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I've started noticing some sneaky math in my local supermarket. When comparing two items, both list the unit price, but they list the price in different units. Example: A 1.4-lb package lists the price per oz, while the 0.8-lb package lists the price per lb. In every case, the intent seems too be an attempt to induce math errors and cause someone to accidentally purchase the package with the higher true unit price.

Just multiply or divide by 10s or 100s or whatever. It's not hard.

Oh.. oh yeah. ;)

That's not going to work with imperial units :-)

And heaven knows programmers can't multiply by 16.

Only bad programmers multiply by 16.

The good ones use a bit shift.

For about the last 15 years the compiler will generated the same code in either case. Thats really old advice!

I've noticed this as well here in Sweden. Take for example bacon - product A is listed in terms of price/slice while the other is listed as price/weight. It's ridiculous.

same in the UK, some items priced per kg and others per 100g - not insurmountable but just another slight hump in the road to easy comparison when you're stood there.

I've noticed a similar thing. I always assumed it was negligence, not malice.

Yeah, Whole Foods in particular often lists the price "per item" (one package of 14.5 oz at $8.25 "per item" and another of 12 oz at $6.10 "per item") which makes a mockery of the unit pricing law. I wrote to the A.G. a few years ago but nothing happened.

I’ve noticed some weird anomalies as well like if there is a small, medium, and large size package the large sized package isn’t always the cheapest per unit - as they assume customer will just assume the largest packages has the lowest per unit price

Some supermarket chains have an ethics policy against shelving like that with mixed units and if you catch it, and report it to a manager they will apologize and correct it.

I recall Kroger used to have a big focus on it, but a quick skim of current ethics policies on their website doesn't show an explicit mention. However they still have "honesty" as a guiding goal, so supposedly it's still worth reporting.

You think someone who can't solve arithmetic problems is going to understand the consequences of compound interest and the TCO of revolving debt?

Those are not all that much related.

I have extreme issues doing any sort of math in my head and have been like this since birth - I failed a general level math course in high school. I can't add 2 two-digit numbers in my head at all, forget multiplication.

However, I understand compound interest and consumer debt very well - I pay my credit card in full every month, I save for retirement, I actively budget. I've also had a career in the computer and Internet business for 25 years. A calculator solves a lot of problems.

Yeah. I suck at arithmetic. For the soda example, I just round down to 3 and say: "about $1". Much faster than worrying about 5 cents.

But I also know how to solve for discount percentages, where most people would not, the article states.

People are mixing "solving arithmetic problems" with "solving arithmetic problems without paper or calculator". Furthermore, GP's example mixes "can't calculate price per pound" with "Can't read a line graph". The former is okay, the more I advance in graduate training as a mathematician, the less I'm able to do mental math. The latter is bad.

There are good reasons for teaching mental math to children, but for adults, really, calculators exist. If you can't calculate price-per-pound with a calculator, you have a low IQ (can't translate a simple written instruction to pressing buttons on your phone) and should be elligible for government aid. But if you can't calculate discounts, compound interest, read graphs, etc -- these can be taught.

Should be taught. For example: banks could condition debt renegotiations to attendance to special financial education classes for adults. Governments could incentivize banks to do so by picking up part of the check for reduced debt.

Back in my college prime and I was knee deep in the stuff, I was the guy that would do an integral in my head and then screw up the subtraction at the end. I've just always been bad at mental arithmetic, and don't put in the practice to get better because it just doesn't affect my life that much.

So yes, we exist.

>...catastrophic consequences of spending $9 per month on dish soap when you could have spent $7.20 instead.

Indeed, for the poor, it doesn't take too many of these $1.80 differences to add up to catastrophe.

I feel like a lot of people don't realize how much small changes can add up. For a lot of people living paycheck to paycheck, they would be broke regardless of whether they earned $1500/ month or $2000/ month, because their spending adjusts upward any chance they get. If you optimize your purchases on the margin, you can escape this trap much easier.

If you can plan, purchase and store in bulk then you can save money on essentials. That's about it. Then you have to get rid of as many subscription based items you can.

> Especially when you consider that most supermarket labels will also include the "price per unit weight" or "price per unit volume"

My local grocery does this.

Whenever it stocks multiple items, though, they are never in the same units of measurement. So, you get one brand of any given priced in $/ounce, and another priced in $/pound, or heaven help me, $/serving.

I have no doubt that they pull this bullshit on purpose, to make comparing prices more difficult.

I guess the problem for the older generation, especially here in the UK, is they were raised on imperial units and have never really gotten used to metric.

Not saying that excuses it, but it could explain part of the reason why some people don't fully grasp the £-per-X calculations

If you could/can work out the change from a pound on a 4s-6½ (four shillings and six and a halfpence) packet of fags (circa 1050s), you'd hardly be likely to have much problem with metric.

[As was: 1 pound = 240 pence = 20 shillings i.e. 1 shilling = 12 pence]

I don't think they had tobacco in the 1050s.

Well I'm not that old so maybe it's different but as an American raised on imperial units I find metric absurdly easy to learn because it is just shuffling a decimal point around.

Going from metric to imperial must be challenging.

I'm also American raised on imperial, and metric conversions often confuse me _because_ of the uniformity of the conversions. Each imperial conversion has a unique number, making it easy for me to associate the unit with the number, but metric tends to get jumbled up.

Note: $9/month (or $7.20/month) of dish soap is a ridiculously vast amount of dish soap for an ordinary family to be using. My wife and I have been on the same $10 (?) bottle of dish soap (yay Costco) for over 5 years and it is just now running out. (Not that this affects your main point.)