The Italian debate on the wage differentials between Northern and Southern Italy has recently moved its research focus from "what is the dimension of the gap" to "how the gap can be measured - what kind of data are needed". Some authors argue that the use of administrative data, based on firm declarations, may yield biased results in the comparative analysis af wage differentials. The point is that the behaviour of Southern firms lead to overestimate the daily wages: firms underreport the number of working days in order to achieve two targets: 1) increase the level of the daily wage in order to the above the minimum contractual wage; 2) reduce the social security contributions and thereby the Labor cost, calculated on the basis of daily wages. The aim of this paper is to test empirically these propositions, using a large employer-employee linked longitudinal panel drawn from the national social security archives (Inps). We find that full-time Southern employees work fewer than Northern employees. If we look at full-time stayers - workers employed by the same firm during all year - we find that among them the number of working days - for the same number of paid months - is much lower for Southern employees. These results confirm the hypothesis that the wage gap expressed in daily or monthly pay from Inps data, is seriously underestimated. Is there a solution? Our proposal is abstract from any consideration about firm behaviour and measure the wage differential only on "stayers" who report al least 287 working days. For these workers the problem of incorrect reporting is minimal.