[Libs-Or] Fwd: [PubLib] REALM study virus numbers

[Diedre Conkling]

Thought people trying to understand the information from the REALM project
might find this look at the numbers helpful.

---------- Forwarded message ---------
From: Cab Vinton <bibliwho at gmail.com>
Date: Mon, Sep 21, 2020 at 9:05 AM
Subject: [PubLib] REALM study virus numbers
To: <PUBLIB at oclclists.org>


Hi, All --


I've received questions from a number of folks asking where I got
the following statistics:

"On average, virus is 99.85% gone after 3 days & 99.97% gone after 6 days."

Thought it better to reply to the whole list rather than individually.

Those numbers come from the Test 4 results & represent the average

reductions after 3 days for all 5 material types (99.85%) & after 6 days
(99.97%).

The Project REALM folks haven't done a particularly good job of translating
the results into terms that are more accessible for non-experts like us.
This is unfortunate as I suspect many libraries are making decisions based
on these results without fully understanding what they're looking at.
Logarithms are not something any of us have to deal w/ on a regular basis.

So some basic math is involved to get from the reductions reported in the
results to the percentage reductions referred to above.

For example, the following results are reported for hardcovers on the last
page of the Test 4 report:

Inoculum 4.85

T0        3.27

2 Days 3.10

3 Days 2.52

4 Days 1.59

6 Days 1.88

The way these work is that every 1 log reduction is the equivalent to a
reduction of 90%, which you can easily see from the right-hand axis with
the actual virus counts. This starts at 1 million at the top
& progressively decreases by 90% with each line (100K, 10K, 1K, etc.)

So, to find the % decrease for hardcovers after 3 days:

1./ Find the net log decrease: 4.85 minus 2.52 = 2.33 log reduction

2./ Plug that number into the following formula in Excel or Google Sheets:
=1-1/POWER(10,2.33) = 99.532% reduction

I then repeated this for the other 4 material types to arrive at
the overall average of 99.85% after 3 days. And then repeated again using
the log numbers provided for 6 days. Reproduced on the following

Google Sheet for the curious:

https://docs.google.com/spreadsheets/d/1Pqcy1FMQikgij5EzZDpi9ihcC3A0TRM3jCI_wdCiLnE/edit?usp=sharing

Note that for 2 of the items, the amount of virus detected
actually increases from Day 4 to Day 6 (hard covers & soft covers). This
isn't
supposed to happen, so it would be great to have an explanation from the
Project REALM folks at some point.

The math the other way is equally simple, btw. Subtract the log10 of the
ending number of virus from the log10 of the starting number EG LOG10(1000)
- LOG10(10) = 2. Plug this into the formula above to confirm that the
reduction is 99% as expected.

Up to each library to determine for themselves, of course, the significance
of the above.  In our case, as I noted earlier, we're

comfortable sticking w/ a 3 day quarantine period. But each to their own.

All best,

Cab Vinton, Director

Plaistow Public Library

Plaistow, NH



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-- 
*Diedre Conkling*
*diedre08 at gmail.com* <diedre08 at gmail.com>

“If you don't like something, change it. If you can't change it, change
your attitude.”―Maya Angelou
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