Airborne spread with linear decline and maximum distance of spread of 1 km

Posted August 7, 2009 by guest
NAADSM 3.1
Local-area or airborne spread
I created a scenario with NAADSM 3.1.22 with airborne disease spread. I selected the "Rate of disease transfer declines linearly from source" option, and used a maximum distance of airborne spread of 1 km. When I ran the scenario, there was no airborne spread, even if the probability of airborne spread was quite high. Can anyone explain what might be happening?
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You've found a bug!

Posted August 7, 2009 by Aaron Reeves
It turns out that you've run across a bug in NAADSM. If you look at the formula for the "DistanceFactor" in section 4.3 of the Model Description, you'll see that for airborne spread with linear decline, using a maximum distance of spread of 1 km would result in division by 0. NAADSM should not allow you to use a value less than or equal to 1 km for maximum distance of airborne spread. It turns out that versions of NAADSM through 3.1.22 do allow this, but if the situation is encountered, the program simply turns off airborne spread altogether, which is why you're not getting any airborne spread of disease. We will add this bug to the published bug list, and will get a corrected version out ASAP.

Bug fixed in version 3.1.23

Posted August 10, 2009 by Aaron Reeves
This bug has now been fixed: please download NAADSM 3.1.23 to obtain the latest version of NAADSM, which corrects this problem. Thanks for your help in identifying the error. If you find any other potential problems or have any other questions, please don't hesitate to post again.