Two categories of establishments can be identified among those that made the transition to the 35-hour working week in late 2000: the establishments that signed a «Robien» or «Aubry 1» agreement and received incentive scheme assistance and those that switched to 35 hours without obtaining this assistance, but benefited from the lower payroll charges provided for by the «Aubry 2» mechanism. The proposed analysis sets out to identify the determinants for job creations in these two categories of establishments that implemented the shorter working week. It is based on detailed data taken from the 2001 Passages (Transitions) survey conducted by DARES (Directorate for the Coordination of Research, Studies and Statistics) and the BVA institute. The effects of the extent of the variation in working hours and the changes to the labour cost and productivity gains are studied in turn. The ex-post evaluation finds that these variables, used in the forward-looking macroeconomic models to explain the employment effectiveness of the shorter working week, significantly influenced the level of job creations by all the establishments that switched to 35 hours. Yet they had a greater impact on the establishments that received incentive scheme assistance. The Passages survey reports that the staff of establishments receiving incentive scheme assistance rose by over 10% as opposed to 4% for the others. To explain this deviation, we break it down into three parts: that associated with the observed features of implementing the shorter working week, that associated with the unobserved characteristics of the establishments and the part due to the selection effect of the most buoyant establishments.