Why are so many things 99.9% effective? Why not 98.9%?

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Why are so many things 99.9% effective? Why not 98.9%?

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Mostly marketing. And if you are 98.9% effective, why not try to shoot for 99.9%?

Because it’s practically 100%, but because there has to be at least ONE exception for every rule, it’s virtually 100% effective, minus a tiny .00001% where it’s not.

I seen this with bleach.

It was proven that Bleach is so destructive to germs that it is (in theory) 100% effective… However, legally you can’t say 100% effective due to the possibility of Human error when using the product which may take away from its god tier destruction.
Also that remnants after use are so small that they are unquantifiable.

So saying 99.9% is effectively saying “I am 100% effective but want to protect myself from Lawsuits when people dun goof using me”

If you claim your product is 100% effective, then you might be in legal trouble if someone can find any case where it wasn’t effective. Making something absolutely perfect is basically impossible.

“99.9% effective” is as close as you can get to saying 100% effective without putting yourself in legal trouble, so that’s what they go for.

98.9% effective sounds worse, so nobody’s going to say that

The methods we have for [sterilizing](https://www.cdc.gov/infectioncontrol/guidelines/disinfection/efficacy.html) things (chemicals, radiation, dry heat, steam, filtration) are never 100% effective, they only cause a *reduction* in the numbers of bacteria. Typically for “sterile” results, methods produce a reduction of 10^12 or 10^9 and there’s “bioburden” testing to prove that the material starts with much fewer bacteria than that. For testing, they usually put in a spore strip with 10^6 bacteria, and run the sterilization method at half duration or half strength, to “prove” that it still kills everything in that spore strip. For “consumer grade”, your 99.9% is actually the equivalent of a 10^3 reduction, and they just write it as a percentage so they don’t have to explain to the public all this stuff I just wrote.

In addition to these levels of “effectiveness”, the testing itself is a statistical testing. You have to open the package to “test” an item, and that “destroys” the item in that it’s no longer sterile and you can’t sell it anymore. So they can’t test *every single item* in a lot, they can only test a few samples. So the answer is a [statistical answer](https://en.wikipedia.org/wiki/Sterility_assurance_level), we’re 99% sure this whole lot is sterile, but 1% chance maybe one of the items is not sterile. So legally they can’t claim 100% if they’re only 99% sure.

90%, 99%, 99.9%…effective translate into 10%, 1%, 0.1% ineffective. Or in exponential notation, 10^(-1), 10^(-2), 10^(-3), etc.

It’s just an easy number. “99.9% effective against bacteria” means that 1 in 1000 survives. 98.9% would mean that 1 in 90.9 survives. One of these reads nicer than the other.

Virtually any task is very challenging under certain conditions (And usually there is chance factor even for simpler cases). If you are trying to excel at a task, it is likely that you will get stuck at a point close to 100%. Why?

– Two numbers like 99% and 99.99% may look very close to each other. However the first one fails 100 times more often than the second one (The failure rates are 1% and 0.01% respectively). So there is a huge range of possibilities in that small-looking window.

– Plus there is this thing called low-hanging fruits. For example, improving the performance from 50.00% to 50.01% is much easier than improving from 99.90% to 99.91%.

– These being said, sometimes these numbers are said arbitrarily. In this way, one cannot blame or sue them so easily. Claiming 100% success is practically impossible.