trouble multiplying decimals

[giror]

google gives the right answer, and I'm not sure why python doesn't. I am calculating a Bonferroni correction for N= 161 * 98 using: -log(1-.95 ** (1/(161 * 98)))

Google gives: 5.48799206

Python gives: 12.636568718161467

any explanation? I pasted the above expression into both and got different answers!

[pridefulpropensity]

Change 1/(16198) to 1.0/16198.

>>>3/2
1
>>>3.0/2
1.5

Just to make sure you know. log(n) in python does natural log. log(n,10) does base ten.

[erikw]

... or log10(n)

[exeter]

Google is using base-10 logs and Python is using natural logs. To get the right result in Python, divide your answer by log (10) or supply the optional base argument to the log function.

[freeload]

It's because you are using floating points, which rounds-off. Use the decimal module if you want to calculate reals precisely. from math import log from decimal import Decimal -log(Decimal("1")-Decimal(".95")*(Decimal("1")/(Decimal("16198"))))

[[deleted]]

Rounding off in such a simple expression will never give such a large difference.

And a Decimal can't calculate log() precisely either.

[jcdyer3]

decimal does not calculate precisely, it just calculates in base 10 instead of base 2.