Monday, April 6, 2009

Quick calculation

My friend; my enemy

Several months ago I discovered I have a slight lactose intolerance. It's nothing critical; I don't rush to the bathroom moments after ingesting dairy. But it does cause me some discomfort and upsets my system. So I picked up some lactose pills and they helped, but then just switched over to soy milk or lactose-free milk. And all was good.

On Saturday I went to the store with my brother-in-law, Brady. Brady and I are a lot alike in certain things: we're both athletic, we both enjoy analyzing data, and we both scrutinize details. (He's almost completed his masters degree in accounting as I write this. This lends itself to us discussing a lot of things that are related to numbers and data analysis.) We get in a lot of discussions about various topics--usually connections or suspected causal relationship, but we're both good at always keeping in mind that there are probably details we don't know and that correlation does not equal causation.

Well, much like XKCD (♥) explains, two nerds--that is, two people interested in numerical data and analysis--make for interesting shopping trips. For most of the trip we were fine: we agreed that a big container of strawberries for $1.79 is a good deal, we agreed that frozen vegetables are good to buy, and we agreed that Sriracha hot sauce is awesome. (We did discuss it's location of origin: he was thinking Vietnam, I thought China. According to Wikipedia, we're both wrong: it's Thai.)

Then we got to the milk. I noticed that the price of a gallon of milk was only $1.89 while the price of a half-gallon of lactose-free milk is $3.19. As Brady and I discussed this, I mentioned that I'm not entirely sure it's cheaper to buy lactose-free milk than it is to just buy regular milk and take lactose pills with it. After a quick discussion we figured that the price difference is actually negated by the lactose pills: it still comes out to around $6 a gallon, at which point Brady actually did make the comment, "Well, monetarily it's about the same, but then there's the question of non-monetary value in which one do you like more."

I went with the regular milk and lactose pills.

I am curious though if our calculations were correct. This has led me to running the numbers on a calculator and using as close to exact numbers as I could (in the store we just estimated to get rough figures). This is what I found:

Cost of gallon of milk: $1.89 for 16 servings
Cost per serving of milk: $0.118125
Cost of lactose pills: $10 for 32 servings
Cost per serving of lactose pill: $0.3125
Actual cost per serving of milk: $0.430625

Cost of half-gallon of lactose-free milk: $3.19 for 8 servings
Cost per serving of lactose-free milk: $0.39875

Difference in cost per serving: +$0.031875

So there's a $0.03 difference per serving this way. As Brady pointed out, there is also the value of regular milk tasting better (+) but also the inconvenience of having to make sure I take lactose pills before hand (-). Since it's hard to say exactly how much each of those is worth, I've decided to let them cancel each other out.

Best deal?

What this comes down to is unless the price of milk drops to $1.38 per gallon (not likely), lactose pills drop to $8.98 for 32 (likely), or lactose-free milk increases to $3.45 per half-gallon (very likely), it's actually better monetarily to buy lactose-free varieties of milk. I don't recall the price of soy milk, but I'll have to check that out next time. If it's under $3 per half-gallon it may come out as the obvious victor. (Assuming non-monetary factors negate each other out.)

Yes, this is actually how I shop. Stop looking at me like that.


nae4blue said...

I use enriched rice dream on my cereal, but it doesn't taste good when cooking with it. I use cow juice sparingly when cooking. A little asphyxiation won't kill me, right? ;)

Lactose pills won't help me, but 12 hour sudafed sort of helps. I have a rhinitis, not an intolerance. I can digest it just fine.

nae4blue said...

by the way, a half gallon of rice milk from walmart costs $3.12 (which I think is going to be cheaper than goats milk)