# probability values

• Jul 8th 2007, 05:29 PM
mkcar
probability values
Hello, I need help, which of the following represent valid probibility values?
0
100%
-0.8
0.001
thanks again
• Jul 8th 2007, 05:34 PM
Jhevon
Quote:

Originally Posted by mkcar
Hello, I need help, which of the following represent valid probibility values?
0
100%
-0.8
0.001
thanks again

when written in terms of a decimal, a valid probability value, call it P, ranges from $0 \leq P \leq 1$

when written as a percentage: 0 % $\leq P \leq$ 100%

Note: when an event is certain, it has a probability of 1 or 100%, when an event is impossible, it has a probability of 0
• Jul 8th 2007, 10:49 PM
CaptainBlack
Quote:

Originally Posted by Jhevon
Note: when an event is certain, it has a probability of 1 or 100%, when an event is impossible, it has a probability of 0

But not the other way around. An event can be possible and still have probability 0, and not be certain and have probability 1.

RonL
• Jul 9th 2007, 03:55 AM
rualin
How? By subjective probability from intuition, guesses, and estimates?
• Jul 9th 2007, 04:43 AM
CaptainBlack
Quote:

Originally Posted by rualin
How? By subjective probability from intuition, guesses, and estimates?

What are you refering to?

RonL
• Jul 9th 2007, 04:45 AM
rualin
Quote:

Originally Posted by CaptainBlack
But not the other way around. An event can be possible and still have probability 0, and not be certain and have probability 1.

RonL

Sorry, I forgot to quote. My question was, "How can it be that an event that's possible has a probability of 0 and one that is not certain has a probability of 1?"
• Jul 9th 2007, 04:56 AM
CaptainBlack
Quote:

Originally Posted by rualin
Sorry, I forgot to quote. My question was, "How can it be that an event that's possible has a probability of 0 and one that is not certain has a probability of 1?"

Suppose the lengths of rods are distributed normaly with mean mu and sd sigma.

Then it is possible for a rod to have length mu+signa/3, but the probability that it does is zero.

This is a toy construction but there are real cases where this can be important (fractal noise
comes to mind).

RonL