santa Clara County.

A common argument for performing unneeded tests is that it will provide a profile useful for tracking the progression of infections. That might be true if the selection of people being tested created a representative sample of the population, but the sample is self-selected. This blog was inspired by a social media posting encouraging people to get tested to get the number up to the threshold so that County residents could get haircuts. That will certainly help get a representative sample (sarcasm)!

If you are curious about the effect of increasing daily testing in SCC from 0.10% to 0.15%, it would increase the number of detected cases by 46 people, from 67 to 113. If you want to explore this, I have provided an ^XLSX spreadsheet^ for you to get started (you must make your own copy to modify values). Because I couldn't find the accuracy of the test kits being used by SCC, I used an unrealistic best-case value of 100% accurate (sensitivity and specificity).

A complexity I chose not to pursue was the infectiousness of the people detected as having an active case of COVID-19. Some research indicates that people are most likely to spread the disease in the days just before and after the onset of symptoms. CDC currently says most infectious during first 5 days, and effectively not infectious after 10. The 35-50% of people who are infected are asymptomatic, although a portion of this may be pre-symptomatic.

The guesstimate is that 80% of the people who are infected will have mild to moderate symptoms. This number was established before the prevalence of asymptomatic people was discovered, and has been updated to include asymptomatic cases with no change in the percentage itself.

----Antibody (serological) test----

A closely related issue is testing for the proportion of the population that has already had COVID-19 by looking for the presence of antibodies in the blood. Because these tests are currently at best only 90-95% accurate, they are effectively useless. With a low percentage of people having been infected, if you test positive, it is several times more likely that you aren't than you are. At 90% accuracy and 5% of the population already infected, only 32% of the positive tests are correct. At 2% infected, that drops to 15.5%. You can also play with this in the ^provided spreadsheet^.

If you are interested in this counter-intuitive effect, it is known as the ^Base Rate Fallacy^ (and other names).

----
An ^abbreviated index by topic and chronologically^ is available.


----Boilerplate on Commenting----
The ^Guidelines^ for comments on this blog are different from those on Town Square Forums. I am attempting to foster more civility and substantive comments by deleting violations of the guidelines.

I am particularly strict about misrepresenting what others have said (me or other commenters). If I judge your comment as likely to provoke a response of "That is not what was said", do not be surprised to have it deleted. My primary goal is to avoid unnecessary and undesirable back-and-forth, but such misrepresentations also indicate that the author is unwilling/unable to participate in a meaningful, respectful conversation on the topic.
A slur is not an argument. Neither are other forms of vilification of other participants.

If you behave like a ^Troll^, do not waste your time protesting when you get treated like one.
We need your support now more than ever. Can we count on you?

Comments

 +   5 people like this
Posted by Sunshine, a resident of Barron Park,
on May 28, 2020 at 7:31 am

Another good one, Doug.
I think the important issue here is that the test is not particularly accurate. Thus, if I test positive, there is quite a high likelihood that I am actually negative. So why would a no symptoms person want to be tested? If your test result is positive, you will be required to stay shut in for 2 weeks for nothing. This might not be too bad if you have a nice garden and like working in it. It could be terrible if you are shut in an apartment with someone who likes to watch FOX all day.
Conversely, if you test negative, you are free to roam (as long as you wear a mask). During that time you may infect others.
As I see it, this discourages people from getting tested. I think I will wait until there is a good blood or serum test that has a much higher accuracy.


 +   3 people like this
Posted by Resident, a resident of Another Palo Alto neighborhood,
on May 28, 2020 at 8:11 am

I purely don't get this.

If I am well why should I go and mix with others who may be sick. Even sitting in a car where there are others who think they are sick sitting in cars, even with health officials who are taking the test on others, I am putting my risk higher of contracting the virus. If I do get infected while getting tested, or if I get infected while stopping at Costco on the way home, I will still be infected even if I test negative.

And what does testing tell me? If I am negative it tells me nothing. If I am positive I will still have to isolate for 2 weeks and treat my symptoms if I have any.

Why are we not being tested for antibodies? That will give me some useful information.

I personally think the likelihood of healthy people going to get tested is very low. Those who go are probably concerned that they may have been exposed.


 +   3 people like this
Posted by Victoria Call, a resident of Community Center,
on May 28, 2020 at 9:06 am

What kind of bounce or 2nd wave would be enough to call for a 2nd shutdown?


 +   8 people like this
Posted by Curmudgeon, a resident of Downtown North,
on May 28, 2020 at 6:53 pm

[[ Deleted. Established troll. In this case, his comment was based on ignoring the crucial word "crucial" in the opening sentence of the blog.]]


 +   1 person likes this
Posted by TimR, a resident of Downtown North,
on May 28, 2020 at 10:21 pm

I've gone past the Stanford Galvez site a number of times now, and have yet to see anyone inside one of the tents. It's a ghost town. If not enough people are getting tests, that's not citizens' fault. It's the county's fault, and they need to do a better job of getting people to testing sites, if that's what they want. Anecdotally, I was watching a YouTube video of a vlogger driving in Bolder, CO (on his way to a trail run), and he went past a huge sign that said "FREE COVID-19 TESTING." Why don't we have that here? It's time we made some metrics for county health officials to meet, in order for them to keep making rules for us to follow.


 +   7 people like this
Posted by Dan, a resident of Midtown,
on May 28, 2020 at 11:52 pm

Testing is a nonsensical metric for re-opening. Testing capacity, yes, that makes sense... but if testing capacity is no longer constrained and infections are getting less, you would expect less tests to get performed, not more. Then again, much of the regulations put in place didn't "follow the science" but instead followed the fear factor. Why, as the danger of overwhelmed hospitals failed to materialize, did we see yet more stricter rules put in place and such random metrics created for ending the SIP order?


 +   4 people like this
Posted by Jennifer, a resident of another community,
on May 29, 2020 at 9:55 pm

I don't get it either. Why would I get tested if I was well? Testing is a non-sensical metric for reopening. Or maybe I'm basing my opinion on my own excellent health. I think a no symptoms person would test out of fear. If you've had health problems in the past, you probably are more fearful. Maybe strengthen your own immune system, and try to reduce your worrying for your own mental well-being. A nice walk through the park...


 +   2 people like this
Posted by Douglas Moran, a Palo Alto Online blogger,
on May 29, 2020 at 11:57 pm

Douglas Moran is a registered user.

Why would an asymptomatic person get tested?

Some people are required, or at least advised, to have periodic (screening) testing. These tend to be people whose jobs entail lots of close contact with other people, especially the general public. I assume that this includes health care workers and police officers.

The recent stats are 95.5% of the tests are negative. This suggests to me that a large proportion of these are screening tests -- not diagnostic tests or monitoring (random) tests.

Interesting problems in stat analysis:

• How do you adjust for the different reasons for testing?

• Are the people taking screening tests good "sentinels" because they are effectively indirect indicators of the people they come in contact with? (I suspect not).

• Since there isn't a statistically useful test for already having had COVID-19, a portion of the people taking screening tests may have had COVID-19 before testing for active infections began, how do you adjust for that? Especially since 35-50% of infected people are asymptomatic, and thus wouldn't have realized it and then there are many more whose symptoms may have been mild and/or ambiguous enough that they weren't tested while their infection was still active.

There seems to be some convergence on important parameters. Testing during the first 2-3 days of an infection is likely to produce a negative result. This is about the time the person become infectious, and the asymptomatic person typically remain infectious through ten days after becoming infected.*
Locally there is currently a less than two-day delay in getting test results.
• How often should screening tests be administered in order to remove an infected person from contact with the public?
• How often to help with contact tracking?
• How often to profile the prevalence of COVID-19?
I know enough to know that these are difficult questions, far beyond my capabilities.

* Asymptomatics being infectious up to day 10: I have seen no confidence level stated for this number. In the early days of the disease, the claim was that symptoms might not appear until 14 or 24 days after infection. The former for 95% of patients and the latter for 99% of patients. However, these numbers seemed to have been estimates from small sample sets combined with experience with similar viruses.


 +  Like this comment
Posted by chris, a resident of University South,
on May 30, 2020 at 5:10 pm

People who have significant contact with the public need to be tested frequently: health care workers, grocery store workers, retail workers. It is more important to test them weekly and check their temperature daily than it is to check the seniors and vulnerable holed up at home.

It will be interesting to see if any of the demonstrations this week, with or without masks, results in any outbreaks.


 +   4 people like this
Posted by Jeremy, a resident of Midtown,
on May 30, 2020 at 9:06 pm

For months we were told there weren't enough tests, and that the available tests had to be rationed for people who were already sick or in vulnerable occupations. Now perhaps tests are more widely available, but I think it was just this month that a few free testing sites opened in Santa Clara County, and at first they were only in San Jose. Joe Simitian as I recall pushed for one or two to open in the Mountain View area. But the number of testing sites, free or otherwise, relative to the county's population seems to be very low, and I am not aware of any organized effort, at least by county or city officials, to get a representative sample of the population tested. I've just seen a few articles in newspapers or online. Plus, people are still being told to shelter-in-place, and not travel long distances, so there is no real incentive to seek out a test unless you are sick or are worried you might have been exposed.

I personally think large-scale testing is crucial to understanding the spread of the virus and ending the shelter-in-place, but I'm not aware that any official at any level in the Bay Area, or really California or the whole country, has put together an organized, focused plan to test a significant portion of the population, and then use the resulting data to learn as much as possible about how the virus spreads, how many people are infected, and ultimately learn more about the virus itself and its possible future trajectory.


 +   2 people like this
Posted by Resident, a resident of Another Palo Alto neighborhood,
on May 31, 2020 at 6:52 am

As far as I am aware, today May 31, is the last day of the Shelter in Place for SCC.

As far as I am aware, this does not mean that we can go back to work tomorrow. It does not mean that friends can get together. It does not mean that grandparents can hug their grandchildren. It does not mean that those who are struggling to work from home in a home with children who are bored are demanding attention from parents, or teens are taking up all the wifi bandwidth can go back to the peace and productivity of office life. It does not mean that we can have a relaxing coffee or beer in air conditioned surroundings away from from home. It does not mean that a couple can meet for a date. It does not mean that we can go to the gym or the pool for exercise.

In fact, I don't know what we can do to make life more bearable. Even for those who were putting a positive vibe on the Shelter in Place and enjoying more time at home, are now feeling the isolation and admitting to cabin fever. Watching all the terrible scenes on tv news I have to wonder just how much of this is from the pent up emotions of the restrictions life has put on society over the recent months.

Our mental health is suffering. Our quality of life is suffering. We see no end in sight. For many it is is the fact that their business may not be able to open. That is a very serious fact as the economics hit not just the business owners, but the local economy. The local economy will hit us all as we see City services reduced. It is not whether or not we can get a haircut, or whether we should support local businesses by getting take out from a local restaurant. It is whether we can go to our downtown and see anything other than shuttered store fronts as one by one they all close permanently. It is whether we see increases in lines at food banks as those who were previously employed are now unable to feed their families. It is whether our children can look forward to going back to school in the fall and know that they will be able to see their friends again. It is whether we as a society can become social once again.

For many who work, they are still able to get their salaries and perhaps make plans to move out of the area to somewhere cheaper.

For others, we perhaps will have lives disrupted in ways unimaginable to those who do not personally experience them. But for all, our lives are not going to look anything like pleasant, let alone normal, for a very long time.

The pandemic is a very bad situation. The closure of life is much more problematic. The guesstimates on the effects of the closure are only just beginning.


 +   1 person likes this
Posted by outside , a resident of Embarcadero Oaks/Leland,
on May 31, 2020 at 9:23 am

Chris: "It will be interesting to see if any of the demonstrations this week, with or without masks, results in any outbreaks."

Yes, but tracking will be difficult. The St Paul mayor announced yesterday morning that all the arrests in his town on Friday night were people from out-of-state.

Web Link


 +   1 person likes this
Posted by Jeremy, a resident of Midtown,
on May 31, 2020 at 11:16 pm

Resident, I, too, thought that Santa Clara County's shelter-in-place order was expiring tonight, but it turns out that the amended order that went into effect May 22 included extending the order indefinitely:

"This new Order will go into effect at 12:01 am on Friday, May 22, 2020, and will remain in effect until the Health Officer amends or rescinds it. This new Order replaces the prior order, which went into effect on May 4, 2020."

Web Link


 +   2 people like this
Posted by Dianne, a resident of Menlo Park,
on Jun 1, 2020 at 11:06 am

Your statement re the accuracy of antibody testing is not quite correct. In the rush to provide serological testing, the FDA approved several commercial kits, plus others went to market without approval. There is a big range in the accuracy of these kits which is the basis for your assertion that “tests are at best 90-95% accurate". That may be true across the board, but high quality ELISA tests such as those performed at Stanford and UCSF are accurate to well over 99% and therefore could be quite useful in informing future policy decisions.


 +  Like this comment
Posted by Douglas Moran, a Palo Alto Online blogger,
on Jun 1, 2020 at 9:15 pm

Douglas Moran is a registered user.

Dianne,
Thanks. It is hard to keep track of the claims and counter-claims and their credibility.
However, the Base Rate problem is still relevant at 99% accuracy: If 2% of the population has had COVID-19, one-third of the positive results are false positives.
I am alert to this problem because it kept popping up during a significant portion of my career.


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