Chronic Diseases in Canada

Volume 26, No. 1, 2005

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Table of Contents

1. Influential Observations in Weighted Analyses: Examples from the National Longitudinal Survey of Children and Youth (NLSCY) - View Abstract
   Jennifer J Macnab, JJ Koval, KN Speechley and MK Campbell

2. The estimation of heritability for twin data based on concordances of sex and disease - View Abstract
   Hongzhuan Tan, Mark Walker, France Gagnon and Shi Wu Wen

3. Breast cancer trends in Manitoba: 40 years of follow-up - View Abstract
   Alain A Demers, Donna Turner, Daojun Mo and Erich V Kliewer

4. Smoker preference for “elastic cigarettes” in the Canadian cigarette market - View Abstract
   Michael O Chaiton, Neil E Collishaw and Aaron J Callard

5. Trends in mortality from diabetes mellitus in Canada, 1986–2000 - View Abstract
   Jinfu Hu, Glenn Robbins, Anne-Marie Ugnat and Chris Waters

Chronic Diseases in Canada (CDIC) is a quarterly scientific journal focusing on current evidence relevant to the control and prevention of chronic (i.e., non-communicable) diseases and injuries in Canada. The journal publishes a unique blend of peer-reviewed feature articles by authors from the public and private sectors that may include research from such fields as epidemiology, public/community health, biostatistics, behavioural sciences and health services. Authors retain responsibility for the contents of their papers, and opinions expressed are not necessarily those of the CDIC Editorial Committee nor of Health Canada nor of the Public Health Agency of Canada.

Abstracts from Feature Articles in Chronic Diseases in Canada Volume 26, No 1, 2005

Article 1: Influential Observations in Weighted Analyses: Examples from the National Longitudinal Survey of Children and Youth (NLSCY)

Authors: Jennifer J Macnab, JJ Koval, KN Speechley, MK Campbell

Abstract:
This paper highlights the impact of survey weights on model fit in multiple linear regression with specific reference to the National Longitudinal Survey of Children and Youth (NLSCY) and provides recommendations for the treatment of influential observations. Multiple linear regression was used to estimate the association between child and family factors in the preschool years and vocabulary development at school age. Analyses were performed with and without survey weights. The model fit was assessed by examining the distribution of the studentized residuals and the change in the regression coefficients that would occur if an observation were removed. Two summary measures of influence, Dffits and Cook's D are reported. The models were refit excluding influential observations. Weighting of the linear model resulted in previously non-influential observations having an undue influence on the estimation of the regression parameters in the weighted model. The influential observations were driven primarily by the size of the survey weight as opposed to unusual values of x and y. Researchers working with large national health surveys such as the NLSCY and the National Population Health Survey (NPHS) are advised to include a detailed influence analysis before any final conclusions are made.

Article 2: The estimation of heritability for twin data based on concordances of sex and disease

Authors: Hongzhuan Tan, Mark Walker, France Gagnon and Shi Wu Wen

Abstract:
Heritability is an important measure in chronic disease epidemiology. Almost all developed ?methods of heritability estimation for dichotomous outcomes in twin data are based on concordance of monozygous (MZ) and dizygous (DZ) twins. However, most existing twin registries, which provide a unique and efficient opportunity to assess the putative genetic basis of diseases, do not have zygosity information. We developed a method that can be used to estimate the heritability for twin data with no information on zygosity. The only conditions on using this method are that the studied disease incidence is not strongly related to sex, and the distribution of zygosity is in accordance withWeinberg's rule. Using asthma twin data which has histological confirmed zygosity, we compared the results of our method with Holzinger's formula. The heritability of asthma was 24.88% (95%CI 21.98% - 27.78%) and 29.83% (95%CI 22.28% - 37.38%) estimated by ourmethod and Holzinger's formula respectively.We conclude that our new method can be used in the estimation of heritability with large twin register data with no zygosity information available.

Article 3: Breast cancer trends in Manitoba: 40 years of follow-up

Authors: Alain A Demers, Donna Turner, Daojun Mo and Erich V Kliewer

Abstract:
This study reports a comprehensive array of breast cancer statistics for Manitoba for a 40-year period. Data from the Manitoba Cancer Registry were combined with the provincial population-based registration file to determine trends in breast cancer incidence, prevalence and mortality rates, as well as survival and the probability of being diagnosed with breast cancer in the next 10 years. The age-standardized incidence rate of breast cancer increased by 0.99/100,000 women per year over the 40 years of follow-up (69.6/100,000 women in 1960, 109.9/100,000 women in 1999).Mortality rates peaked in 1986 (35.7/100,000 women), while the 1999 mortality rate (26.0/100,000 women) was almost comparable to the 1960 rate (22.4/100,000 women). No significant trend in mortality rate was observed over the 40-year period. The 5-year prevalence rate of breast cancer increased by 8.6/100,000 women per. Between 1960-64 and 1995-99, 5-year survival increased from 0.62 to 0.86. The probability of being diagnosed with breast cancer in the next 10 years increased the most for women 60 years of age. The breast cancer burden in Manitoba is rapidly evolving mainly because of the increasing incidence and the better survival of cases.

Article 4: Smoker preference for “elastic cigarettes*” in the Canadian cigarette market

Authors: Michael O Chaiton, Neil E Collishaw, and Aaron J Callard

Abstract:
Elastic cigarettes are characterized by yields of constituents that increase proportionally faster than smoke volume as cigarettes are smoked more intensely. Elasticity may function to overcome physical limitations in increasing puff volume during nicotine-seeking behaviour. The purpose of this study was to determine if there are elastic cigarettes in the Canadian cigarette market, and to determine smoker preference for elastic cigarettes. Elasticity was calculated for 115 brands in the Canadian filtered cigarette market for puff volumes of 44 and 56 ml. Puff volumes, nicotine and tar deliveries were obtained from earlier published documents.1 Sales data were used as a proxy for smoker preference. Ordinary least squares regression was used to determine the association of sales and elasticity in the Canadian cigarette market. The cigarette brands ranged from a mean elasticity value of 1.21 to 0.67. Of the 115 Canadian cigarette brands tested, 23 brands had a mean elasticity value significantly over 1.00, making them elastic. After adjusting for brand, the average elastic cigarette sold an average of 361 million cigarettes while an inelastic cigarette sold 89.5 million cigarettes (p < 0.0001). The difference in sales between elastic and inelastic cigarettes was independent of tar yield and filter type. Elasticity was not associated with tar yield (p = 0.2734). There are elastic cigarettes in the Canadian cigarette market and the results suggest a possible smoker preference for elastic cigarettes. Utilizing elasticity may be valuable in the development of future harm reduction strategies.

Article 5: Trends in mortality from diabetes mellitus in Canada, 1986–2000

Authors: Jinfu Hu, Glenn Robbins, Anne-Marie Ugnat, and Chris Waters

Abstract:
The purpose of this study was to examine trends in diabetes mellitus (DM) mortality rates in Canada, including analysis at the provincial level, during the period 1986–2000. The study population included Canadians aged 35 and over. Age-standardized mortality rates (ASMRs) were computed. Linear regression was used to calculate the average annual percentage change (AAPC) by age, sex and province. The results showed a substantial increase in DM mortality rates among those aged 35 and over, particularly for men; the AAPC indicated an increase of 2.4% for men and 0.7% for women. When the mortality rates were plotted for three time periods, the rates increased with each successive age group and period for both sexes. Mortality from DM increased significantly in both sexes in Canada between 1986 and 2000, particularly in men.