At first glance, some of the recent housing market numbers for Harrisonburg and Rockingham County seem a bit contradictory.

Looking at all residential home sales combined (detached + attached), the median sales price has dipped slightly over the past year.

But if we break the market into its two main segments, something interesting appears:

• Detached home prices are still rising
• Attached home prices are also rising

How Can Both Things Be True At The Same Time?

The key lies in the proportion of detached vs. attached homes that are selling.

Last year:

• 62% of home sales were detached homes
• 38% were attached homes

This year so far:

• 50% of home sales are detached homes
• 50% are attached homes

That difference from year to year matters because detached homes typically sell for more than attached homes.  When a smaller share of higher priced detached homes make up the overall pool of sales, the combined median price can decline slightly even if prices within each category are rising.

This Confusing Concept Has A Name?

One astute reader and statistician even informed me that there is a name for this!?!

This statistical effect is called Simpson's Paradox, where trends within separate groups can appear different from -- or even the opposite of -- the trend when the data is combined.

In our local housing market, it shows up like this:

• Detached home prices = rising
• Attached home prices = rising
• Combined (detached + attached) home prices = slightly down

This overall downward trend is not a result of homes losing value, but a result a shift in the types of homes that are selling.

What Does This Mean For You?

If you own a detached home, the recent data suggests values are still trending upward.

If you own an attached home, values are also continuing to rise.

And if you are looking at the overall median price, remember that it reflects the mix of homes selling, not just price movement within each category.

As always, understanding the context behind the numbers is just as important as the numbers themselves.