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PubMed1999

Interpreting age, period and cohort effects in plasma lipids and serum insulin using repeated measures regression analysis: the CARDIA Study.

Observed changes in health-related behaviours and disease risk factors may arise from physiological or environmental changes, or from biases due to sampling or measurement errors. We illustrate problems in the interpretation of such changes with longitudinal data from the Coronary Artery Risk Development in Young Adults (CARDIA) study. Mean plasma cholesterol was 14 mg/dl higher in 27- than in 20-year-old black men cross-sectionally, but longitudinally it declined by 4 mg/dl during the 7 years. To sort out these contradictory assessments of the effect of age/passage of time, we estimated age and period effects under the assumptions that age effects are a smooth function of age independent of period, and that period effects are changes common to persons across all ages. Simple estimates the age effect, such as the cross-sectional age slopes, may be confounded by cohort effects, by interactions of time and age after baseline, or by the occurrence of non-linearities in response after baseline. We note examples of each potential type of bias. The data and background literature support the assumption that cohort effects do not seriously compromise interpretation for these variables in the CARDIA study. Strong secular decreases in plasma cholesterol, apparently due to population-wide dietary change, mask increases with ageing. Age increases in triglycerides are largely explained by increases in body fatness. For these data, we cautiously accept the cross-sectional age slope as an estimate of ageing and the age-matched time trend as an estimate of secular trend.

Adolescent↗
PubMed1994

Increasing prevalence of Helicobacter pylori infection with age: continuous risk of infection in adults rather than cohort effect.

It remains unclear whether acquisition of Helicobacter pylori is due to a continuous risk of acquiring the infection or a cohort effect. In this prospective 3-year cohort study, the seroprevalence, conversion, and reversion of H. pylori infection as determined by IgG antibodies was examined. The cohort consisted of 316 randomly selected, nonpatient subjects aged 18-72 years who each provided at least 2 suitable samples. Seroprevalence of H. pylori increased from 21% in the third decade to 50% in the eighth decade. Crude annual seroconversion rate was 1% and the "spontaneous" seroreversion rate was 1.6%. Age was the only identified risk factor for H. pylori infection. A continuous risk of acquisition of 1%/year rather than a cohort effect best explains the pattern of H. pylori infection in this Canadian population. Seroconversion continues in adult life, and spontaneous reversions do occur, especially in the later decades.

Adult↗
PubMed2004

The increase of female breast cancer incidence in Japan: emergence of birth cohort effect.

During recent decades, breast cancer incidence has been increasing in Japan. According to the latest reports from several cancer registries in Japan, the breast has become the leading cancer site in female cancer incidence. To analyze the trend of breast cancer incidence in detail, we summarized female breast cancer incidence in Miyagi Prefecture, Japan during 1959-1997, and evaluated the period and cohort effect on breast cancer incidence using the age-period-cohort model. Age-specific and age-standardized rates have increased over successive calendar periods. Around 1980, an accelerated increase in these incidence rates took place. A full model including age, period and cohort was best fitted to the trend of incidence. In the model, the effects of period and cohort were statistically significant. The nonlinear effect for cohort indicates an increasing trend, beginning with the cohort in 1888-1897, and the nonlinear effect for period showed a clear increase in risk with calendar period. Furthermore, the full model including a linear component showed a steadily upward trend in the cohort effect. Based on our own epidemiologic studies previously conducted in Miyagi Prefecture, and other published reports, the cohort effect is likely to be related to the change in prevalence of women with risk factors such as low parity and insufficient breastfeeding. We believe that the emergence of the cohort effect is an important finding, although the period effect may also persist. The significant cohort effect may give a caution for continuous increase of breast cancer incidence in Japan.

Adult↗
PubMed2003

Influence of cohort effects on patterns of suicide in England and Wales, 1950-1999.

BACKGROUND: Age- and gender-specific suicide rates in England and Wales have changed considerably since 1950. AIMS: To assess whether cohort effects underlie some of these changes. METHOD: Graphical displays to assess age-period-cohort effects on suicide for the period 1950-1999. RESULTS: Successive male birth cohorts born after 1940 carried with them, as they aged, a greater risk of suicide than their predecessors although this effect diminished for the 1975 and 1980 birth cohorts. There was less clear evidence of any increased risk of suicide in post-war female birth cohorts. CONCLUSIONS: Succeeding generations of males born in the post-war years have experienced increasing rates of suicide at all ages, an observation in keeping with patterns seen in other countries. If these trends continue into middle- and old-age they will lead to a great increase in overall male suicide rates.

Adolescent↗
PubMed1989

Decline of height with age in adults in a general population sample: estimating maximum height and distinguishing birth cohort effects from actual loss of stature with aging.

The decline in stature with age among adults is well documented. Although part of this represents a birth cohort effect, actual height declines among older individuals are known to contribute to the effect. In this study we used longitudinal changes in the heights of adults in a general population sample to determine the rate of decline in height over time in individuals of different ages. This allowed an estimation of the age at which decline in height begins, a value close to age 40 in both sexes. It also allowed derivation of equations from which the maximum height of subjects can be estimated on the basis of their sex, current height, and age. These equations should prove useful when examining the effect of aging per se on physiological measurements that are height dependent. The data also allow one to compare the magnitude of the effect of year of birth with that of the actual decline in height seen among the elderly. We estimate that approximately 60% of the smaller stature of older male subjects and 45% of the smaller stature of older female subjects is a birth cohort effect deriving from the secular trend toward greater stature; the remainder is a result of an actual decrement in height after the age of 40.

Adult↗
PubMed1990

Period and cohort effects on divorce rates.

The author analyzes the demographic dimensions of divorce. A model is developed to estimate "the isolated effects of marriage duration, marriage cohort, and divorce period...on divorce probabilities over time [using data for Austria, Denmark, Finland, the Netherlands, Sweden, and Switzerland].... Crude divorce rates (divorces per 1,000 married couples) in the six countries for the period 1946-1984 are reviewed, followed by a very brief review of some of the literature on period and cohort effects in general, and divorce in particular. The [second] section describes the data. The third section presents and discusses the results."

Austria↗
PubMed1984

Period, age, and cohort effects on substance use among American youth, 1976-82.

Period, age, and cohort effects on substance use are differentiated for American youth 18 to 24 years old during the period from 1976 to 1982. The data are provided by the Monitoring the Future project, an ongoing study which employs a cohort-sequential design. Weighted least squares regression is used to find plausible and parsimonious models to account for the observed variation in 12 different classes of drugs, both licit and illicit. The point is made that there are no definitive ways to differentiate among the types of effects; thus, any interpretation is open to debate. Period effects involving increased use occurred for cocaine, amphetamines, and methaqualone, while decreases occurred for barbiturates, tranquilizers, and psychedelics other than LSD. Marijuana showed a curvilinear period effect, first increasing then decreasing. Effects of age were more complex. There were increases in the year after high school for daily cigarette use, but not for monthly use. Monthly and daily alcohol use increased with age. A measure of heavy drinking showed a curvilinear trend, first increasing and then decreasing. Annual use of cocaine showed an increase between the ages of 18 and 21. Annual use of narcotics other than heroin showed a linear age decrease. Clear class (or cohort) effects appeared for cigarette use, with each successive class smoking less.

Adolescent↗
PubMed2003

Pertussis resurgence in Canada largely caused by a cohort effect.

BACKGROUND: Beginning in 1990 Canada experienced a resurgence of pertussis. Changes in incidence and hospitalization according to age in the province of Quebec between 1983 and 1998 were examined to assess the presence of a cohort effect resulting from a poorly protective vaccine. METHODS: The source of data on incident cases was pertussis notifications to the Quebec Ministry of Health and Social Services. Hospitalization data were extracted from the administrative database that collects information on each hospitalization. RESULTS: The mean annual incidence before 1990 was 3.8 cases per 100,000 population which increased to 37.2 thereafter. Infants had the smallest increase (2.7-fold) when compared with children between 1 and 19 years who experienced a 9- to 15-fold increase and with adults (22.5-fold). The mean annual hospitalization rates increased from 2.7 per 100,000 before 1990 to 5.2 afterward. Ninety percent of hospitalizations occurred in children <5 years of age. The proportion of cases in 0- to 4-year-old children decreased, whereas it increased steadily in all other age groups during the entire study period. Between 1990 and 1998 the median age of cases shifted from 4.4 to 7.8 years. Pertussis affected predominantly children who were immunized with a vaccine introduced in the mid-1980s. The evolution of the age distribution of cases paralleled the aging of this cohort with a slow but steady drift of disease from early childhood to adolescence. CONCLUSION: The sudden increase in pertussis incidence in Canada can be largely attributed to a cohort effect resulting from a poorly protective pertussis vaccine used between 1985 and 1998.

Adolescent↗
PubMed1991

Sex-specific trends in mortality for the aging population in Norway: a model with age, period, and cohort effects.

During the last few decades, the citizens of most western countries have experienced an increased expectation of life. The rate of this change differs for males and females. We evaluated reasons for this pattern, using a Poisson model, with age, period, and cohort effects applied to vital statistics data from the Central Bureau of Statistics of Norway. The data comprised the total Norwegian population aged 45 to 104 years who died in the period from 1966 until 1986. Differential changes in life expectancy for the elderly are explained by differences in sex-specific cohort effects. We also observed an extra-Poisson variation, or overdispersion, in the distribution for females.

Age Factors↗
PubMed1987

The changing relationship between age and suicide rates: cohort effect, period effect or both?

Massive changes in suicide rates over time have been recognized in the United States. An attempt has been made to describe these changes with age-period-cohort analyses. A variety of approaches has led us to conclude that suicide rates of non-white males, white and non-white females can be described adequately without a cohort effect. Recent suicide trends lead to the conclusion that a model based on a rising rate in more recently born white male cohorts coupled with an independent age effect could be rejected. If a cohort effect is postulated for more recent birth cohorts, it would require that the cohort suicide rate is decreasing with each successive birth cohort. Models based on high suicide rates in recent cohorts and additive age effects are probably misleading for future predictions. An association was noted between recent changes in the teenage and young adult suicide rates and rates of depression. Both may be the product of similar social influences.

Adolescent↗
PubMed2002

Is the increase in allergic respiratory disease caused by a cohort effect?

BACKGROUND: Changes in lifestyle or environmental factors are responsible for the increasing prevalence of allergic respiratory disease. Establishing the time at which the increase began may provide a clue as to what factors possibly could have contributed to the increase. Many cross-sectional studies have shown that the prevalence of allergic sensitization decreases with increasing age. This could reflect the natural course of allergic sensitization. Alternatively, this could reflect that the increase in sensitization is caused by a cohort effect, i.e. an increase among subjects born during recent decades. OBJECTIVE: The aim was to investigate age-specific changes in the prevalence of allergic sensitization in a cohort of adults. METHODS: A total of 599 subjects aged 15 to 69 years participated in a cross-sectional general population study in 1990. In 1998 they were invited to a follow-up, and 64.4% (386/599) were reexamined. Serum samples obtained from the participants in 1990 and 1998 were analysed for specific IgE to six common inhalant allergens with the same assay. RESULTS: The prevalence of allergic sensitization (specific IgE to at least one allergen) increased among subjects who were less than c. 30 years at baseline (1990), i.e. subjects born during the 1960s or later, while the prevalence was unchanged among subjects who were more than c. 30 years at baseline. CONCLUSIONS: The results support the notion that the increasing prevalence of allergic respiratory disease is caused by a cohort effect. Thus, changes in lifestyle or environmental factors that occurred around or after 1960 may have contributed to this increase.

Adolescent↗
PubMed1997

Helicobacter pylori and the birth cohort effect: evidence of a continuous decrease of infection rates in childhood.

OBJECTIVE: To investigate whether the decrease in rate of Helicobacter pylori infection in subsequent birth cohorts has continued during the last decades. METHODS: Determination by ELISA of IgG H. pylori antibodies in 314 serum samples from Dutch children (age 6-8 yr, n = 154) and young adolescents (age 12-15 yr, n = 160), collected in 1978 and 1993. RESULTS: The prevalence of H. pylori declined from 19% to 9% at age 6-8 yr and from 23% to 11% at age 12-15 yr. For the whole study population, a decline from 21% to 10% (p = 0.01) was observed between 1978 and 1993. On the basis of these data and an incidence of infection with H. pylori of 0.3% per year during the same period, a model for both past and future prevalence rates of H. pylori in the Dutch population was calculated. The outcome demonstrates a decrease from more than 50% around World War II to less than 20% for the whole population around year 2040. CONCLUSIONS: H. pylori infection rates in childhood have continued to decline until recent decades, demonstrating a persistent birth cohort effect. This decline will result in a very low prevalence of H. pylori infection in the Dutch population during the next decades, becoming even lower as the observed decline in children and young adolescents continues.

Adolescent↗
PubMed2000

Age-related decreases in the prevalence of myopia: longitudinal change or cohort effect?

PURPOSE: The prevalence of myopia shows a decline with age in cross-sectional studies. This pattern may represent an increase in the prevalence of myopia in younger generations, possibly through increased exposure to near work, or an intrinsic age-related decline in myopia prevalence. Data were analyzed from published studies to determine which of these two alternatives better explains the data: a cohort effect of changing prevalence by decade or a longitudinal effect of changing prevalence as a function of age. METHODS: Prevalence data were taken from three studies conducted in the late 1980s and compared with those obtained indirectly from a national survey conducted in the early 1970s. The prevalence of myopia was then plotted as a function of age and year of birth. RESULTS: The pattern of change in the prevalence of myopia as a function of age was consistent across all studies when data were scaled relative to the prevalence of myopia at age-range midpoints from 44.5 to 49.5 years. The pattern of change was not consistent as a function of year of birth. When the data were scaled relative to the prevalence of myopia among those with years of birth from 1940 to 1942 and plotted by year of birth, results from the early 1970s were offset from those of later studies by approximately 18 years. CONCLUSIONS: The decline in the prevalence of myopia in older adults between the early 1970s and the late 1980s can be better explained by age than by year of birth. The prevalence of myopia appears to decrease because of an intrinsic age-related decrease in the amount of an individual's myopia rather than because of a cohort effect of increasing prevalence over time. The hypothesis that increasing environmental exposures to near work in recent decades have changed the prevalence of myopia is not supported by this analysis.

Adolescent↗
PubMed1988

Period, age, and cohort effects on substance use among young Americans: a decade of change, 1976-86.

In an earlier article in this Journal, we reported analyses that differentiated among period, age, and cohort effects on substance use among American youth 18 to 24 years old, from the high school classes of 1976 to 1982 during the period of 1976 to 1982. The present analyses extend the classes and years to 1986, and the age range to 18-28. A cohort-sequential design is employed, based on annual surveys of nationally representative samples of high school seniors, plus annual follow-up surveys of each senior class. Twelve different classes of drugs, both licit and illicit, are examined. Several different types of period, age, and cohort effects over the last decade are identified. Alcohol use (monthly and occasions of heavy use), and the use of marijuana, cocaine, amphetamines, methaqualone, barbiturates, LSD, psychedelics other than LSD, and tranquilizers all showed period effects. Occasions of heavy drinking, cigarette smoking, monthly and daily use of alcohol, and annual prevalence of cocaine, amphetamines, barbiturates, LSD, and narcotics other than heroin showed age effects. Class effects were seen for cigarette smoking and daily marijuana use.

Adolescent↗
PubMed1995

Age, period, and cohort effects on pulmonary function in a 24-year longitudinal study.

This paper proposes the use of two-factor models (age-period and age-cohort models) to estimate age, period, and cohort effects on pulmonary function by using the data collected in a 24-year longitudinal study in the Netherlands from 1965 to 1990. The analysis included 18,363 pulmonary function measurements on 6,148 subjects aged 20-54 years at the initial visit. The subjects were grouped into four birth cohorts (before 1923, 1923-1934, 1935-1946, and after 1946) and four survey periods (1965-1972, 1973-1978, 1979-1984, and 1985-1990). In the age-cohort model, the decrement in forced expiratory volume in 1 second (FEV1) associated with a yearly increase in age was 28.3 +/- 3.7 ml/year for a man 176 cm tall and 16.0 +/- 1.9 ml/year for a woman 163 cm tall. The estimated acceleration of decline with aging was significant for both men (beta = -0.212; standard error = 0.079 ml) and women (beta = -0.346; standard error = 0.058 ml). Compared with that of the cohort born before 1923, the average level of FEV1 was estimated to increase by 156, 277, and 379 ml, respectively, for the three younger cohorts in men (p = 0.01) and by 133, 213, and 328 ml for the three younger cohorts in women (p < 0.01). In the age-period model, the estimated linear age effect on FEV1 was 36.2 +/- 4.2 ml/year for a man and 30.5 +/- 2.3 ml/year for a woman. The age quadratic term was significant for women, but not for men. Average FEV1 was estimated to be increased by 141, 169, and 250 ml, respectively, for the periods 1973-1978, 1979-1984, and 1985-1990 in men and by 131, 138, and 219 ml in women. These period effects were significant for both men and women. In summary, this study applied the two-factor models to estimate cross-sectional and longitudinal effects of aging on FEV1 and demonstrated significant period and cohort effects, which could be attributed in part to changes in air pollutants, respiratory infections, vaccinations, types of cigarettes, diet, and lifestyles over time.

Age Factors↗
PubMed1996

Age-period-cohort effects on stroke mortality in Sweden 1969-1993 and forecasts up to the year 2003.

BACKGROUND AND PURPOSE: Variations in stroke mortality could be explained by changes in factors that act around the time of death (period effect) and by risk factors that are present in early life (cohort effect). The aim of this study was to analyze mortality rates for stroke in Sweden during the period 1969 through 1993 and to predict mortality trends until the year 2003, taking into account age, cohort, and period effects. METHODS: Age-period-cohort models were used to analyze stroke mortality in Sweden between 1969 and 1993 and to predict age-specific death rates and total number of deaths for the periods 1994 through 1998 and 1999 through 2003. RESULTS: Mortality rates in the age group 25 to 89 years decreased from 203 to 143 per 100,000 for men and from 185 to 113 per 100,000 for women over the study period (average annual decrease of 1.3% for men and 1.9% for women). The decline was present in all age groups. The full age-period-cohort model provided an acceptable fit in both sexes. Predictions based on these models gave a mortality rate of 122 and 92 per 100,000 for the period 1999-2003 in men and women, respectively. Despite an aging and increasing population, the total number of stroke deaths in Sweden is predicted to decline by approximately 10% in both men and women from 1989-1993 to 1999-2003. CONCLUSIONS: Both factors, cohort and calendar period, contain relevant information to explain the decline in stroke mortality trends in Sweden. Predictions indicate that the decline of both age-specific and total mortality will continue.

Adult↗
PubMed1999

Cancer mortality in Russia and Ukraine: validity, competing risks and cohort effects.

BACKGROUND: The dramatic increase in mortality in Russia and Ukraine in the late 1980s and 1990s has been due to increases in certain causes of death, particularly cardiovascular disease and accidents and violence. In contrast, there has been a slight fall in mortality from cancer. METHODS: This paper presents an analysis of trends and patterns in cancer mortality and examines four possible explanations for its recent fall: changes in data collection; cohort effects; competing mortality from other causes of death; and improvements in health care. RESULTS: All contribute to some extent to the observed changes, with each affecting predominantly different age groups. There is evidence of a significant underrecording of cancer deaths among the elderly especially in rural areas and of significant changes in coding practices in the early 1990s. Competing mortality from cardiovascular diseases and accidents can explain some reduction in male deaths from cancer in middle age. Birth cohort effects can explain some reduction among males after early middle age and among females at all ages. The impact of changes in health care are more difficult to identify with certainty but there is evidence of reduced deaths from childhood leukaemia. IMPLICATIONS: Recent changes in mortality in Russia are complex and their understanding will require a multidisciplinary approach embracing demography, epidemiology and health services research.

Adolescent↗
PubMed2001

Increasing trend of type I diabetes in children and young adults in the province of Turin (Italy). Analysis of age, period and birth cohort effects from 1984 to 1996.

AIMS/HYPOTHESIS: This study aimed to determine if the incidence of Type I (insulin-dependent) diabetes mellitus increased over time and if the time trend is attributable to linear trend, calendar period or birth cohort effects. METHODS: This study was based on a cohort of subjects aged 0-29 years from 1984 to 1996, who resided in the province of Turin, Italy. Poisson regression models were used to estimate the effect of sex, age, calendar time and cohorts on incidence rates. RESULTS: The mean incidence rate in the age group of 0 to 29 years was 7.78 of 100,000 person-years (95% confidence interval: 7.26-8.32), with a lower risk in women than in men [rate ratio (RR): 0.76 (0.67-0.88)]. We found a trend of incidence increasing over time (annual increase of risk 2.25 % a year, 95 % CI 0.44-4.10). In Poisson regression analysis we found that the best model was the one with sex, age and a linear term attributable to either calendar period or cohort effects. The linear term corresponds to a RR of 1.12 (1.02-1.22,p = 0.015) for each 5-year age span. CONCLUSION/INTERPRETATION: We found that the time trend in this Mediterranean area was similar in magnitude to that reported in northern European countries. The increase was linear in pattern, each birth cohort and each calendar period showing a higher risk than the preceeding one with some evidence of two steep increases in the incidence of Type I diabetes centered on the years 1964 and 1984. We found that the incidence of diabetes increased in the 0 to 14 age group and also in the older age group of 15 to 29 years and that the age-period models were not statistically significantly better than the age-cohort models.

Adolescent↗