When something is 98% effective, what does that mean?
For example, a cleaning product that says it’s 98% effective at removing germs from surfaces.
* Does it clean 98% of all germs away and leave 2% behind?
* Of all known germs, are 2% of them totally immune to this cleaner, so it can only hope to clean the other 98%?
* Is it that out of 100 people, 98 will be able to clean the surface to 100% clean and the other 2 won’t?
* Does the product work on 98% of surfaces, but there are some surfaces, say the metal bolts on a toilet, that the product just can’t clean?
* Is the product actually 100% effective, but in tests 2% of people drank it instead of wiped the counter with it?
How can you really tell what an effectiveness rating is measuring?
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I’ve always just assumed they did that so they didn’t get sued for claiming it was 100% effective.
Very few things in biology are 100% consistent, so when it comes to effectiveness, manufacturers account for outliers…the especially resistant bugs…in their effectiveness claims. From statistics, we would expect an outlier to be in the range of 1.5-3% (up to 5%, depending on who you talk to).
If manufacturers claimed 100% effectiveness, they’d be getting sued every time someone could prove they used the product as directed and still got sick.
> For example, a cleaning product that says it’s 98% effective at removing germs from surfaces.
they come up with this rating based on controlled labratory tests… where the scenario is all uniform (same chemical, same volume, same bacteria, same application method) except in variations of either type of bacteria or the concentration of the chemical
> Does it clean 98% of all germs away and leave 2% behind?
yes, there’s more to it, but that’s what it means
> Of all known germs, are 2% of them totally immune to this cleaner, so it can only hope to clean the other 98%?
No. It can be immune, but we/they don’t know the cause in every case
> Is it that out of 100 people, 98 will be able to clean the surface to 100% clean and the other 2 won’t?
No, the effectiveness is of the chemical, not the cleaning ability of the person.
> Does the product work on 98% of surfaces, but there are some surfaces, say the metal bolts on a toilet, that the product just can’t clean?
No, should be applicable to all surfaces, unless the instructions specify exceptions. There will be differences in performance if you clean a smooth surface vs a fuzzy surface
> Is the product actually 100% effective, but in tests 2% of people drank it instead of wiped the counter with it?
no, funny scenario, but no
> How can you really tell what an effectiveness rating is measuring?
only by how much you trust the company to report it’s product with accuracy. that’s why there’s laws and regulations to leverage risk vs compliance.
>Does it clean 98% of all germs away and leave 2% behind?
This is the main idea. For specifics you’d have to look up the testing methodology which probably has to comply with some legal norms.
This is less about mathematics and more about just pure language.
How come 10% more of 100 is 110 but 10% less of 110 is 99?
It’s just because percentages are relative to a quantity. The word “effective” doesn’t explicitly reveal the product’s intent. Given the body of your question, it’s usually used to say 98% of germs don’t survive application of the cleaning agent, but I can’t promise you that’s always the case. Percentages are dimensionless (unlike say 30 mph) so you can assign it anything you want.
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