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Advanced Member
Math  probablity question
<sigh> it's been 30 years or so since I've done this and I'm drawing a blank.
What are the odds of a supposedly random event:
that happened 5 times
that happened on a Saturday each of those 5 times
over 2 years.
Intuitively I know this is virtually impossible, the odds must be astronomical, but some people just have to have some numbers to finally convince them...

Moderator
Re: Math  probablity question
If it's truly random,
and the events are not dependent on each other,
and the only variable here is on which day of the week the event occurs
then it appears that the likelihood of the event taking place on 5 Saturdays, rather than any other day is
1/7 * 1/7 * 1/7 * 1/7 * 1/7
right?
Is that astronomical? It's hard to say without some context. Certainly it's the same as "5 Tuesdays", or the same as the sequence "Monday, Tuesday, Wednesday, Thursday, Friday".
And certainly the odds for it happening next on a Saturday are still just 1/7, given the same assumptions.
(Now often, on further investigation, the event isn't truly random, or once established is periodic. In those cases, the math does not apply)

Advanced Member
Re: Math  probablity question
Sorry for the confusing context, this should make it clearer.
Here is the situation.
A script runs once every day .
The script has failed 5 times.
Each time it was on a Saturday.
True, the odds are 1/7 that the next time will be a Saturday, but I'm asking what are the odds that this is truly random given that this has run 730 times (2 years), failed 5 times  and each time on a Saturday? Are the odds 1/16807 no matter how many years? That can't be right.

Moderator
Re: Math  probablity question
I suspect you are correct  there is probably a definite cause and it is not random.
And if it is not random, then odds aren't relevant.

Moderator
Re: Math  probablity question
David,
Are you wanting to 1) calculate the odds, 2) get ideas on how to get at root cause, or 3) both?
Here is one resource:
http://en.wikipedia.org/wiki/Binomial_distribution

Advanced Member
Re: Math  probablity question
Just the odds. I want to first prove that it is virtually impossible that this is "random".

Moderator
Re: Math  probablity question
[img]/images/graemlins/smile.gif[/img] Even if you calculate the odds which will never be zero  what do you gain?
Are these Saturdays unique relative to holidays and/or business cycles such as endofmonth processing?

Moderator
Re: Math  probablity question
How long does the script take to run and what type of failure occurs? Is it dependent on the Inter, or intra net? What is the mean time between failures?
Personal Comment
Success is the ability to go from one failure to another with no loss of enthusiasm.
~ Winston Churchill ~
...Rich Wagner

Senior Member
Re: Math  probablity question
x = number of possibilities during time period
q = total time period
n = number of occurrences
(x/q)^n
So in this formula we have 104 possible days out of a total of 730 days where it could occur on a saturday and each of the 5 times it occurred was on a Saturday.
(104/730)^5
or
(52/365)^5= .000058688417057480754356176255798719
1/.000058688417057480754356176255798719 = 17,039.
So your odds are roughly 1 in 17,039. Not exactly astronomical. You should focus more on demonstrating the pattern and finding out what else was going on on those saturdays. Antivirus scans? Software releases pushed out across network?
"I think when you spread the wealth around, it's good for everybody."  Toledo, Ohio, Oct. 12, 2008

Advanced Member
Re: Math  probablity question
Thanks All.
And apologies about my vagaries, the issue/data/problem is sensitive and I couldn't reveal much.
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