# Urgent Probability Help

• Sep 23rd 2007, 07:54 AM
samantha_malone
Urgent Probability Help
heyy i need some help with probability.. its not really one question, im just asking for some general help wiht the following:

what is conditional probability? any formulas? when do i need to use conditional probability?

what about formulas for independent and dependent events? when do i use these formulas? here is where i am confused because i dont know when to use conditional probablity, or when events are dependent or independent.

and when do i use combination and permutation formulas? what are they looking for in these types of problems?

and then there's that whole thing about trees, when do i use a tree to find probability?

i hope someone can help me! thanks a lot! (Angel)
• Sep 23rd 2007, 08:34 AM
Jhevon
Quote:

Originally Posted by samantha_malone
heyy i need some help with probability.. its not really one question, im just asking for some general help wiht the following:

what is conditional probability? any formulas? when do i need to use conditional probability?

what about formulas for independent and dependent events? when do i use these formulas? here is where i am confused because i dont know when to use conditional probablity, or when events are dependent or independent.

and when do i use combination and permutation formulas? what are they looking for in these types of problems?

and then there's that whole thing about trees, when do i use a tree to find probability?

i hope someone can help me! thanks a lot! (Angel)

for general information like this, it is perhaps better if you consult google or wikipedia. you will get better help here with specific questions. but, hey, maybe someone would be kind enough to answer these.
• Sep 23rd 2007, 08:41 AM
Jhevon
Quote:

Originally Posted by samantha_malone
heyy i need some help with probability.. its not really one question, im just asking for some general help wiht the following:

what is conditional probability? any formulas? when do i need to use conditional probability?

i guess i'm kind enough to answer at least one.

condition probability is any probability that we are looking for after any conditions are fulfilled. So, let's say we have events A and B. and let's say i don't really care about the probability of B by itself, but I am interested in the probability of B given that A has happened. so A happening is a condition that i want fulfilled in order to find the probability of B. so we say the Probability of B occurring given that A has occurred is a conditional probability and it is denoted $P(B|A)$

Definition:

If $P(A)>0$, then $P(B|A) = \frac {P(A \cap B)}{P(A)}$
• Sep 23rd 2007, 08:50 AM
Jhevon
Quote:

Originally Posted by samantha_malone
what about formulas for independent and dependent events? when do i use these formulas? here is where i am confused because i dont know when to use conditional probablity, or when events are dependent or independent.

a lot of the formulas change depending on whether events that you are looking at are dependent or independent events. i will just tell you the formula for the union.

two or more events are independent (or mutually exclusive or disjoint) if the outcome of any one does not affect the others.

two or more events are dependent if the outcome of one or more affects the outcomes of the others

If $A$ and $B$ are independent events, then

$P(A \cup B) = P(A) + P(B)$

If $A$ and $B$ are dependent events, then

$P(A \cup B) = P(A) + P(B) - P(A \cap B)$
• Sep 23rd 2007, 09:03 AM
Jhevon
Quote:

Originally Posted by samantha_malone
and when do i use combination and permutation formulas? what are they looking for in these types of problems?

we use permutation when we want to find the number of arrangements of, say n, objects, and here order matters

we use combinations when we want to know the number of different groups of say r objects that can be formed from n objects. for instance, how many groups of 3 people can we form from a group of 6 people? here, order doesn't matter
• Sep 23rd 2007, 09:40 AM
Plato
Quote:

Originally Posted by Jhevon
two or more events are independent (or mutually exclusive or disjoint) if the outcome of any one does not affect the others.
If $A$ and $B$ are dependent events, then
$P(A \cup B) = P(A) + P(B) - P(A \cap B)$

Unfortunately there is some bit of vocabulary confusion there.
Independent events are not the same as mutually exclusive or disjoint events.

If $A$ and $B$ are mutually exclusive events, then $(A \cap B) = \emptyset$ and $P(A \cup B) = P(A) + P(B)$.

If $A$ and $B$ are independent events, then $P(A \cap B) = P(A)P(B)$ .

For all events $A \;\&\;B$ then $P(A \cup B) = P(A) + P(B) - P(A \cap B)$.