Starting from the characterization of the past time evolution of market pri
ces in terms of two fundamental indicators, price velocity and price accele
ration, we construct a general classification of the possible patterns char
acterizing the deviation or defects from the random walk market state and i
ts time-translational invariant properties, The classification relies on tw
o dimensionless parameters, the Froude number characterizing the relative s
trength of the acceleration with respect to the velocity and the time horiz
on forecast dimensionalized to the training period. Trend-following and con
trarian patterns are found to coexist and depend on the dimensionless time
horizon. The classification is based on the symmetry requirements of invari
ance with respect to change of price units and of functional scale-invarian
ce in the space of scenarii. This "renormalized scenario" approach is funda
mentally probabilistic in nature and exemplifies the view that multiple com
peting scenarii have to be taken into account for the same past history. Em
pirical tests are performed on about nine to thirty years of daily returns
of twelve data sets comprising some major indices (Dow Jones, SP500, Nasdaq
, DAX, FTSE, Nikkei), some major bonds (JGB, TYX) and some major currencies
against the US dollar (GBP, CHF, DEM, JPY). Our "renormalized scenario" ex
hibits statistically significant predictive power in essentially all market
phases. In contrast, a trend following strategy and trend + acceleration f
ollowing strategy perform well only on different and specific market phases
. The value of the "renormalized scenario" approach lies in the fact that i
t always selects the best of the two, based on a calculation of the stabili
ty of their predicted market trajectories.