Data sources, methods, and limitations

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The 1991 to 2001 Canadian census mortality follow-up study is a probabilistically linked cohort consisting of a 15% sample (n = 2,735,152) of the non-institutionalized population aged 25 or older, all of whom were enumerated via the 1991 census long-form questionnaire.  This cohort was tracked for mortality from June 4, 1991 through December 31, 2001.  Because names were not captured on the census database, but were needed to link to the mortality data, creation of the cohort required two probabilistic linkages.  First, eligible census respondents were linked to a nominal list (name) file (abstracted from 1990 and 1991 tax-filer data and then encrypted) using common variables such as date of birth, postal code, and spousal date of birth (if applicable); 80% of eligible respondents were successfully matched.  Then, the census plus encrypted names were matched to the Canadian Mortality Database.  Based on 1991 deaths, which could be identified independently in the Canadian Mortality Database and/or the name file, ascertainment of deaths in the cohort followed for mortality was estimated to be 97% overall.  Specifically, more than 260,000 deaths over the 10.6-year follow-up period were linked to the sample.²⁶

The 1991 Census defined immigrants as people who were, or who had been, landed immigrants in Canada.  A landed immigrant is not a Canadian citizen by birth, but has been granted the right to live in Canada permanently by Canadian immigration authorities.  In  this study, the Canadian-born population (non-immigrants) is the reference group.  The analysis excluded refugee claimants and non-permanent residents, that is, people on employment or student authorizations.

To examine the duration aspect of the healthy immigrant effect, immigrants were classified by period of immigration and by world region of birth.  The period-of-immigration categories were:  before 1970 (established), 1970 through 1980 (medium-term), and 1981 through June 1991 (recent).  The world regions were defined as:  United States,  Caribbean/Central and South America, Western Europe, Eastern Europe, Sub-Saharan Africa, North Africa/Middle East/West Asia, South Asia, South East Asia, East Asia, and Oceania (Appendix Table A). These are non-standard 1991 Census classifications of place of birth, established in order to achieve a balance between creating homogeneous categories for epidemiological research and having a manageable number of groups.  For example, for conciseness, South, Central, West and East Africa were combined, whereas North Africa, West Asia and the Middle East were grouped because the people in these regions share cultural and epidemiological characteristics.  South Asia, South East Asia and East Asia were categorized separately according to the 1991 Census definition, except that Singapore, which is part of South East Asia in the census definition, was included in East Asia because of socio-economic and cultural similarity.  For Europe, the standard 1991 Census groupings of West, South and North Europe were combined with the Scandinavian countries as Western Europe, except that Albania and Yugoslavia, which are part of South Europe in the census definition, were included with Eastern Europe.  South and Central America (including Mexico) and the Caribbean were combined.  The United States of America was singled out as a place of birth instead of being part of North America.  Greenland and St. Pierre and Miquelon, the other two components of North America, were included with Oceania.  However, Oceania was dropped from the analyses by world region of birth because of the small sample size (n=4,600).

The immigrant population can be very diverse, even within the same world region of birth.  To make the results more specific, immigrants from three countries—China (including Hong Kong), India and the United Kingdom—were selected for more in-depth analysis.  Since the 1980s, the People's Republic of China and India have become the leading sources of immigrants to Canada, whereas the United Kingdom was a major source of immigrants in earlier years.  Because the baseline data were obtained in 1991, before the influx of immigrants from the People's Republic, those in the sample who were born there most likely lived in Hong Kong before migrating to Canada.  For this analysis, the People's Republic of China and Hong Kong were grouped.

This study also examines mortality in the three gateway Census Metropolitan Areas (CMA): Toronto, Montreal and Vancouver.   For these analyses, the reference group was the Canadian-born population in each CMA.

Age- and sex- specific mortality rates by 5-year age group (at baseline) were used to derive the age-standardized mortality rate (ASMRs), using the population structure of the census mortality follow-up cohort as the standard.  ASMRs and rate ratios were calculated by sex and by key immigration characteristics overall and for circulatory diseases (ICD9 codes 390 to 459) and cancer (ICD9 codes 140 to 239), which are the top two causes of death in Canada.²⁷ASMRs were calculated at the national level by sex for:

1. total population
2. Canadian-born population (reference)
3. total immigrant population and by period of immigration.
4. immigrant population by world region of birth and then by period of immigration. 
5. immigrant population for selected countries of birth

These calculations were done separately for all-cause and for cause-specific mortality.  The steps were repeated for the selected CMAs, except for period of immigration, which was not always possible because of small sample sizes.  Rate ratios were calculated to determine if the ASMRs for various immigrant subgroups were significantly different from those for the respective Canadian-born population—the overall healthy immigrant effect.  The duration effect was determined based on whether a lessening of immigrants' health advantage was observed, as reflected in rising ASMRs with increased years in Canada as indicated by period of immigration.

Data were suppressed according to Statistics Canada's data quality guidelines.  The coefficient of variation was used to ensure that the ASMR estimates could be released; estimates with a coefficient of variation larger than 33.3% were suppressed.