Angband Forums can we look at the spell fail percentages
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 November 8, 2017, 14:32 #1 Sky Veteran   Join Date: Oct 2016 Location: somewhere on the south coast Age: 45 Posts: 1,113 can we look at the spell fail percentages call me anal but if i see 34% spell fail i expect the spell to fail 34% of the time. i managed to fail 5 times in a row a 24% spell fail. that happen .. maths .. 0.1% of the time. i would be ok with that if not that i keep failing strings of 3/4 failures on 5% spell fails. with ANY class, ANY spell. i zap a rod 6 times and fail the 5% activation 5 times out of 11 tries - that is NOT 5% fail. how exactly are spell fail rates calculated? because i'm at the point that i already know a 50% spell fail will need to be tried at least 5 times before it passes. __________________ "i can take this dracolich"
 November 8, 2017, 14:45 #2 PowerWyrm Veteran     Join Date: Apr 2008 Posts: 2,398 RNG weirdness. I've managed to fail a 1% spell five times in a row once, which means the RNG rolled 0 five consecutive times when pulling randomly a number between 0 and 99. __________________ PWMAngband variant maintainer - check http://powerwyrm.monsite-orange.fr (or http://www.mangband.org/forum/viewforum.php?f=9) to learn more about this new variant!
 November 8, 2017, 14:56 #3 Sky Veteran   Join Date: Oct 2016 Location: somewhere on the south coast Age: 45 Posts: 1,113 just cast rememberance, 31% fail, 3 castings = 3 fails, 2 castings 2 fails, 3 castings 2 fails. thats 7 fails to 1 pass. this are the only instances of my casting this spell; i'm not ignoring cases where the spell passes. __________________ "i can take this dracolich"
November 8, 2017, 14:58   #4
Sky
Veteran

Join Date: Oct 2016
Location: somewhere on the south coast
Age: 45
Posts: 1,113
Quote:
 Originally Posted by PowerWyrm RNG weirdness. I've managed to fail a 1% spell five times in a row
and you don't think there's anything wrong with that ?
1/10,000,000,000 fail rate. i would think it's *more likely* that the fail rate RNG doesn't actually work the way it should.
__________________
"i can take this dracolich"

 November 8, 2017, 15:24 #5 Derakon Prophet     Join Date: Dec 2009 Posts: 8,421 People have done in-depth analyses of Angband's RNG. It's really quite good. You just notice the strings of failures far more than you notice the strings of successes. Every time I've decided to take notes on how frequently things actually succeed/fail over the long term, my results have closely matched the listed percentages.
 November 8, 2017, 17:27 #6 Pete Mack Prophet   Join Date: Apr 2007 Location: Seattle, WA Posts: 4,832 Donated: \$40 A .1% chance is actually pretty high. You should notice it roughly every 1000 times you make a decision--so about once a game. In any case, a 24% fail rate is not a spell you can count on during a fight. Getting ready for a fight, sure.
November 8, 2017, 18:14   #7
kandrc
Swordsman

Join Date: Dec 2007
Posts: 281
Quote:
 Originally Posted by Sky i managed to fail 5 times in a row a 24% spell fail. that happen .. maths .. 0.1% of the time.
Actually, it's not. .24^5 ~= .0001 is the probability that if you flip your weighted coin exactly 5 times you will get exactly 5 heads (where heads is our failed roll case). It is not the probability of flipping 5 heads in a row over the course of n > 5 flips. That analysis is far more complicated. See this page:

The exact solution (scroll down to "Jabberwocky" and see the last formulation above it) involves the difference of a pair of infinite series of a bunch of ugly binomial coefficients.

Now you can say, "But I started at zero when I tried to cast the spell, then I rolled 5 heads in a row, therefore .24^5." While this is not strictly incorrect, it is a biased observation. How many times does 1/1000 not happen in a game? This is Derakon's point. We (humans) evolved into pattern-recognizing machines because seeing patterns was good for our survival; it's easy to show that we err on the side of caution (seeing patterns where they don't actually exist). This is why you notice 5 fails in a row, but you fail to notice .76^n < .24^5 implies n >= 26 successes in a row.

 November 8, 2017, 21:47 #8 Sky Veteran   Join Date: Oct 2016 Location: somewhere on the south coast Age: 45 Posts: 1,113 if it were not that this is a spell which isn't cast in combat. i just now cast holy word 4 times (3 fails), then 4 times (3 fails). that's a 75% fail rate, and the game says 31%. i've literally just used this spell under "test" conditions - due to the fail rate, the high mana cost, and the fact that it's only really useful when strongly wounded. i understand the difference between an open set and a closed set. i failed consistently between 66% and 75% of the castings when the fail rate is 31%. or do you think every set i observe is an outlier? __________________ "i can take this dracolich"
 November 8, 2017, 22:23 #9 Gwarl Knight   Join Date: Jan 2017 Posts: 572 You need a sample size of at least 25 before you can go about drawing conclusions from it. Also kandrc's explanation is very good.
 November 9, 2017, 20:58 #10 Sky Veteran   Join Date: Oct 2016 Location: somewhere on the south coast Age: 45 Posts: 1,113 no it's not, because we haven't drawn an infinite sequence. i.e. i have not "not taken into consideration all the time where the observed even DID NOT take place", but i ran a character from CL1 to CL47 and every time i cast high fail rate spells, it's never what's listed. this, incidentally, applies to every character i play. yes i know what observation bias is. __________________ "i can take this dracolich"

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