Their math vs your math

When I was shopping for cell phones, I briefly considered AT&T's early nights feature. According to the website, I could start my unlimited nighttime minutes at 7 p.m. And it was only 30 cents a day.


Given that 9 p.m. is a little late to get started on your major calls, this was initially very appealing. After all, who can't spare 30 cents a day, right?


Then I wondered, how did AT&T arrive at 30 cents? Sure enough, a little math showed me that the company was averaging the $8.99 monthly fee over the course of a 30-day month.


But, really, this feature doesn't apply to weekends, which already have unlimited minutes. So why count them?


Instead, I divided the $8.99 by 22 days of actual use. (Assuming an average of 8 Saturdays and Sundays in any given month.) The cost was still only 40 cents per day. Still a pretty good price, right?


But, really, the feature doesn't apply to the whole day. It is really just for two hours -- from 7 p.m. to 9 p.m. After that, my minutes would be unlimited anyway. So why stretch the cost over a whole day? Instead of 40 cents a day, it would be 40 cents for the two hours -- or 20 cents per hour.


Granted, the rate is only 20 cents/hour if you use the phone between 7 and 0 p.m., Monday through Friday. Otherwise, the cost per use is going to go up -- maybe closer to $1 an hour.


Technically, this cost is also only true if you also use every minute of your cell plan. If you routinely have time left over in your plan, the early evening/weekend minutes are at least partially redundant. And the more minutes you have left over, the less you really needed to pay that $8.99 for the month.


And, hey, there's every chance that you use most of your minutes and consistently get on the phone in the evenings. There's every chance that 20 cents an hour -- or 40 or 60 -- is worth it for you.


But what you have to remember is that AT&T marketed this feature as 30 cents a day. By my calculation, the best you can get is 20 cents per usable hour.


That's quite the disparity. Even more startling, both numbers are correct.


It's a good reminder that numbers are mutable. They can be massaged, toyed with, and just generally manipulated. And that's exactly what most retailers do.


We already know that it's in their best interests to sell. That means they are intent on giving you the most attractive terms, not necessarily the most accurate. It's their job to sell; it's your job to do the math as it pertains to you.