91 research outputs found
Bias and variance reduction procedures in non-parametric regression
No full textThe purpose of this study is to determine the effect of three improvement methods on nonparametric kernel regression estimators. The improvement methods are applied to the Nadaraya-Watson estimator with cross-validation bandwidth selection, the Nadaraya-Watson estimator with plug-in bandwidth selection, the local linear estimator with plug-in bandwidth selection and a bias corrected nonparametric estimator proposed by Yao (2012), based on cross-validation bandwith selection. The performance of the different resulting estimators are evaluated by empirically calculating their mean integrated squared error (MISE), a global discrepancy measure. The first two improvement methods proposed in this study are based on bootstrap bagging and bootstrap bragging procedures, which were originally introduced and studied by Swanepoel (1988, 1990), and hereafter applied, e.g., by Breiman (1996) in machine learning. Bagging and bragging are primarily variance reduction tools. The third improvement method, referred to as boosting, aims to reduce the bias of an estimator and is based on a procedure originally proposed by Tukey (1977). The behaviour of the classical Nadaraya-Watson estimator with plug-in estimator turns out to be a new recommendable nonparametric regression estimator, since it is not only as precise and accurate as any of the other estimators, but it is also computationally much faster than any other nonparametric regression estimator considered in this stud
Modified tests for exponentiality in the presence of Type II right censored data
No full textThe exponential distribution is one of the most popular choices of model, especially in survival analysis and reliability theory. Typically, to test the hypothesis that the underlying distribution of lifetimes are exponentially distributed based on censored data, the standard goodness‐of‐fit tests used in the complete sample case are modified. However, none of the characteristic or Laplace function based tests have been adapted to incorporate censored data. In this paper, we modify three well‐known tests for exponentiality on the basis of either the characteristic function or Laplace transform to accommodate Type II right censored data. We compare the power of the three newly changed tests to some existing modified tests in an independent identically distributed (i.i.d.) and a model‐based setup through a Monte Carlo study, where it was found that the newly adjusted Baringhaus and Henze test outperformed all other tests considere
Ethnic disparity in defensive coping endothelial responses: The SABPA study
No full textOBJECTIVES
Defensive coping (DefS) in Blacks has been associated with greater cardiovascular risk than in their White counterparts. We examined associations between endothelial function mental stress responses and markers of vascular structure in a bi-ethnic cohort.
METHODS
We examined vascular function and structure in 368 Black (43.84±8.31years) and White Africans (44.78±10.90years). Fasting blood samples, 24h blood pressure, left carotid intima-media thickness of the far wall (L-CIMTf), and left carotid cross-sectional wall area (L-CSWA) values were obtained. von Willebrand factor (VWF), endothelin-1 (ET-1) and nitric oxide metabolite (NOx) responses to the Stroop mental stress test were calculated to assess endothelial function. DefS was assessed using the Coping Strategy Indicator questionnaire. Interaction between main effects was demonstrated for 283 participants with DefS scores above the mean of 26 for L-CIMTf.
RESULTS
Blunted stress responses for VWF (men 16.71% vs. 51.10%; women 0.85% vs. 42.09%, respectively) and NOx (men -64.52% vs. 74.89%; women -76.16% vs. 113.29%, respectively) were evident in the DefS Blacks compared to the DefS Whites (p<0.001). ET-1 increased more in Blacks (men 150% and women 227%, p<0.001) compared to the Whites (men 61.25% and women 35.49%, p<0.001). Ambulatory pulse pressure, but not endothelial function markers, contributed to L-CIMTf (ΔR(2)=0.11 p<0.001), and L-CSWA (ΔR(2)=0.08, p<0.001) in DefS African men but not in any other group.
CONCLUSIONS
Blunted stress-induced NOx and VWF responses and augmented ET-1 responses in DefS Blacks indicate endothelial dysfunction. DefS may facilitate disturbed endothelial responses and enforce vascular remodelling via compensatory increases in pulse pressure in Black men. These observations may indicate an increased risk of cardiovascular incidents via functional and structural changes of the vasculature in DefS Blacks
The state of the psychological contract and employees’ intention to leave: The mediating role of employee well-being
No full textChanges in the work environment can influence employees’ perception of the employment relationship, their psychological contracts, and consequently, their work-related attitudes and behaviour. The study aimed to demonstrate the mechanism through which the state of employees’ psychological contract impacts employees’ intention to leave employment and the role of employee well-being as a mediator. Cross-sectional data were obtained from employees representing various organisations (N = 246). Structural equation modelling demonstrated that employee well-being mediates the relationship between the state of the psychological contract and an employee’s intention to leave the organisation. The study stresses the importance of designing individual-level interventions to enhance well-being in the workplace, which should form part of the retention strategy of any organisatio
Troponin T release is associated with silent myocardial ischaemia in black men: The SABPA Study
No full textBackground
High sensitivity cardiac troponin T (hs-cTnT) is a validated marker of myocardial damage and may reflect the degree of silent myocardial ischaemia (SMI) and ventricular strain. Our aim was to compare hs-cTnT levels in black and white South Africans taking SMI into consideration. We further explored the capability of hs-cTnT to predict the presence of compensatory systolic hypertension in this South African cohort.
Methods
A bi-ethnic sex cohort (n = 404) with similar socioeconomic status (198 black participants and 206 white participants, aged 20–65 years) participated in this target population study where 24 h ambulatory blood pressure, electrocardiogram and overnight fasting cardiometabolic variables were measured.
Results
Hypertension, higher glycated haemoglobin levels and more frequent and longer SMI events were observed more often in the black participants. Multivariate linear regression analysis showed positive associations between SMI events [Adj. R2 = 0.19; β 0.35 (0.08–0.62); p < 0.01], SMI event maximum duration [Adj. R2 = 0.17, β 0.43 (0.16–0.70), p < 0.01], SMI total duration [Adj. R2 = 0.12; β 0.37 (0.10; 0.65), p = 0.05] and hs-cTnT in black males only.] A lower hs-cTnT cut-point ≥4.2 pg/ml for 24 h systolic hypertension was predicted in the black participants compared with ≥5.6 pg/ml in the white participants (area under the curve 0.66–67 (95% CI: 0.57–0.75), p < 0.001) with a respective sensitivity/specificity of 64/68% and 61/71%.
Conclusions
hs-cTnT may be a potential marker of SMI in the prediction of systolic blood pressure increases, as well as clusters of risk factors for cardiovascular disease. Ethnic- and possibly sex-specific references values for hs-cTnT should be considered for risk stratificatio
Prediction of 24-hour sodium excretion from spot urine samples in South African adults: a comparison of four equations
No full textRepeated 24-hour urine collection is considered to be the gold standard for assessing salt intake. This is often impractical in large-population studies, especially in low–middle-income countries. Equations to estimate 24-hour urinary salt excretion from a spot urine sample have been developed, but have not been widely validated in African populations. This study aimed to systematically assess the validity of four existing equations to predict 24-hour urinary sodium excretion (24UNa) from spot urine samples in a nationally representative sample of South Africans. Spot and 24-hour urine samples were collected in a subsample (n = 438) of participants from the World Health Organisation Study on global AGEing and adult health (SAGE) Wave 2 in South Africa in 2015. Measured 24UNa values were compared with predicted 24UNa values from the Kawasaki, Tanaka, INTERSALT and Mage equations using Bland–Altman plots. In this subsample (mean age 52.8 ± 16.4 years; body mass index 30.2 ± 8.2 kg/m2; 76% female; 73% black African; 42% hypertensive), all four equations produced a significantly different population estimate compared with the measured median value of 6.7 g salt/day (IQR 4.4–10.5). Although INTERSALT underestimated salt intake (−3.77 g/d; −1.64 to −7.09), the other equations overestimated by 1.28 g/d (−3.52; 1.97), 6.24 g/d (2.22; 9.45), and 17.18 g/d (8.42; 31.96) for Tanaka, Kawasaki, and Mage, respectively. Bland–Altman curves indicated unacceptably wide levels of agreement. Use of these equations to estimate population level salt intake from spot urine samples in South Africans is not recommende
Goodness-of-fit tests in the Cox proportional hazards model
No full textWe consider a variety of tests for testing goodness-of-fit in a parametric Cox proportional hazards (PH) model and compare their performance. Aspects of the model under test include the baseline distribution and time-invariance of covariates. We also test for the PH model itself against a certain generalization. This is done through an extensive Monte Carlo study where we simulate the performance of the tests for these three paired hypotheses. The results show that the tests based on the empirical characteristic function and those based on the empirical Laplace transform have the best overall power performance. It is also found that the distributions of the considered test statistics do not depend on the specific functional form of the covariate functio
An elderly, urban population: Their experiences and expectations of pharmaceutical services in community pharmacies
No full textObjective: The aim of this study was to determine the pharmaceutical services experiences of an elderly, urban population in relation to their expectations in community pharmacy.
Design: and setting: The study was a cross-sectional descriptive empirical study and was conducted by means of a structured questionnaire, designed and administered by there searcher, in face-to-face interviews at an old age residence in the participants' own dwellings (N = 242). The management of this specific village approached the researcher about healthcare concerns for their residents and granted permission for this study to be conducted.
Main outcome and results: The sample population of elderly patients (n = 67) had a higher expectation of community pharmacists, in terms of pharmaceutical services, than what they had experienced. Significant differences were found in all aspects examined. Effect size, amongst others, revealed that expectations were not met in discussions about the effect of other medicines on their chronic medicine (d = 1.94); whether they had any medicines left from previous issues (d = 1.77) and questions regarding existing chronic conditions (d = 1.69). There was an association between questions, regarding the use of chronic medicines at pharmacies and at other healthcare professionals (d =0.26), as well as the supply of written information at pharmacies and other healthcare professionals (d = 0.42). The study reveals that this population has expectations of their community pharmacists that are not met.
Conclusions: Pharmacists can be active members of the healthcare team and restore their professional image in the eyes of the community, if they renew their attitudes, have confidence in their abilities and understand their role in patient care. The community pharmacist should focus on the health-related quality of life of the individual patient and identify the immediate healthcare needs of their unique community, with specific reference to vulnerable populations like the elderly. Pharmacists should establish themselves as the go-to healthcare professional
C679X loss-of-function PCSK9 variant lowers fasting glucose levels in a black South African population: a longitudinal study
No full textAims
To determine the longitudinal association of the loss-of-function (LOF) PCSK9 variants (C679X and A443T), proxies of PCSK9 inhibitor drugs, with LDL-C, fasting glucose and glycated hemoglobin.
Methods
We conducted a five year, longitudinal study, nested within the Prospective Urban and Rural Epidemiology study, among 737 apparently healthy, male and female black South Africans of the North West province. Genotyping of the C679X and A443T PCSK9 variants was achieved using Taqman assays from Applied Biosystems. Generalized estimating equations were used to determine longitudinal association of the A443T and C679X PCSK9 variants with LDL–C, fasting glucose and glycated hemoglobin.
Results
C679X and A443T variant carriers were associated with significant reductions in LDL-C of −0.98(−1.29, −0.67) mmol/L; p < 0.001) and −0.39(−0.57, −0.20) mmol/L; p < 0.001) respectively, compared to the non-carriers. Only C679X variant was independently associated with reductions in fasting glucose of −0.37 (−0.61, −0.13) mmol/L; p = 0.002) compared to non-carriers. However, the association of the selected variants with glycated hemoglobin were not significant. C679X and A443T carriers were associated with −0.07 (−0.23, 0.09) %; p = 0.400), 0.05 (−0.13, 0.22) %; p = 0.599) of glycated haemoglobin respectively.
Conclusion
Our results indicated that carriers of A443T and C679X variants exhibit sustained low LDL-C levels over 5 years and have varied effects on T2D biomarkers compared to non-carrier
The long-term effects of methamphetamine exposure during pre-adolescence on depressive-like behaviour in a genetic animal model of depression
No full textMethamphetamine (METH) is a psychostimulant and drug of abuse, commonly used early in life, including in childhood and adolescence. Adverse effects include psychosis, anxiety and mood disorders, as well as increased risk of developing a mental disorder later in life. The current study investigated the long-term effects of chronic METH exposure during pre-adolescence in stress-sensitive Flinders Sensitive Line (FSL) rats (genetic model of depression) and control Flinders Resistant Line (FRL) rats. METH or vehicle control was administered twice daily from post-natal day 19 (PostND19) to PostND34, followed by behavioural testing at either PostND35 (early effects) or long-lasting after withdrawal at PostND60 (early adulthood). Animals were evaluated for depressive-like behaviour, locomotor activity, social interaction and object recognition memory. METH reduced depressive-like behaviour in both FSL and FRL rats at PostND35, but enhanced this behaviour at PostND60. METH also reduced locomotor activity on PostND35 in both FSL and FRL rats, but without effect at PostND60. Furthermore, METH significantly lowered social interaction behaviour (staying together) in both FRL and FSL rats at PostND35 and PostND60, whereas self-grooming time was significantly reduced only at PostND35. METH treatment enhanced exploration of the familiar vs. novel object in the novel object recognition test (nORT) in FSL and FRL rats on PostND35 and PostND60, indicative of reduced cognitive performance. Thus, early-life METH exposure induce social and cognitive deficits. Lastly, early-life exposure to METH may result in acute antidepressant-like effects immediately after chronic exposure, whereas long-term effects after withdrawal are depressogenic. Data also supports a role for genetic predisposition as with FSL rat
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