Intercourse debut age: Poor resources, problem behavior, or romantic appeal? A population-based longitudinal study

The most important predictors of early intercourse debut are reported to be poor social resources and early developing problem behaviors. In this study we have a new, additional emphasis on variables related to self-concept and social acceptance. In a population-based longitudinal study, 1399 Norwegians were followed over a 7-year span. We analyzed data using multivariate Cox-regression techniques. Early intercourse debut was part of a broader spectrum of problem behaviors, including early alcohol intoxication and early-developing conduct problems. A new finding was that a positive self-concept in the domain of ýromantic appealý was also a strong predictor, but only for boys. We suggest that the findings may have important implications for prevention and more research should be conducted along this line

Low Serum Levels of 25-Hydroxyvitamin D Predict Hip Fracture in the Elderly: A NOREPOS Study

Background: Despite considerable interest, the relationship between circulating 25-hydroxyvitamin D and the risk of hip fracture is not fully established. Objective: The objective of the study was to study the association between serum 25-hydroxyvitamin D concentrations [s-25(OH)D] and the risk of hip fracture in Norway, a high-latitude country that has some of the highest hip fracture rates worldwide. Methods: A total of 21 774 men and women aged 65–79 years attended 4 community-based health studies during 1994–2001. Information on subsequent hip fractures was retrieved from electronic hospital discharge registers, with a maximum follow-up of 10.7 years. Using a stratified case-cohort design, s-25(OH)D was determined by HPLC-atmospheric pressure chemical ionization-mass spectrometry in stored serum samples in hip fracture cases (n = 1175; 307 men, 868 women) and in gender-stratified random samples (n = 1438). Cox proportional hazards regression adapted for the case-cohort design was performed. Results: We observed an inverse association between s-25(OH)D and hip fracture; those with s-25(OH)D in the lowest quartile (<42.2 nmol/L) had a 38% [95% confidence interval (CI) 9–74%] increased risk of hip fracture compared with the highest quartile (≥67.9 nmol/L) in a model accounting for age, gender, study center, and body mass index. The association was stronger in men than in women: hazard ratio 1.65 (95% CI 1.04–2.61) vs hazard ratio 1.25 (95% CI 0.95–1.65). Conclusion: In this prospective case-cohort study of hip fractures, the largest ever reported, we found an increased risk of hip fracture in subjects in the lowest compared with the highest quartile of serum 25-hydroxyvitamin D. In accordance with the findings of previous community-based studies, low vitamin D status was a modest risk factor for hip fracture.publishedVersio

Exact inference intheproportionalhazard model: possibilities and limitations

It is suggested that inference under the proportional hazard model can be carried out by programs for exact inference under the logistic regression model. Furthermore a different type of exact inference is developed under Type II censoring. Performance of logistic exact and exact inference is compared to large sample Wald, score and likelihood inference by coverage and power calculations. The logistic exact confidence intervals have coverage well above the nominal level in most computation. The exact inference under Type II censoring turn out to be less conservative but numerically very complex. Large sample methods works well with remarkably small data but score and likelihood ratio methods are preferable to inference by Wald statistics

Stratified case-cohort analysis of general cohort sampling designs

We first point out that variance estimates for regression coefficients in exposure stratified case-cohort studies (Borgan et al., 2000) can easily be obtained from influence terms routinely calculated in standard software for Cox regression. We also place the estimators proposed by Chen (2001) for a general class of cohort sampling designs within the Borgan et al. framework by allowing for post-stratification on outcome. This facilitates simple variance estimation for this class of cohort sampling designs. Finally, we extend the approach of Chen to accommodate stratified designs with surrogate variables available for all cohort members, such as stratified case-cohort and counter-matching designs