The association between chronotype and wages at mid-age

Sleep has been shown to affect economic outcomes, including wages. The mechanisms by which sleep affects wages remain unclear. We examine the relationship between chronotype – morning larks, evening owls – and wages at mid-age. We propose a novel model relating chronotype to wages in consideration of human, social, and health capital constructs. Empirically, we explore the effects of chronotype mediated through life course choices, such as work experience, trust, and health behaviour. The data come from the 46-year-old follow-up study of the Northern Finland Birth Cohort (1966) and from registers of the Finnish Tax Administration. We find evening chronotype to have a significant indirect negative effect on wages, which occurs through accumulating less work experience and through poor health outcomes. The effect is largest for male workers, with a total indirect effect on average wages of − 4%. We also provide evidence that chronotype has a long-term association with wages between 29 and 50 years of age. We conclude that evening-type workers are less suited to typical working hours and accumulate less human, social and health capital which in turn negatively affects their wages. Our findings are of great socio-economic importance because evening chronotypes make up a significant part of the population.     

Age structure changes and extraordinary lifespan in wild medfly populations

The main purpose of this study was to test the hypotheses that major changes in age structure occur in wild populations of the Mediterranean fruit fly (medfly) and that a substantial fraction of individuals survive to middle age and beyond (> 3-4 weeks). We thus brought reference life tables and deconvolution models to bear on medfly mortality data gathered from a 3-year study of field-captured individuals that were monitored in the laboratory. The average time-to-death of captured females differed between sampling dates by 23.9, 22.7, and 37.0 days in the 2003, 2004, and 2005 field seasons, respectively. These shifts in average times-to-death provided evidence of changes in population age structure. Estimates indicated that middle-aged medflies (> 30 days) were common in the population. A surprise in the study was the extraordinary longevity observed in field-captured medflies. For example, 19 captured females but no reference females survived in the laboratory for 140 days or more, and 6 captured but no reference males survived in the laboratory for 170 days or more. This paper advances the study of aging in the wild by introducing a new method for estimating age structure in insect populations, demonstrating that major changes in age structure occur in field populations of insects, showing that middle-aged individuals are common in the wild, and revealing the extraordinary lifespans of wild-caught individuals due to their early life experience in the field.     

A simulation approach for power calculation in large cohort studies based on multistate models

Realistic power calculations for large cohort studies and nested case control studies are essential for successfully answering important and complex research questions in epidemiology and clinical medicine. For this, we provide a methodical framework for general realistic power calculations via simulations that we put into practice by means of an R‐based template. We consider staggered recruitment and individual hazard rates, competing risks, interaction effects, and the misclassification of covariates. The study cohort is assembled with respect to given age‐, gender‐, and community distributions. Nested case‐control analyses with a varying number of controls enable comparisons of power with a full cohort analysis. Time‐to‐event generation under competing risks, including delayed study‐entry times, is realized on the basis of a six‐state Markov model. Incidence rates, prevalence of risk factors and prefixed hazard ratios allow for the assignment of age‐dependent transition rates given in the form of Cox models. These provide the basis for a central simulation‐algorithm, which is used for the generation of sample paths of the underlying time‐inhomogeneous Markov processes. With the inclusion of frailty terms into the Cox models the Markov property is specifically biased. An “individual Markov process given frailty” creates some unobserved heterogeneity between individuals. Different left‐truncation‐ and right‐censoring patterns call for the use of Cox models for data analysis. p‐values are recorded over repeated simulation runs to allow for the desired power calculations. For illustration, we consider scenarios with a “testing” character as well as realistic scenarios. This enables the validation of a correct implementation of theoretical concepts and concrete sample size recommendations against an actual epidemiological background, here given with possible substudy designs within the German National Cohort.     

A 27-year study of brown trout population dynamics and exploitation in Lake Songsjøen, central Norway

From 1968–1984 (period I), a brown trout Salmo trutta , population in a 70-ha oligotrophic lake in central Norway was exploited using larger mesh gill-nets selectively removing the larger fish. From 1985–1994 (period II), intermediate sized fish were removed using smaller-mesh sizes gill-nets. Fishing mortality and CPUE were correlated positively with effort and numbers of fish >3 years old for period II. The gill-net catchability was correlated negatively with spawner biomass and number of trout >3 years old. The significant positive correlation between natural mortality and stock biomass and spawning stock biomass indicated density-dependent mortality. The significant correlation between spawning stock and recruitment described by the Ricker model, indicated density-dependent recruitment of 1-year-old trout. The fishing regimes in the two periods affected the population dynamics and density differently. Selective removal of smaller fish permitted the larger fish to survive, and was beneficial in reducing fish density and maintaining stocks at low levels, consequently, achieving the expected increase in fish growth rates.     

Automated Assay of Telomere Length Measurement and Informatics for 100,000 Subjects in the Genetic Epidemiology Research on Adult Health and Aging (GERA) Cohort

The Kaiser Permanente Research Program on Genes, Environment, and Health (RPGEH) Genetic Epidemiology Research on Adult Health and Aging (GERA) cohort includes DNA specimens extracted from saliva samples of 110,266 individuals. Because of its relationship to aging, telomere length measurement was considered an important biomarker to develop on these subjects. To assay relative telomere length (TL) on this large cohort over a short time period, we created a novel high throughput robotic system for TL analysis and informatics. Samples were run in triplicate, along with control samples, in a randomized design. As part of quality control, we determined the within-sample variability and employed thresholds for the elimination of outlying measurements. Of 106,902 samples assayed, 105,539 (98.7%) passed all quality control (QC) measures. As expected, TL in general showed a decline with age and a sex difference. While telomeres showed a negative correlation with age up to 75 years, in those older than 75 years, age positively correlated with longer telomeres, indicative of an association of longer telomeres with more years of survival in those older than 75. Furthermore, while females in general had longer telomeres than males, this difference was significant only for those older than age 50. An additional novel finding was that the variance of TL between individuals increased with age. This study establishes reliable assay and analysis methodologies for measurement of TL in large, population-based human studies. The GERA cohort represents the largest currently available such resource, linked to comprehensive electronic health and genotype data for analysis.     

Helicobacter pylori and the birth cohort effect: evidence for stabilized colonization rates in childhood

Background: The prevalence of Helicobacter pylori has declined over recent decades in developed countries. The increasing prevalence with age is largely because of a birth cohort effect. We previously observed a decline in H. pylori prevalence in 6‐ to 8‐year‐old Dutch children from 19% in 1978 to 9% in 1993. Knowledge about birth‐cohort‐related H. pylori prevalence is relevant as a predictor for the future incidence of H. pylori‐associated conditions. Aim: The aim of this study was to investigate whether the birth cohort effect of H. pylori observed between 1978 and 1993 continued in subsequent years. Methods: Anti‐H. pylori IgG antibodies and anti‐CagA IgG antibodies were determined in serum samples obtained in 2005/2006 from 545 Dutch children aged 7–9 years who participated in the Prevention and Incidence of Asthma and Mite Allergy birth cohort. The H. pylori and CagA antibodies were determined by enzyme‐linked immunosorbent assays that have been extensively validated in children, with a 94% sensitivity for H. pylori colonization and a 92.5% sensitivity for colonization with a cagA‐positive strain. Results: Of the 545 children (M/F 300/245), most (91.5%) were of Dutch descent. The H. pylori positivity rate was 9% (95% CI 6.6–11.4%). The prevalence of CagA antibodies was 0.9% (95% CI 0.1–1.6%). No significant differences were demonstrated in H. pylori and cagA prevalence in relation to gender or ethnicity. Conclusion: The prevalence of H. pylori in childhood has remained stable in the Netherlands from 1993 to 2005, suggesting a stabilization of the previously decreasing trend in subsequent birth cohorts. This finding may reflect stabilization in determinants such as family size, housing, and hygienic conditions (or offset by day care). If confirmed in other populations in developed countries, it implies that colonization with H. pylori will remain common in the coming decades. Remarkably however, the rate of colonization with cagA⁺H. pylori strains has become very low, consistent with prior observations that cagA⁺ strains are disappearing in Western countries.     

Negative and Non-Positive Epidemiological Studies

Aim. To identify and discuss validity aspects on so called negative and non-positive studies. Methods. Arguments and examples are drawn from experiences in occupational health epidemiology regarding the interpretation of more or less equivocal study results. Results and conclusions. A negative study may be defined as showing a result that goes against the investigated hypothesis of an increased (or prevented) risk. Traditionally, studies with a risk estimate (relative risk or odds ratio) above but close to unity are also referred to as negative, given a narrow confidence interval (CI) that includes unity. A risk estimate above unity with the CI including unity is non-positive, however, but an estimate below unity with upper CI bond exceeding unity might be seen as possibly negative or non-negative. A weaker “significance” than usually required should perhaps be accepted when evaluating serious hazards. In contrast to positive studies, the negative and non-positive studies tend to escape criticism in spite of questionable validity that may have obscured existing risks (or preventive effects). Even stronger arguments can be made in criticising negative and non-positive studies than positive studies, for example, regarding selection phenomena, and observational problems regarding exposure and outcome. Negative confounding should be considered although usually weak. In case-control studies, so-called over-matching may obscure an existing risk as could the “healthy worker effect” in cohort studies. Small scale non-positive studies should be made available for meta-analyses and when considering studies that do not convincingly show a risk; those who are exposed should be given the “benefit of the doubt.”     

Quantitative observations on the population ecology of Branchiura sowerbyi (Oligochaeta,Tubificidae)

Cohorts of Branchiura sowerbyi were reared at different temperatures and initial population densities in order to obtain data suitable for the interpretation of population dynamics in field populations. Percent hatching from cocoons reaches its maximum at 25 °C and decreases towards lower and higher temperatures. Embryonic development time, T_E, was measured and the relative threshold temperature, 10 °C, calculated by extrapolation. The degree day requirement for embryo development is 195 °C d. The time of first cocoon laying (T_gm) was observed and the ratio T_E/T_gm was seen to fit with that of other tubificid species cultured so far. Embryo mortality is rather high, while worm mortality is low or very low. Fecundity increases from 15 to 20 °C but decreases at 25 °C. A mathematical model for the simulation of population densities with four size-stage compartments is suggested. It could be used for the optimization of worm uptake (simulated as stage specific mortality) in mass cultures reared for the production of Branchiura, to be used as food for fish fingerlings.