This morning, we were talking to Modern Domestic about how hard it is for people to make rational, informed decisions about money – on everything from grocery shopping to refinancing a mortgage.
She said when she shops for Wonk the Plank’s Cheerios and has to pick between the different box sizes, she learned (under Wonk the Plank’s tutelage) to make sure she was always paying the lowest cost for the cereal and to not be fooled by different specials or box sizes.

Mmmmmmmmm...
Before we were dating, she would just pick the lowest posted unit price (the little calculation most stores post along with the cost of the item) and keep on shopping. She had no idea that 2 for 1 specials and other promotions can throw those numbers off and require the buyer to do some mental math. Still, at least she was checking the unit prices, something some folks never bother to do.

24.3 cents per fluid ounce!
So if people botch small and uncomplicated choices all the time, imagine the capacity for expensive mistakes when we set Cheerios aside and consider something like a mortgage, full of fine print and bewildering different options.
Wonk the Plank decided to give Modern Domestic a little test. “Suppose,” we said, “Wonk the Plank offered to pay you $5 a year for the next ten years. That’s a total of fifty dollars. But we won’t do it for free. How much would you be willing to pay us today for that $50?”
Modern Domestic thought hard and told us she would offer $25. She was clever enough to realize that money in the future will be worth less than it is today, but still, she was mostly taking a guess. “Is that way too much to offer?” she wanted to know.
That is actually a pretty reasonable try, we told her, but the point is that there’s an exact answer (with certain assumptions about interest rates, to all you haters out there) about the net present value of Wonk the Plank’s $5 payments.

Yikes!
Nobody panic, it’s just a formula for figuring out the present value of a stream of future payments. In our example, we’re going to plug in $5 for C, 4% as our interest rate i and 10 as the number n of times we’ll pay Modern Domestic the money.
$38.60! So while Modern Domestic’s guess wasn’t too bad, it could still cost her thousands if she were guessing about refinancing a mortgage or paying off a car loan. Folks, do the math before you make a such a decision. If you’re not sure how to do it, enlist someone you trust (NOT anyone trying to sell you something) to help you figure it out. You can still shop for Cheerios on your own, but for decisions with more serious implications, you should get help.
Posted by wonktheplank
Posted by wonktheplank 
Posted by wonktheplank 
