The 12-month median is a figure used to represent the value of properties within a market (suburb and property type). It is based on sales over a 12-month period and therefore is often a more stable/reliable estimate of value.
A median is the middle figure in a set of ordered values. The 12-month median is based on sale prices of properties over a 12-month window.
There are some additional adjustments applied to this figure based on the nature of properties selling and varying methods of defining the precise extents of the 12-month window. But essentially, it's the middle sale figure.
If you're trying to calculate capital growth over a long period of time, the 12-month median is preferred to the TV. It's also handy if you want to see the gradual trend in price changes.
But if you're looking to invest in a suburb, you might prefer to use a more up-to-date metric such as the TV.
Because it covers a long period of time, the 12-month median has a much smoother curve on the historical chart than the Typical Value (TV). However, it may under-quote values in fast-rising markets.
This is because recent sales at noticeably higher prices will be combined with older, cheaper prices in the same 12-month window to come up with a summary figure. The old prices will bias downward the quoted figure because recent expensive prices will be for only part of the year.
In relatively consistent markets, i.e., those not rising rapidly or in free-fall, the 12-month median provides the most accurate gauge of value for properties in a suburb.
Some alternative sources for this kind of data include: