Nov
16
2006

  Woman Is Smoking Crack

I was surfing to find information on the Roth 401(k) eligibility rules when I came across this webpage:

Simple Subtraction Method

Curious about an easier way to subtract, I watched the video. This woman suggests that instead of subtracting from right-to-left as we all learned in school (or at least those of us old enough to have avoided being taught to use "strategies") that it is easier to subtract from left-to-right. She claims that her way is easier since you avoid carrying your ones all the way across.

As I was watching her do the math, I failed to see how this would be any quicker than the usual way (in fact, I suspect that even "strategies" would be quicker than this). Not only must one carry the ones (albeit only to the next number), but one must look at each set of numbers twice; the first time to determine if a carry will be required, the second time to actually subtract.

But I was willing to give this method a try, perhaps it really was faster. Um, no. I made five sets of 6-digit subtractions with random numbers. The first time I would subtract right-to-left. The second time left-to-right. Not only was the old way faster, but MUCH faster. Here are my times (in seconds): 18.89/27.15, 12.40/17.95, 13.51/20.96, 11.72/15.12, 6.59/12.17. On average, I can complete three old-fashioned calculations for every two the new way.

I considered whether this was simply a matter of familiarity and perhaps if I switched to left-to-right calculation from now on, I would eventually calculate faster with this method. But I rejected that--no matter how familiar with the method, one still must first calculate column left, then before writing the number check to see if a carry is needed for column right. Then write the borrowed number in column left, add the carry to column right if necessary, then start again with column right becoming column left. With the old method, you just always borrow from column left if column right requires it. No need to assess what's going on in column left before committing to the answer in column right. And if the argument is that it is simply easier to always carry a 1 to the right (as opposed to subtracting one from the left and carrying the 1 to the current column), that doesn't hold water either. The new method doesn't eliminate the need to subtract one from a number--it just requires that one subtract in their head from the answer line rather than write a lower number the top line. Isn't it intrinsically easier to do all the borrow/carries on the same line rather than manage those calculations on two separate lines?

The moral of this story: There's worse-than-useless advice on the web. And I shudder to think of how many people might watch that video and say, "Hey, this really *is* easier than subtracting from right-to-left! Look at this, hon--I'm going to start subtracting this way from now on!".

Then again, what do I know. Maybe some people are so bad at the right-to-left that this way is faster. But if anyone wants to do an empirical experiment like I did I would be interested in the results.