Cunningham et al. BMC Cancer 2010, 10:674 http://www.biomedcentral.com/1471-2407/10/674 contribute to differences between groups where there are differences in screening coverage. Differences in breast cancer incidence between social groups vary by age [15,16]. This is likely to relate to var- iations in risk factor distribution in social groups over time (a cohort effect) as well as certain risk factors hav- ing a differential effect by age. Two risk factors for which this is the case are childbearing and obesity [17,18]. It appears that childbearing has an initial cancer promoting effect, and then a later protective effect, so that young age at first birth initially increases breast cancer risk above childless peers, but then reduces breast cancer risk in later life [8,19]. Obesity reduces breast cancer risk pre-menopausally, but increases breast cancer risk following menopause [20]. This paper examines variation in breast cancer inci- dence by socioeconomic status and ethnicity in New Zealand over 25 years from 1981. There are a number of reasons for investigating this issue. Firstly, there are known differences between social groups in New Zealand in terms of risk factor patterns and screening, and so differences in breast cancer incidence between groups are expected. Moreover, reproductive patterns and other important risk factors are changing over time and differentially between socioeconomic and ethnic groups. We would therefore expect that ethnic and socioeconomic patterns in breast cancer incidence may also be changing over time. These differences may also vary with age, as they do between social groups in other countries. Secondly, “expected” patterns of breast cancer incidence between social groups based on risk factor distribution are not always seen, pointing to alternative aetiological explanations [21,22]. In New Zealand, Māori (the Indigenous population) women have higher parity and earlier childbearing than European women, a pattern which would perhaps predict a lower incidence of breast cancer, and yet earlier studies have found that Māori women have a similar incidence of breast cancer to New Zealand European women (who have similar rates of breast cancer to women in other developed countries) [23-25]. It is therefore particularly important to examine ethnic trends in New Zealand, to further elucidate the unexpectedly high rates amongst Māori women when compared to European women. Thirdly, recent work in New Zealand suggests an emerging trend of higher breast cancer mortality among Māori women and women of lower socioeconomic sta- tus [26], but it is difficult to know whether this is due to increasing incidence, decreasing survival, or a combina- tion of the two, without an analysis of incidence (and survival) data over time. The linkage of census and cancer registry data in New Zealand allows a rigorous analysis of socioeconomic and ethnic trends over an entire country and through a 25 year period. This Page 2 of 10 linkage removes the potential for bias occurring when socioeconomic status and ethnicity are measured differ- ently in numerator (usually cancer registry) compared with denominator (usually census) data. The aims of this paper are: To examine differences in breast cancer incidence between ethnic and socioeconomic groups To examine whether differences in incidence between social groups in New Zealand vary over time and by age Methods Data The datasets were created through linking records from the New Zealand Cancer Registry (a population based cancer register which collects information on all malig- nant tumours except basal and squamous cell skin cancers, with mandatory notification since 1994) and records from a 5 yearly census of population and dwell- ings. Five closed cohorts were created of the New Zeal- and usual resident population (all ages) on census night 1981, 1986, 1991, 1996, 2001, followed up for incident cancer(s) until the subsequent census or in the case of the 2001 cohort, until 31 December 2004 (the most recent data available at the time of the study’s record linkage). Cohorts were created using probabilistic record linkage software (QualityStage) to anonymously link cen- sus and cancer register records within a geographic area (meshblock or census area unit) on sex, date of birth, eth- nicity, and country of birth. Details on the linkage meth- ods, process and outputs are detailed elsewhere [27,28]. Briefly, 73.2% (1981-86) to 81.7% (2001-04) of eligible cancer registry records were linked to a census record, with 95.2% (1981-86) to 96.9% (2001-04) of these linked census-cancer records estimated to be true links. To adjust for underestimation of rates using the linked datasets, and to correct for any linkage bias whereby the percentage of eligible cancer records linked varied by socio-demographic characteristics, we calculated weights for strata of age, sex, ethnicity, region, small area depri- vation, and time since census. All analyses presented in this report use these weights. Approval was granted for this project under the Statis- tics New Zealand Data Integration Policy [29], and the Wellington Ethics Committee granted ethics approval for CancerTrends (Ref 04/10/093). Variables Exposures There are four main ethnic groups in New Zealand: 1. the Indigenous people (Māori) who migrated to New Zealand c.1000 AD; 2. those of European origin who commenced migration in the 1800s; 3. people from islands in the Pacific region, with the main migration occurring between 1945 and 1980; and 4. a substantial