1,721,020 research outputs found
Combining the Liu-type estimator and the principal component regression estimator
No full textIn this study a new two-parameter estimator which includes the ordinary least squares, the principal components regression (PCR) and the Liu-type estimator is proposed. Conditions for the superiority of this new estimator over the PCR, r-k class estimator and Liu-type estimator are derived. Furthermore the performance of this estimator is compared with the other estimators in different conditions with simulation studies
A new alternative estimation method for Liu-type logistic estimator via particle swarm optimization: an application to data of collapse of Turkish commercial banks during the Asian financial crisis
No full textIn the existence of multicollinearity problem in the logistic model, some important problems may occur in the analysis of the model, such as unstable maximum likelihood estimator with very high standard errors, false inferences. The Liu-type logistic estimator was proposed as two-parameter estimator to overcome multicollinearity problem in the logistic model. In the existing previous studies, the (k, d) pair in this shrinkage estimator is estimated by two-phase methods. However, since the different estimators can be utilized in the estimation of d, optimal choice of the (k, d) pair provided using the two-phase approaches is not guaranteed to overcome multicollinearity. In this article, a new alternative method based on particle swarm optimization is suggested to estimate (k, d) pair in Liu-type logistic estimator, simultaneously. For this purpose, an objective function that eliminates the multicollinearity problem, provides minimization of the bias of the model and improvement of the model's predictive performance, is developed. Monte Carlo simulation study is conducted to show the performance of the proposed method by comparing it with existing methods. The performance of the proposed method is also demonstrated by the real dataset which is related to the collapse of commercial banks in Turkey during Asian financial crisis
Particle swarm optimization based ridge logistic estimator
No full textLogistic regression is a commonly used method when the dependent variable is dichotomous. However, it is known that the presence of multicollinearity significantly affects maximum likelihood estimations in logistic regression models. In this case, unstable estimates, in other words, parameter estimates with high variances, are obtained. To deal with this problem, a ridge-type estimator was proposed by Schaefer et al. Ridge regression shrinks the maximum likelihood estimation vector of regression coefficients, allowing a bias but providing a smaller variance. However, the selection of shrinkage parameter lambda in ridge logistic regression is an important matter. In this study, a new alternative approach based on particle swarm optimization is introduced to obtain an optimal shrinkage parameter. The performance of the new approach is evaluated by simulation studies and a real dataset application
Identification of influential observations based on binary particle swarm optimization in the cox PH model
No full textProper identification of influential observations should be an integral and significant part of the Cox modeling process. This is because the failure to identify influential observations may have negative effect on the estimates acquired from the process. Furthermore, in survival analysis, influential observations frequently propose that the model has imperfections, including poorly specified factor, non-proportional hazards, loss of important information and/or a variable that has been omitted. Nevertheless, many procedures have been developed for the identification of a single influential observation based on the leave-one-out method. However, the results from leave-one-out diagnostic techniques are often misleading as a result of swamping and masking problems in the presence of multiple influential observations in the dataset. In this paper, identification of the optimal set of influential observations problem has been considered as the combinatorial optimization problem and a new simultaneous approach for identification of the optimal set of influential observations is proposed based on Binary Particle Swarm Optimization (BPSO) approach in the Cox PH model. The performance of the proposed BPSO-based approach and conventional diagnostic techniques have been compared according to various evaluation criteria by simulation studies. The performance of the BPSO-based approach has been also demonstrated by the clinical real dataset
A New Estimator for Cox Proportional Hazard Regression Model in Presence of Collinearity
No full textWe propose a new approach to estimate the parameters of the Cox proportional hazards model in the presence of collinearity. Generally, a maximum partial likelihood estimator is used to estimate parameters for the Cox proportional hazards model. However, the maximum partial likelihood estimators can be seriously affected by the presence of collinearity since the parameter estimates result in large variances. In this study, we develop a Liu-type estimator for Cox proportional hazards model parameters and compare it with a ridge regression estimator based on the scalar mean squared error (MSE). Finally, we evaluate its performance through a simulation study
Predictive factors for the efficacy of Er:YAG laser treatment of urinary incontinence
No full textObjectives: To determine the efficacy and predictive factors for the success of Er:YAG laser treatment in patients with urinary incontinence (UI). Methods: Eighty-two patients with UI were treated by Er:YAG laser in this cohort study. The patients were evaluated by ICIQ-SF and KHQ-UI before and after the procedure. Improvement was categorized as: none (0-25%), mild (26-50%), moderate (51-75%), or high (76-100%). The duration of the treatment effect was evaluated at follow-up in relation to maximum improvement time (MIT) and total improvement time (TIT). Results: Forty-two patients were determined to have SUI and 40 patients MUI. The mean ISIQ-SF and KHQ-UI scores significantly improved after the procedure (p < 0.0001). The SUI patients responded to the laser treatment significantly better (p < 0.008). Younger women had significantly better results (p < 0.008), while premenopausal women (p < 0.032) and women in the early postmenopausal years (p < 0.032) also saw a positive response to the Er:YAG laser treatment. Women with a lower BMI had greater improvement (p < 0.011). The total laser energy expenditure during the sessions may also be a predictive parameter for the success of Er:YAG laser treatment of UI (p = 0.059). MIT and TIT were significantly longer among the patients in the high-improvement group. Conclusion: Er:YAG laser treatment of the symptoms of UI, especially SUI, is more efficacious and of longer duration for younger, premenopausal or early postmenopausal women with normal BMI
Serum kisspeptin levels correlated with anti-mullerian hormone levels in women with and without polycystic ovarian syndrome
No full textObjective: Todetermine serum kisspeptin and anti-mullerian hormone (AMH) levels in women with and without polycystic ovary syndrome (PCOS). Materials and methods This was a prospective, cross-sectional, comparative study that included 70 women with PCOS and 58 non PCOS controls. PCOS patients were diagnosed according to the Rotterdam criteria. Age, body mass index (BMI), number of menstrual cycles per year, and the Ferriman-Gallwey Score were determined for each woman. Serum levels of kisspeptin, follicle stimulating hormone (FSH), luteinizing hormone (LH), thyroid stimulating hormone (TSH), estradiol, total testosterone, dehydroepiandrosterone sulfate(DHEA-S), AMH, fasting glucose and insulin were determined. Results Women with PCOS were younger (p < .001), with higher BMI (p = .027) and glucose values (p < .001); while displaying less number of menstrual cycles per year (p < .001). Although serum kisspeptin levels were similar in both groups, age was negatively (r= -0.33,p = .00018) and serum AMH levels were positively correlated (r = 0.25,p = .0039) with the serum kisspeptin levels in women with the PCOS. After adjusting for age, serum kisspeptin levels were comparable in both groups (p > .05). Serum LH, AMH, DHEA-S and total testosterone glucose, insulin levels and HOMA-IR values were significantly higher in women with PCOS as compared to controls (allp < .05). Conclusions Serum kisspeptin levels were similar in women with and without PCOS but positively correlated with AMH serum levels in PCOS women
Liu-Type Logistic Estimator
No full textIt is known that multicollinearity inflates the variance of the maximum likelihood estimator in logistic regression. Especially, if the primary interest is in the coefficients, the impact of collinearity can be very serious. To deal with collinearity, a ridge estimator was proposed by Schaefer et al. The primary interest of this article is to introduce a Liu-type estimator that had a smaller total mean squared error (MSE) than the Schaefer's ridge estimator under certain conditions. Simulation studies were conducted that evaluated the performance of this estimator. Furthermore, the proposed estimator was applied to a real-life dataset
Serum kisspeptin levels along reproductive period in women: is it a marker for aging?
No full textObjective To demonstrate the change in serum kisspeptin levels during the reproductive period in healthy women and to investigate the relationship with other reproductive hormones. Methods One hundred thirty-one healthy women with normal menstrual history were included and serum kisspeptin, follicle-stimulating hormone (FSH), luteinizing hormone (LH), thyroid-stimulating hormone (TSH), estradiol (E-2), and anti-Mullerian hormone (AMH) levels were determined on cycle day 3. The data were analyzed in 5-year age groups. Results Serum kisspeptin levels of all women were found to be significantly and negatively correlated with age (r= -0.458). The kisspeptin levels were the highest in the group of women aged between 20 and 24 years compared to other age groups above 25 years (p < .01, p < .001, p < .0005, p < .0005). There was not any significant correlation between serum kisspeptin levels and AMH, FSH, LH, TSH, E-2, and body-mass index (BMI), respectively. The Scatter and Violin plots showed that most of the women over 35 years of age had serum kisspeptin levels under the level of 500 pg/ml. The kisspeptin levels of women over 35 years of age clustered closely as opposed to the kisspeptin levels of those below the age of 35, which were scattered. The median serum kisspeptin levels were found to be high in women below the age of 35 (p < .0005). Conclusion In healthy women, serum kisspeptin level is the highest in the group of women aged between 20 and 24 years and declines with age. It tends to be below the level of 500 pg/ml in women over the age of 35
Er:YAG laser treatment of urinary incontinence after failed TOT/TVT procedures
No full textObjective: To determine if the Er:YAG laser can improve the symptoms of SUI patients after previously failed TOT/TVT procedures. Study Design: This retrospective study includes the data of patients who were recruited from two different out-patient clinics of Obstetrics and Gynecology Department. 25 women with persistent SUI after failed TOT/TVT operations and 25 women who previously did not receive either any type of surgical treatment procedure or non-invasive treatment modalities for SUI. Er:YAG laser with 2940 nm was used in the treatment procedure for SUI setting. The patients were evaluated on the basis of ICIQ-SF before and after the procedure. The severity of SUI symptoms was graded. According to the differences in the ICIQ-SF between before and after the procedure, the percentage of improvement was graded as good responders (>= 50 %) or poor responders (<50 %). The duration of the treatment effect was evaluated in follow-ups with relation to maximum improvement time (MIT) and total improvement time (TIT). Results: The SUI patients who previously had failed TOT/TVT operations, had significantly higher initial ICIQ-SF score (p = 0.013). Non-ablative Er:YAG laser treatment significantly and similarly improved the severity of SUI symptoms in both groups (p = 0.000 for failed TOT/TVT group and p = 0.001 for non-TOT/TVT group, respectively). The women who were good responders, were younger (p = 0.012) and had less number of years in menopause (p = 0.011). The effect of Er:YAG laser treatment lasted longer among the SUI women in the good responders group (p = 0.000 for MIT and p = 0.000 for TIT, respectively). Conclusions: Non-ablative Er:YAG smooth mode laser is an alternative choice of treatment for the SUI patients who previously had failed TOT/TVT procedures. Its effect lasts longer especially in younger and early postmenopausal women. (C) 2020 Elsevier B.V. All rights reserved
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