qcomer – The concept to which you are referring to in statistics is called “stationarity”. In general terms this means that the statistical measures apply over a time frame (or set of samples) for which the system remained stable, with no fundamental underlying changes. If there are external forces which change the system, the previous statistics do not apply.
PS suggests that she wants to project future prices by only considering data prior to what was a fundamental underlying change to the system in 1997 (i.e. change in tax treratment of the sale of personal residences).
