[lyner]

Everyone can appreciate what a 10% discount means.
It is more difficult but fairly straightforward to work out what the original price was if you paid  £10 for an item and it was discounted by 10%.
BUT
What does 99% less fat mean?
What does 100 times less mean?
Why don't journalists ever use fractions these days?
"Only 1% of the fat in normal drinks" means something to me.
"1/100 of the weight of fuel" is, likewise, unambiguous.
Am I just a boring old pedant or have I a point here?
What happened to meaningful communication?

This may not be the best place for the subject to be posted but I am looking for some support amongst other old pedants / mathematicians / physicists.

[Bored chemist]

99% less fat means it doesn't taste as good.
100 times less mans the opposite of 100 times more.
Let's face it if we have journalists using numbers at all either
1 we should be grateful that they are trying to convey real data or
2 we should accept that since they are jounalists the numbers are probably wrong so it doesn't matter
(I'm now going to get slaughtered by all the journalists.)

Seriously you should never give a percentage without making it clear what it's a percentage of.

My favourite example of the misuse of percentages is a well-known brand of chicken soup. The low fat variety says in big letters that it's 95% fat free. I have little doubt of that, but I wonder why they don't also advertise that the product is (IIRC) 94% chicken free.
5% fat
6% chicken
Looks to me like it's mainly water.

[another_someone]

My concern is with the idea that physicists are precise about anything.  These are the people, who in past years, were happy to do all their calculations on a slide rule.  As for quantum physicists, they are quite happy to say the cannot say precisely where anything is. [].

[DoctorBeaver]

I get annoyed with the "If you do so-and-so, you're 50% more likely to contract such-and-such disease than someone who doesn't".

Sounds a lot, doesn't it? Enough to put you off doing so-and-so for life!

But wait... what if only 0.5% of people get that particular ailment anyway? That means that if you do so-and-so you still only have a 0.75% chance of getting such-and-such. That puts a somewhat different light on things.

So, I agree with BoredChemist - when a %age is used, it should be made explicit just what it is a %age of.

And yes, I agree about the 95% less fat. 95% less fat than a walrus is still one hell of a lot of blubber!

[lyner]

These are the people, who in past years, were happy to do all their calculations on a slide rule.

Actually, it was ENGINEERS who always used slide rules - I was one of them.  Bridges seldom fall down.
Anyway, using a slide rule shows you to use the appropriate number of sig figs in your result.
Modern students / scientists quote 10 sig figs if that's what their calculator gives them -regardless of the likely accuracy of the most critical measurement.
If a good scientist says that something is true and quotes the probable accuracy then go for believing  it.
Never knock the slide rule - slide rules rule!!!

On the other hand, some of the modern medical statistical analysis is pretty good and should not be dismissed - unless a Politician is using it.