De trending - Effect on business cycle lenght

[kangkan]

The Bureu of economic research used to modified the methodlogy to calculate the duration of recession in 1929. Before 1929,they had used detrended data .Post 1929 they are using the measures of aggregate economic acitivity. Does the methodology change affect the duration of recession that is calculated.

I am getting that it makes expanions and contraction look longer..please confirm. Abel bernanake page 308