By the numbers

tridentRemember the old commercial for Trident sugarless gum?  “Four out of five dentists surveyed recommend sugarless gum for their patients who chew gum.”

Here’s the way my mind works.  When I heard that line, I always wanted to know: (a) whether that meant they only surveyed five dentists, (b) whether the dentists were paid for their opinion,  and (c) why the fifth dentist didn’t recommend sugarless gum.

Did that dentist know something we should know?

These days, while teaching a required course in research and statistics, I’ve been reminded of how numbers can be used to influence or deceive.  I know that the majority of my students have no intention of ever being researchers.  But I suspect that most of them will at some point be involved in conversations where a bit of savvy with numbers — what some call “numeracy” — would be helpful.

Thus, I’ve decided to put up an occasional post to foster numeracy.  Maybe that seems like a strange thing to include on a blog like this one.  But I firmly believe that to be intelligent participants in important conversations, in which numbers and stats are likely to be bandied about, Christians need an intuitive grasp of how numbers work.

And besides, it’s fun.

Well, to me, anyway.

So, for our first numeracy post, here’s an example for all you Kohl’s shoppers out there.  One of their regular promotional gimmicks is called “Kohl’s cash”: for every $50 you spend, you receive a certificate worth $10 toward your next purchase.

Question: Getting $10 in Kohl’s cash after a $50 purchase is equivalent to a discount of what percent?  (I apologize to all of you who still flinch at word problems in math.)

I recently put this question to my students.  Many immediately answered, “20 percent.”  And many others, sensing some kind of trick, said nothing.  But the correct answer (ignoring sales tax) is just under 17 percent.

Why?  Because the $10 is only good on the next purchase, for which you will have to spend at least another $10.  That means your minimum outlay is $60, not $50.

And $60 is being optimistic.  Think about it.  You’re standing in the store with, say, $40 worth of goods.  You say to yourself, “Hey, I might as well buy something else to get it to $50.   After all, I’m going to get $10 back, so it will almost be like getting it for free.”  (Nope.  Not really.)

So you find something you weren’t really going to buy, so you can get the Kohl’s cash.  It’s not exactly $10, so now your bill is more like $55.  But still, you’re happy to get your certificate!

And when you go back to the store to redeem it (assuming you haven’t forgotten and let it expire!), you don’t spend exactly $10 — again, maybe something more like $15.  Your total outlay is now $70.  And your “discount” after you get your 10 bucks back?  It’s down to 14 percent.

Kohl’s, of course, knows this.  It must work for them, or they wouldn’t do it.  (Face it, they’re not exactly giving away free money out of the goodness of their corporate heart.)

Here’s the bottom line, if you want the best deal.  Do you have less than $50 of merchandise on your shopping list?  Then think very carefully about whether it’s worth spending extra just to get the Kohl’s cash, especially if it means adding something you weren’t going to buy.  “But it’s almost free!” is an illusion that’s likely to push you into buying something you don’t need.

And if you have more $60 worth to buy, just spend $50 now, and come back with the certificate to get the rest.  Resist the temptation to buy more.

A little numeracy can go a long way.  Let the buyer beware.