I'll start by sparing you the details. Nationwide home prices need to fall another 10%-15%. If you want to know how I came to that conclusion, keep reading.

KISS
As much as possible, I try to follow Marty Whitman's philosophy that "rarely do more than three or four variables really count. Everything else is noise."

With housing, three variables tell you most of what you need to know: price-to-income, price-to-rent, and months of supply. So let's see what they tell us.

1. Price-to-income



Sources: Case Shiller Housing Index, Census Bureau.

Home prices have tanked measured against incomes. That's obvious. But two points here should stick out: One, price-to-income levels are still about 5%-8% above average. Two, the ratio is headed back up already. Don't get too comfortable with that. Since the ratio stabilized at an above-average level, the rebound is likely just a temporary blip caused by housing stimulus. "Temporary" is the key word there. You can't permanently exit the largest housing bubble in history at above-average prices. More on that in a second.

2. Price-to-rent



Sources: Case Shiller Housing Index, Census Bureau.

Prices against rents are actually back inline with average levels. That's encouraging. Four years ago, renting was far cheaper than owning. It's a much more equitable trade-off today. Embrace that -- it's what a normal market should look like.

Just don't get too excited. Supply is what's really important here. And it doesn't look good.

3. Months of Supply 



Source: Census Bureau.

This chart is the most important. First, notice the recent spike on the far right. Both the spike and the drop to its immediate left are the result of the expiration of the first-time homebuyers credit. There was a massive buying rush before the deadline, which temporarily juiced sales. After the deadline passed, sales plummeted. Same goes for the drop and spike in late 2009, which shows a period when everyone thought the credit was about to expire, only to learn it would be extended. Congress has become the undisputed champion of creating obnoxious volatility.

The red line in this chart denotes six months, which is usually seen as a neutral level of supply. As a rough rule, prices tend to fall when there's more than six months supply, and vice versa. The temporary trip below six months supply earlier this year might explain why prices began rising.

But with the expiration of the housing credit, we're now solidly back above six months. If you want a good reason to expect housing prices to fall in the coming quarters, there it is. It's also a bad omen for homebuilders like KB Homes (NYSE: KBH) and Pulte Group (NYSE: PHM) -- there's simply too much supply out there to justify building more homes.

And the problem is actually deeper than it looks. This chart only shows actual on-market supply. What it doesn't show is so-called "shadow inventory," which are foreclosed homes that banks own but haven't put up for sale, or homes that are still occupied by homeowners who are about to get foreclosed on.

The size of the shadow inventory can only be estimated -- it's called "shadow" for a reason -- but credible estimates range between 2 million and 8 million units. Yes, that's a huge gap, which is testament to how uncertain things are. What's important is that inventory is still way too high. How high? No one knows. But it's high. And as long as that's the case, the housing bears have the ball.

To get inventory numbers back to healthy levels, homes need to be bought, of course. That happens when prices become so cheap that renters start buying, and to a larger extent when new households are formed.

Unfortunately, household formation is abysmal right now. Over the last decade, an average of 1.3 million households were created every year. In 2008, that dropped to 772,000. In 2009, it was a mere 398,000. What caused the drop? For one, the young and unemployed, especially new college grads, are living with their parents. Others are doubling-up with roommates. At a conference in Vancouver last week, analyst and blogger Barry Ritholtz noted that if you focus on household formation and new home construction at current rates, it could take as much as 12 years to absorb current inventory. Warren Buffett has quipped several times that best way to solve this is by encouraging teenagers to cohabitate, a program "not likely to suffer from a lack of volunteers."

Tying it all together
When I look at these three metrics, I see price-to-income levels that need to fall 5%-8%, price-to-rent levels that look pretty good, and a supply situation that's somewhere between bad and horrific.

Excess supply will cause prices to fall. How far? My best guess is 10%-15%. That would bring prices meaningfully below average against both incomes and rents, which would create a level where homeownership becomes affordable for new households and a better deal than renting. That's when you'll see real demand stabilizing the big picture.

And falling below average is, I think, a realistic expectation. Prices have to fall below average before buyers' animal spirits come back to life. That can last a while. Investor Vitaliy Katsenelson recently gave a great presentation where he notes that valuations "usually stay below average for eight years in a sideways cycle." He was referring to the stock market, but the theory should at least roughly extend to housing. Bubble wounds take time to heal.

Could I be wrong? Of course. Billionaire investor John Paulson, who called the housing crash, thinks home prices will rise 8%-12% in 2011, which explains why he's invested billions in Bank of America (NYSE: BAC), Citigroup (NYSE: C), and Wells Fargo (NYSE: WFC). But I try to keep it simple. And my simple metrics make me think housing isn't out of the woods just yet.

Disagree? Sound off in the comments box below.

Check back every Tuesday and Friday for Morgan Housel's columns on finance and economics.

Fool contributor Morgan Housel doesn't own shares of any of the companies mentioned in this article. The Motley Fool has a disclosure policy.