# Rule of Thumb of the Day

Via Alex Tabarrok, a pair of researchers asked people how big the economy would be if it grew 5% a year for 25 years:

Only around 10–15% of the participants gave estimations between 50% less and 100% more than the true value…furthermore, the majority of the false estimations were systematically below the true value …which was underestimated by 88.9–92.1% of the participants.

Of course, this is actually a fairly tough calculation even if you’re mathematically inclined and understand the whole compound interest thing.  I guessed vaguely that 5% growth would produce a doubling in about 12 years, so the economy would quadruple in 25 years.  Wrong!  Turns out that doubling takes 14 years, so the answer isn’t 300%, it’s 238%.  But Alex made this worth my time by teaching me a new rule of thumb I hadn’t heard of before:

A good way of approximating is to use the rule of 70.  If x is the growth rate then the doubling time is approximately 70/x.  Thus, with a growth rate of 5% we expect a doubling (100% increase) in 14 years and a quadrupling in 28 years so a bit more than a tripling in 25 years (200% increase) is a good guess.

I love good rules of thumb, and this one makes me slightly more knowledgable than I was five minutes ago.  Thanks, blogosphere!

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### WE'LL BE BLUNT

It is astonishingly hard keeping a newsroom afloat these days, and we need to raise \$253,000 in online donations quickly, by October 7.

The short of it: Last year, we had to cut \$1 million from our budget so we could have any chance of breaking even by the time our fiscal year ended in June. And despite a huge rally from so many of you leading up to the deadline, we still came up a bit short on the whole. We can’t let that happen again. We have no wiggle room to begin with, and now we have a hole to dig out of.

Readers also told us to just give it to you straight when we need to ask for your support, and seeing how matter-of-factly explaining our inner workings, our challenges and finances, can bring more of you in has been a real silver lining. So our online membership lead, Brian, lays it all out for you in his personal, insider account (that literally puts his skin in the game!) of how urgent things are right now.

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