a knowledge of both official languages. As well, levels of knowledge have been shown to change dramatically within the same age cohort over time. (For example, there is a significant decline in knowledge of French among English-speaking Canadians in the decade after they graduate from high school.) Data for this descriptor were first collected in the census of 1931. "Language used most often at work" This is the language used most often by the respondent in carrying out job functions at his or her place of employment during the week prior to the census. In the case of persons not currently working, the language used most often during the longest period of employment in the year preceding the census. This question is only asked of persons over the age of fifteen. Data for this descriptor were first collected in the census of 2001. "Mother tongue" Mother tongue is defined by Statistics Canada as the "first language learned at home during childhood and still understood by the individual at the time of the census." Because some children are born into marriages between parents who use different languages in the home, the census allows individuals to indicate multiple mother tongues. Data for this descriptor were first collected in the census of 1931. Descriptors derived from the primary descriptors "Language Continuity Index" This index has been used by the Office of the Commissioner of Official Languages to determine the health of French-language minority populations outside Quebec. The index "represents the relation between the number of persons speaking French most often at home and the number of persons for whom French is the mother tongue. A continuity index of less than 1 means that French registers more losses than gains in its exchanges with other languages." The exact number reveals the percentage of the French-language population (as measured by mother tongue) that continues to use French as its home language. The remainder of this population now uses English as its preferred language in the home, which is a strong indicator of assimilation. Although the Commissioner has not used the index for other purposes, it could presumably be used the same way for any language group within any province. This descriptor is calculated using data on mother tongue and home language, which were first collected in the census of 1971. Therefore it is possible to calculate statistics for this descriptor from that date forward. "Official