contamination data collected by several French institutions. Regarding methylmercury, seafood contamination data have been collected through different analytical surveys (MAAPAR, 1998-2002; IFREMER, 1994-1998) and were used in Tressou et al. (2004) and Cr´ epet et al. (2005) combined with the INCA survey. In this paper, a methodology using the SECODIP data is developed (see Boizot, 2005, for a full description of this database). The company TNS SECODIP has been collecting the weekly food acquisition data of about five thousand households since 1989. All participating households register grocery purchases through the use of EAN bar codes but other grocery purchases are registered differently: the fresh fruit and vegetable purchases are recorded by the FL sub-panel while fresh meat, fresh fish and wine purchases are recorded by the VP sub-panel. The households are selected by stratification according to several socioeconomic variables and stay in the survey for about 4 years. TNS SECODIP provides weights for each sub-panel and each period of 4 weeks to make sure of the representativeness of the results in terms of several socioeconomic variables. TNS SECODIP also defines the notion of household activity which refers to the correct and regular reporting of household purchases over a year. For each household, the age and gender of each member of the household are retained in our decomposition model with some socioeconomic variables: the region, the social class (from modest to well-to-do), the occupation category and level of education of the principal household earner. For methylmercury risk assessment, the households of the VP panel are considered; in the 2001 data set, there are H = 3229 active households (corresponding to 9288 individuals) and T = 53 weeks during which the households may or may not acquire seafood. The weekly purchases of seafood are clustered into two categories (”Fish” and ”Mollusks and Shellfish”) for which the mean contamination levels are calculated from the MAAPAR-IFREMER data and are given in table 1. Household intake series (( y$_{h,t}$ )$_{h}$$_{=1}$$_{,...,H}$$_{;}$$_{t}$$_{=1}$$_{,...,T}$) are computed as the cross product between weekly purchases of seafoods which are assimilated to weekly consumptions, and mean contamination levels. They are expressed in micrograms per week ( µg/w ). The food ”purchase-consumption” assimilation is of course arguable and will be the main subject of the final discussion (see section 4). An additional assumption concerns the household size, denoted by n$_{h,t}$ for the household h Table 1 around here, see page 21 6