Dependent censoring in piecewise exponential survival models.

There are often reasons to suppose that there is dependence between the time to event and time to censoring, or dependent censoring, for survival data, particularly when considering medical data. This is because the decision to treat or not is often made according to prognosis, usually with the most ill patients being prioritised. Due to identifiability issues, sensitivity analyses are often used to assess whether independent censoring can lead to misleading results. In this paper, a sensitivity analysis method for piecewise exponential survival models is presented. This method assesses the sensitivity of the results of standard survival models to small amounts of dependence between the time to failure and time to censoring variables. It uses the same assumption about the dependence between the time to failure and time to censoring as previous sensitivity analyses for both standard parametric survival models and the Cox model. However, the method presented in this paper allows the use of more flexible models for the marginal distributions whilst remaining computationally simple. A simulation study is used to assess the accuracy of the sensitivity analysis method and identify the situations in which it is suitable to use this method. The study found that the sensitivity analysis performs well in many situations, but not when the data have a high proportion of censoring.

Weaning preterm infants: a randomised controlled trial.

OBJECTIVE: To assess the effect on growth and iron status in preterm infants of a specially devised weaning strategy compared with current best practices in infant feeding. The preterm weaning strategy recommended the early onset of weaning and the use of foods with a higher energy and protein content than standard milk formula, and foods that are rich sources of iron and zinc. SUBJECTS AND DESIGN: In a blinded, controlled study, 68 preterm infants (mean (SD) birth weight 1470 (430) g and mean (SD) gestational age 31.3 (2.9) weeks) were randomised to either the preterm weaning strategy group (n = 37) or a current best practice control group (n = 31), from hospital discharge until 1 year gestation corrected age (GCA). MAIN OUTCOME MEASURES: Weight, supine length, occipitofrontal head circumference, and intakes of energy, protein, and minerals were determined at 0, 6, and 12 months GCA. Levels of haemoglobin, serum iron, and serum ferritin were assayed at 0 and 6 months GCA. RESULTS: Significant positive effects of treatment included: greater increase in standard deviation length scores and length growth velocity; increased intake of energy, protein, and carbohydrate at 6 months GCA and iron at 12 months GCA; increased haemoglobin and serum iron levels at 6 months GCA. CONCLUSIONS: The preterm weaning strategy significantly influenced dietary intakes with consequent beneficial effects on growth in length and iron status. This strategy should be adopted as the basis of feeding guidelines for preterm infants after hospital discharge.

Systematic review concerning the age of introduction of complementary foods to the healthy full-term infant.

OBJECTIVES: To evaluate existing data from published studies investigating outcomes (growth, nutritional status and morbidity) in relation to the age of introduction of complementary foods to the healthy full-term infant. To assess the methodological quality of current scientific evidence on which infant feeding recommendations are based. SEARCH STRATEGY: A database was created following a search of electronic databases MEDLINE and BIDS, a handsearch of relevant journals and consultation with international experts in infant feeding practices. The search was conducted during a 6-month period and encompassed the years 1982-1998. INCLUSION CRITERIA: Randomised/non-randomised controlled trials and cohort studies investigating the relationship to the health of full-term infants of the introduction of complementary foods to childhood health. DATA COLLECTION AND ANALYSIS: Assessment of studies for inclusion and their compliance with methodological criteria was undertaken independently by two reviewers and discrepancies were resolved by discussion. RESULTS: From over 400 published papers identified, 33 met specific inclusion criteria. Significant differences in baseline characteristics of subjects in included studies meant that data were not amenable to meta-analysis. The consensus of the authors was that, of the 33 included publications, 13 contained data supporting the current WHO recommendations which attempt to accommodate all infants, including those whose nutritional requirements are not met by exclusive breast-feeding for 6 months. An equal number contained data that would support a recommendation for delaying the introduction of complementary foods until 6 months of age within the study population. The remaining seven were unable to provide evidence to support a change from the current WHO infant feeding recommendations which state that 'infants should be fed exclusively on breast milk from birth to 4-6 months of age'. None of the studies met all the methodological criteria. CONCLUSIONS: This review has shown that there is a lack of clear evidence to either support or refute a change to the current recommendations for the age of introduction of complementary foods to the breast milk or formula fed infant. Whilst exclusive breast feeding for the first 6 months of life can support growth and development in some infants, sub-groups have been identified within certain populations who may require complementary feeding prior to this age.

Nutritional composition of home-prepared baby meals in Madrid. Comparison with commercial products in Spain and home-made meals in England.

The nutritional value of 50 home-prepared meat-based baby meals for infants of 7 and 8 months old in Madrid was evaluated. Chemical analysis showed the meals to have a low energy density (mean 50 kcal 100 g-1), mainly due to the low fat content of the meals (mean 2.5 g 100 kcal-1). Protein content was high in proportion to the energy provided (mean 9.1 g 100 kcal-1). The addition of fat would have considerably improved the relation between the three macronutrients. About 20% of the meals had a higher than desired sodium level (> 200 mg 100 kcal-1) when assessed against guidelines for commercial baby meals. The English home-made baby meals had a higher energy density, a lower protein content, the same low fat content, and a wider range between minimum and maximum values observed for each nutrient compared to the Spanish home-prepared meals. The commercial baby meals had a higher energy density, a better ratio between the macronutrients and a smaller variation in the nutrient content.

[Feeding habits of a sample of children under 20 months of age in Madrid]. / Hábitos alimenticios de una muestra de niños menores de 20 meses en Madrid.

OBJECTIVE: To know the feeding habits of children of different socioeconomic groups during the first year and a half of life in Madrid. PATIENTS AND METHODS: The mothers of 344 children between 3 and 19 months who attended two vaccination centers of the Regional Health Authorities of Madrid were interviewed in detail using the dietary history method supplemented with questions on milk feeding and weaning. The sample was stratified into three socioeconomic groups, low middle and high, each group comprising about a third of the sample. RESULTS: In general, children were fed according to actual pediatric recommendations. Deviations were that breast-feeding was stopped too early, complementary feeding was introduced somewhat early and mothers were often found not to understand gluten nor did they know which products contain it. The children in this study were remarkably similarly fed regardless of socioeconomic group. CONCLUSIONS: The pediatrician should know where mothers may deviate from the recommendations to prevent feeding errors because o misunderstanding or ignorance.

A Weibull-based score test for heterogeneity.

The Weibull distribution is a natural starting point in the modelling of failure times in reliability, material strength data and many other applications that involve lifetime data. In recent years there has been a growing interest in modelling heterogeneity within this context. A natural approach is to consider a mixture, either discrete or continuous, of Weibull distributions. A judicious choice of mixing distribution yields a tractable and flexible generalization of the Weibull distribution. In this note a score test for detecting heterogeneity in this context is discussed and illustrated using some infant nutrition data.

Weaning practices in children up to 19 months of age in Madrid.

The weaning practices of 344 children in Madrid, aged 3-19 months, were investigated by controlled interview of their mothers or caretakers. The results were compared with national and international guidelines for the introduction of complementary foods; there are no Spanish guidelines. We found only minor differences between the practices in three different socioeconomic groups. Generally, mothers in Madrid followed national and international guidelines well. However, more attention should be paid to aspects such as the timing of the introduction of the first complementary food, the nature of gluten, the addition of salt and sugar to baby meals and the introduction of potentially allergenic foods. Also, manufacturers of baby foods that are sold in Spain could play a useful role by reducing the number of ingredients used in products advised for the initial phase of weaning and by avoiding the use of salt and sugar in their products.

Healthy eating for infants--mothers' attitudes.

Mothers' perceptions of desirable nutritional practices in infant feeding were examined using a questionnaire consisting of open and closed questions. A total of 1004 mother-infant pairs were recruited from a mixture of urban and rural areas in England. The sample represented a cross-section of socioeconomic groups and educational backgrounds. Mothers' attitudes to healthy eating for infants revealed some misconceptions; 83% felt that a high fibre intake was important or very important and 87% that a low fat intake was important or very important, while 20% considered that plenty of calories was not important. Other health guidelines were appropriately applied and most mothers considered a wide variety of foods, plenty to drink and a low sugar and salt intake to be important. These beliefs were representative of the sample population, irrespective of the socioeconomic group, location, age and education of the mother.

Type of milk feeding in infants and young children up to 19 months of age in three socio-economic groups in Madrid.

The feeding practices of 344 children in Madrid between 3 and 19 months were investigated by controlled interview or their mothers of care-takers. Four different types of milk were given to the children: human milk, infant formula, follow-on formula and cow's milk. Three periods could be defined when infants were most likely to change from one milk type to another. The majority of babies changed from human milk to infant formula some time in the first three months of life. At between four and six months, about half the sample changed to a follow-on formula and, finally, around the baby's first year formula was substituted for cow's milk. Differences in feeding behaviour could be observed among socio-economic groups, but were not statistically significant. Feeding patterns closely followed international recommendations for infant feeding.

Flight deck design and pilot selection: anthropometric considerations.

Safe and successful operation of flight displays and controls is, in part, dependent on the anthropometric characteristics of the pilots with respect to the design of a particular aircraft. This paper describes the approach required to optimise this fit and provides guidelines for both those responsible for design and those who select pilot recruits. The major results reported are those for a British population, although the aircraft considered (Boeing 737-200, 747, 757 and Lockheed TriStar) are used by airlines throughout the world. The study shows that limitations in design considerably reduce the pool of potential recruits with the appropriate anthropometric characteristics. The selection criteria, based on functional seated eye height, might exclude 73% of the British, 19-65-year-old female population and 13% of the male population.

A repeated measurements model with applications in psychology.

The multivariate Burr distribution is discussed in relation to the analysis of repeated measures data in psychology. The ease of coping with censored and missing observations is particularly highlighted. A numerical example involving test scores of low IQ institutional patients is used to illustrate the method.