Ok, this might sound a bit stupid, but I find these kind of statements "x % slower/faster" pretty confusing.

Let's say I have algorithm A and algorithm B. Algorithm A takes 50 seconds to complete a task and algorithm B takes 25 seconds.

Now I could make the following statements:

  1. "B is twice as fast as A"
  2. "B is 100 % faster than A" (speedA - speedB)/speedB = 100%
  3. "A is 50 % slower than B" (speedA - speedB)/speedA = 50%

Am I right about this?

  • $\begingroup$ I'm not sure that the tag logic is the appropriate one for this question. It looks more like a question about language than about mathematics. $\endgroup$
    – Fabian
    Aug 25, 2012 at 12:41
  • 1
    $\begingroup$ Yes, you are right...as long as you explain yourself clearly. $\endgroup$
    – DonAntonio
    Aug 25, 2012 at 12:42
  • $\begingroup$ @Fabian I wasn't very happy with the tag "Logic". Thanks to Asaf for retagging! $\endgroup$
    – tsauerwein
    Aug 25, 2012 at 12:56
  • 1
    $\begingroup$ Talk about confusing? Out here in the real world we see things like "ten times less" and so on. $\endgroup$
    – GEdgar
    Aug 25, 2012 at 13:11

1 Answer 1


Yes, you're right about that.

In an ideal world, possibly, we would always use logarithms rather than ratios (percentages) when measuring relative differences. Then, supposing we standardized on the base-2 logarithm we could say

B is $1$ doubling faster than A.

A is $1$ doubling slower than B.

or if we used base-10 logarithms:

B is $0.3$ decades faster than A. ($\log_{10}\frac{50}{25}=0.3$)

A is $0.3$ decades slower than B. ($\log_{10}\frac{25}{50}=-0.3$)

The problem with that "ideal" world is that everyone who needed to speak about relative differences (a rather large fraction of the population) would have to know how logarithms work, which may or may not be a realistic goal.

  • $\begingroup$ "may or may not" hahahahaha $\endgroup$
    – Almo
    May 10, 2019 at 19:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.