Tensile tests have been carried out by deforming polycrystalline samples of substitutional Al-2.5% Mg alloy at room temperature at a range of strain rates. The Portevin-Le Chatelier (PLC) effect was observed. From an analysis of the experimental stress versus time series data we have inferred that the dynamics of the PLC effect in a local finite time is controlled by a finite number of degrees of freedom and this effective dimension becomes reduced with increasing strain.