İstatistik Bölümü Koleksiyonu

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https://hdl.handle.net/20.500.12428/2233

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Leverages and Influential Observations in a Regression Model with Autocorrelated Errors
(Taylor & Francis Inc, 2015) Özkale, M. Revan; Söküt Açar, Tuğba
This article deals with the general form of the hat matrix and the DFBETA measure to detect the influential observations and the leverages in the linear regression model with more than one regressor when the errors are from AR(1) and AR(2) processes. Previous studies dealing with the influential observations and the leverages in the constant mean model and regression through the origin model are obtained as special cases. To demonstrate the utility of the hat matrix and the DFBETA measure, two numerical examples based on the ice cream consumption data with AR(1) errors and the Fox-Hartnagel data with AR(2) errors are analyzed. The results show that the parameter of the autoregressive process affects the influential and leverage points.
A new approach in modelling the circular data: circular ridge estimator
(Taylor & Francis Ltd, 2023) Söküt Açar, Tuğba
Different regression models that use circular data supported on the unit circle are rare. Regression parameters for circular data have mostly been estimated using the least-squares method. This paper addresses multicollinearity between the circular regressors. The ridge estimator is suggested as an alternative to the least-squares estimator in circular-linear regression model. The models fitted by the circular least-squares and circular ridge estimators are compared on real and simulated datasets. The mean squared error and the coefficient of determination are used to assess the models' adequacy. The findings demonstrated that the fitted models might not be significant if the circularity of the data is ignored. Circular regression on circular data shows the model to be significant. Although the two estimators' coefficients of determination for circular models are quite close, the circular ridge estimator with the optimum biasing parameter has a smaller scalar mean square error than the circular least-squares estimator.
Regression diagnostics methods for Liu estimator under the general linear regression model
(Taylor & Francis Inc, 2020) Söküt Açar, Tuğba; Özkale, M. Revan
This paper introduces the regression diagnostic methods for the Liu estimator under the general linear regression model with the autocorrelated error structure which is modeled by first-order autoregressive process. To identify the leverage observations, quasi-projection matrix is used, and to identify the influential observations DFFITS and Cook's D statistics which are single case deletion methods are used for the Liu estimator. For a special model with two explanatory variables, the leverage attitudes of the first observation are examined theoretically according to the autocorrelation coefficient and the Liu parameter. The proposed diagnostic methods are investigated through a numerical example.
Influence measures in ridge regression when the error terms follow an Ar(1) process
(Springer Heidelberg, 2016) Söküt Açar, Tuğba; Özkale, M. Revan
Influence concepts have an important place in linear regression models and case deletion is a useful method for assessing the influence of single case. The influence measures in the presence of multicollinearity were discussed under the linear regression models when the errors structure is uncorrelated and homoscedastic. In contrast to other article on this subject, we consider the influence measures in ridge regression with autocorrelated errors. Theoretical results are illustrated with a numerical example and a Monte Carlo simulation is conducted to see the effect autocorrelation coefficient, strength of multicollinearity and sample size on leverage points and influential observations.
Climate change effects on abundance and distribution of the European eel in Türkiye
(Wiley, 2024) Mestav, Burcu; Yalçın Özdilek, Şükran; Acar, Zahide; Gökkaya, Kemal; Partal, Nurbanu
Spatial and temporal distribution of European eel (Anguilla anguilla) during 1967–2020 was used to model European eel distribution in response to climate variability. Modeling integrated predictions from two distinct models for inferences. First, modeling of the relationship between temperature and precipitation and European eel catch values using multiple time-series regression analyses showed that climate variables contributed to eel distribution in Türkiye. Eel catch became restricted to western and southern coasts of Türkiye and total catch decreased between 1967 and 2020. From 1967 to 2020, favorable climatic conditions for European eels in Türkiye were characterized by moderate temperatures. Furthermore, projections for 2050 and 2070 suggested that suitable habitat would be lost in Türkiye. Our findings underscore the urgent need for immediate and widespread implementation of effective conservation policies to mitigate threats to European eel in Türkiye and globally.
Jeotermal Akışkanın Hidrokimyasal Karakterizasyonunun Belirlenmesi: Simav (Kütahya) Örneği
(Çanakkale Onsekiz Mart Üniversitesi Fen Bilimleri Enstitüsü, 2019) Çardak, Mine; Şanyüksel Yücel, Deniz; Ay, Mustafa Ertan; Söküt Açar, Tuğba; Erol Tınaztepe, Özlem
Yüksek sıcaklık ve basınç altında jeotermal akışkan ile uzun süre etkileşim içerisinde olankayaçtaki birçok metal(loid) çözünerek suya geçmektedir. Jeotermal akışkanın içerdiğiyüksek metal ve iz element konsantrasyonu nedeni ile çevresindeki su kaynakları üzerindeolumsuz etkileri bulunmaktadır. Kütahya ili Simav ilçesinde Eynal, Naşa ve Çitgöl jeotermalalanları yer almaktadır. Jeotermal akışkan termal turizm, konut ve sera ısıtması amaçlı olarakkullanılmakta, atık suyu Simav çayına deşarj edilmektedir. Bölgede reenjeksiyon kuyusubulunmamaktadır. Bu çalışmada Simav ilçesindeki jeotermal akışkanın hidrokimyasalkarakterizasyonunun belirlenmesi ve olası çevresel etkilerinin değerlendirilmesiamaçlanmıştır. Bu amaç ile Eynal, Naşa ve Çitgöl jeotermal alanlarından 2014 ve 2015yıllarında yağışlı ve kurak dönem olmak üzere 2 kez ve toplam 22 jeotermal sondajdan suörneklemesi yapılmıştır. Elde edilen hidrokimyasal analiz sonuçları ulusal standartlarlakarşılaştırılarak, istatistiksel olarak değerlendirilmiştir. Jeotermal akışkanın sıcaklığı 60.05-88.8 °C, elektriksel iletkenlik değeri 1596-2271 μS/cm arasında ölçülmüştür. Jeotermalakışkanın katyon dizilimi Na+> K+> Ca2+> Mg2+, anyon dizilimi ise SO42-> HCO3-> Clşeklinde olup, akışkanın su tipi Na-SO4-HCO3'tır. As (684 μg/l), B (4203 μg/l), Fe (612 μg/l)ve Mn (531 μg/l) konsantrasyonu limit değerlerin üzerindedir. Su Kirliliği KontrolüYönetmeliği'ne göre jeotermal akışkan Na+, SO42-, As ve B konsantrasyonuna göre çokkirlenmiş sular sınıfında yer almaktadır.
The influence of olive leaf extract on the activity of antioxidant enzymes in the Hemolymph of Galleria mellonella (L.) (Lepidoptera: Pyralidae)
(Tokat Gaziosmanpasa University, 2023) Kaya, Serhat; Türkdoğan, Seranay; Söküt Açar, Tuğba
The present study has aimed to examine the effects of olive leaf extract on the antioxidant enzyme activity of the invertebrate model organism Galleria mellonella L. (Lepidoptera: Pyralidae) hemolymph. For this purpose, after the olive leaves collected from Çanakkale province were dried at room temperature, the ethanolic extract was obtained with a soxhlet. After the ethanol was removed from the obtained extract in the rotary evaporator, the obtained dry matter was prepared in the phosphate buffer salt, 72 mg mL-1 as the highest dose and the lowest 0.010 mg mL-1 and injected into the larvae reared for the experiment. Hemolymph was collected at 4, 8, 12, 16, and 24-hours post-injection. Catalase (CAT) and superoxide dismutase (SOD) enzyme activities and malondialdehyde (MDA) formation levels were determined spectrophotometrically in the collected hemolymph. According to the results obtained, olive leaf extract decreased CAT and SOD activity in G. mellonella hemolymph at certain doses and increased the amount of MDA. For all enzymes, group, dose, and group-dose interactions were statistically significant. In addition, as a result of the examination made in terms of time, it was seen that the measurement of enzyme activities at the 24th hour was significant. The results of our study show that the antioxidant capacity of the olive leaf cannot be transferred to other living things and even reduces the antioxidant capacity of other organisms.
Kibria-Lukman Estimator for General Linear Regression Model with AR(2) Errors: A Comparative Study with Monte Carlo Simulation
(2022) Söküt Açar, Tuğba
The sensitivity of the least-squares estimation in a regression model is impacted by multicollinearity and autocorrelation problems. To deal with the multicollinearity, Ridge, Liu, and Ridge-type biased estimators have been presented in the statistical literature. The recently proposed Kibria-Lukman estimator is one of the Ridge-type estimators. The literature has compared the Kibria-Lukman estimator with the others using the mean square error criterion for the linear regression model. It was achieved in a study conducted on the Kibria-Lukman estimator's performance under the first-order autoregressive erroneous autocorrelation. When there is an autocorrelation problem with the second-order, evaluating the performance of the Kibria-Lukman estimator according to the mean square error criterion makes this paper original. The scalar mean square error of the Kibria-Lukman estimator under the second-order autoregressive error structure was evaluated using a Monte Carlo simulation and two real examples, and compared with the Generalized Least-squares, Ridge, and Liu estimators. The findings revealed that when the variance of the model was small, the mean square error of the Kibria-Lukman estimator gave very close values with the popular biased estimators. As the model variance grew, Kibria-Lukman did not give fairly similar values with popular biased estimators as in the model with small variance. However, according to the mean square error criterion the Kibria-Lukman estimator outperformed the Generalized Least-Squares estimator in all possible cases.
Identification of Leverage Points in Principal Component Regression and r-k Class Estimators with AR(1) Error Structure
(Çanakkale Onsekiz Mart University, 2020) Söküt Açar, Tuğba
The determination of leverage observations have been frequently investigated through ordinary least squares and some biased estimators proposed under the multicollinearity problem in the linear regression models. Recently, the identification of leverage and influential observations have been also popular on the general linear regression models with correlated error structure. This paper proposes a new projection matrix and a new quasi projection matrix to determination of leverage observations for principal component regression and r-k class esti mators, respectively, in general linear regression model with first-order autoregressive error structure. Some useful properties of these matrices are presented. Leverage observations obtained by generalized least squares and ridge regression estimators available in the literature have been compared with proposed principal component regression and r-k class estimators over a simulation study and a numerical example. In the literature, the first leverage is considered separately due to the first-order autoregressive error structure. Therefore, the behaviours of first lever ages obtained by principal component regression and r-k class estimators has been also investigated according to the autocorrelation coefficient and biasing parameter through applications. The results showed that the leverage of the first observation obtained by principal component regression and r-k estimators is smaller than that obtained by generalized least squares and ridge regression estimators. In addition, as the autocorrelation coefficient goes to -1, the leverage of the first transformed observation decreases for PCR and r-k class estimators, while its increases while the autocorrelation coefficient goes to 1.
Mother-Hatchling Isotopic Relationship in Green Turtle: Isotopic Niche-based Modelling
(Biodiversity Research Center, Acad Sinica, 2024) Yalçın Özdilek, Şükran; Sönmez, Bektaş; Mestav, Burcu
The isotopic discrimination between a mother and her hatchlings has been modelled in various vertebrates, including sea turtles. In addition to the linear relation between δ13C and δ15N isotope values of the mother – hatchling couple, there is missing data on the combined effects of both isotopes, which represent ecological niches of a species. The stable δ13C and δ15N isotope signatures of live hatchlings and their mother's epidermis tissues were used in green turtles. The samples were taken from three main breeding beaches, Akyatan, Sugözü and Samandağ in Türkiye during the 2020 nesting season. δ15N and δ13C values of hatchlings were not significantly different from those of mothers. Significant relationships were found between hatchlings and their mothers in terms of δ13C value. Furthermore, when the hatchling isotopic niche size is known, the condition probabilities of estimating the mother's isotopic niche size were 85.16% and 92.88% with the 0.95 and 0.99 alpha levels. In addition to showing a linear relationship between hatchlings and their mother's single isotopic composition, the current study offers a novel insight that proposes a niche overlap concept using two isotopes to comprehend the mother-hatchling relationship of green turtles living in the eastern Mediterranean.
Destinasyon İmaj Başarımının Ziyaretçi Memnuniyeti ve Davranışı Üzerine Etkisi: Çanakkale Örneği
(Çanakkale Onsekiz Mart Üniversitesi, 2023) Balyemez Yılmaz, Menije; Mestav, Burcu
Bu çalışmanın amacı, yerli turistlerin destinasyon imajı başarımını algılama düzeylerini ve bunların ziyaretçi memnuniyeti ve davranışı üzerindeki etkisini ortaya koymayı planladık. Bu sayede destinasyon imajını pozitif olarak geliştirmek isteyen mahalli idareciler ve hedef anakütleye yönelik tutundurma faaliyeti gerçekleştirmek isteyen işletmeler için rehber olmasını amaçladık. Araştırmanın evrenini, Kasım ve Aralık 2022 tarihleri arasında Tarihi Alan’ı ziyaret eden 18 yaş ve üstü yerli ziyaretçiler oluşturmaktadır. Anket sayısı Kolayda Örnekleme yöntemi kullanılarak belirlenmiş olup anket 400 kişiye uygulanmıştır. Çalışmadan elde edilen sonuçlara göre, ankete katılanların büyük bir kısmının destinasyondan genel anlamda memnun olduğu, destinasyon imajı başarımı ile ziyaretçi memnuniyeti arasında yüksek bir ilişki olduğu saptanmıştır. Ayrıca ziyaretçi memnuniyeti ile gelecekteki ziyaretçi davranışı ve destinasyon imajı başarımı ile gelecekteki ziyaretçi davranışı arasında anlamlı ilişki, genç popülasyonun memnuniyet düzeyinin daha fazla olduğu ve destinayon imajı başarımını daha yüksek algıladığı ortaya konmuştur. Şehitliklerin temizliği ve bakımı korunması gerekenler kümesinde yer almışlardır. Bilgilendirme panoları, alandaki yön tabelaları, çöp toplama hizmeti ve çevrenin temizliği ziyaretçiler açısından başarılı bulunmasına rağmen diğer destinasyonlarla yapılan karşılaştırmada performans açısından daha düşük olarak değerlendirilmişlerdir. Bu nedenle bu alanlar üzerinde daha fazla çalışma yapılarak geliştirilmesi gereken konular olarak saptanmışlardır.
A 21-year recovery trend in green turtle nesting activity: 2002-2022
(John Wiley and Sons Inc, 2023) Sönmez, Bektaş; Mestav, Burcu; Yalçın Özdilek, Şükran
Long-term monitoring and conservation are crucial for effective protection, since sea turtles migrate cyclically every 2-3 years and lay multiple nests during a breeding season. Therefore, this study aimed to investigate the long-term trend of nesting activities of green turtles over a 21-year period (2002-2022) on Samandag beach.An early morning survey was conducted every year during both nesting and hatching seasons. The trend analyses in nesting activities were performed using Mann-Kendall and Sen's slope tests together with the augmented Dickey-Fuller stationarity test.The nest count and female abundance increased significantly, whereas clutch size, fecundity, and hatching success tended to decrease insignificantly. The percentage change between the most recent and oldest 3-year mean nest count and mean female abundance, which is performed to comply with the Red List assessment method used by the International Union for Conservation of Nature Marine Turtle Specialist Group, was 769% and 764% respectively.Samandag nesting population accounts for one-third of the Mediterranean's total nest count and one-fourth of its total female abundance. The decrease in clutch size and fecundity may be due to the recruitment of individuals with smaller body sizes or malnutrition as a result of population growth. There was an increase in the number of dead embryos and a decline in hatching success. This may be explained by density-dependent population regulation as a result of an increase in the number of nests.It may be argued that adherence to a uniform strategy and the maintenance of continuous protection through collaboration with governmental and non-governmental organizations with scientifically rigorous methodology are significant factors contributing to the high recovery.This study emphasizes the necessity of adopting a comprehensive conservation approach encompassing both foraging habitats and the significant role played by the population in Samandag in the proliferation of Mediterranean green turtles.
Evaluation of the role of enzyme activities in the silvering process of European eels (Anguilla anguilla)
(Springer, 2023) Ertürk Gürkan, Selin; Mestav, Burcu; Yalçın Özdilek, Şükran
This study was carried out to determine the relationships between various morphological parameters, body condition data, and digestive and stress enzyme activities due to silvering in European eel (Anguilla anguilla L.) collected from four locations in Turkey. The yellow and silver eels were caught from Asi River, Köyceğiz, Güllük Bay, and Bafa Lake in November–December 2020. Total body length, eye diameter, and pectoral fin length were measured as morphological parameters. In addition, body fat percentage, gonado-somatic index, and Fulton’s condition factor were calculated, and also weight was assayed as body condition data. The variations of α-amylase, lipase, trypsin, and pepsin enzymes, and superoxide dismutase, catalase, and glutathione peroxidase enzyme activities were measured. The results supported the differences of yellow and silver stage samples in terms of both morphological and physiological parameters and also showed that the digestive and stress enzyme activities of the silver and yellow stage samples were significantly different (p ≤ 0.05). We think this study will make a physiological contribution in terms of digestive and stress enzyme levels to better understand and elaborate the silvering stage of eels.
Genomic Prediction Accuracies for Growth and Carcass Traits in a Brangus Heifer Population
(MDPI, 2023) Peters, Sunday O.; Kızılkaya, Kadir; Sinecen, Mahmut; Mestav, Burcu; Thiruvenkadan, Aranganoor K.; Thomas, Milton G.
The predictive abilities and accuracies of genomic best linear unbiased prediction (GBLUP) and the Bayesian (BayesA, BayesB, BayesC and Lasso) genomic selection (GS) methods for economically important growth (birth, weaning, and yearling weights) and carcass (depth of rib fat, apercent intramuscular fat and longissimus muscle area) traits were characterized by estimating the linkage disequilibrium (LD) structure in Brangus heifers using single nucleotide polymorphisms (SNP) markers. Sharp declines in LD were observed as distance among SNP markers increased. The application of the GBLUP and the Bayesian methods to obtain the GEBV for growth and carcass traits within k-means and random clusters showed that k-means and random clustering had quite similar heritability estimates, but the Bayesian methods resulted in the lower estimates of heritability between 0.06 and 0.21 for growth and carcass traits compared with those between 0.21 and 0.35 from the GBLUP methodologies. Although the prediction ability of the GBLUP and the Bayesian methods were quite similar for growth and carcass traits, the Bayesian methods overestimated the accuracies of GEBV because of the lower estimates of heritability of growth and carcass traits. However, GBLUP resulted in accuracy of GEBV for growth and carcass traits that parallels previous reports.
Dichotomy in morphology of the same genetic lineage of green turtles
(British Herpetological Society, 2023) Sönmez, Bektaş; Mestav, Burcu; Kırbeci, Selma; Yalçın Özdilek, Şükran
Morphological studies of marine turtles are important to provide insight into changes in their developmental environment. This study aimed to determine green turtles' Chelonia mydas morphological differences within the same genetic lineage in the eastern Mediterranean MED3 management unit and to find the best conversion equations between carapace size. A total of 106 adult green turtles (curved carapace length [CCL] range 79–105 cm) were measured on the five major nesting beaches of the eastern Mediterranean during 2020 and 2021. Morphological differences were tested with PERMANOVA and the relationship among body sizes was tested by linear regression. In the eastern Mediterranean green turtles, the mean CCL and SCL (straight carapace length) were 88.5 cm and 83.5 cm, respectively. There were no statistically significant differences in any of the examined morphological characteristics of green turtles collected from five nesting beaches. In the clustering analysis, however, it was found that all the turtles fell into two distinct groups: larger (> 95.2 cm) and smaller (< 85.2 cm) turtles. As well, the conversion equations between CCL and SCL showed a high coefficient of determination (R2 = 0.938). We suggest that the conversion equations may be applied to all green turtles belonging to this population and nesting in the eastern Mediterranean.
Body size-related polymorphic foraging strategy in adult green turtles
(Elsevier Ltd, 2023) Yalçın Özdilek, Şükran; Sönmez, Bektaş; Mestav, Burcu
Green turtles (Chelonia mydas) are among the prominent migratory and conservation-needing animals. Missing data on the foraging strategy of green turtles limits effective conservation actions in the Eastern Mediterranean. We applied the stable δ13C and δ15N isotope signatures together with body size to evaluate the foraging ecology of green turtles. Five tissue samples were utilized, including keratin, epidermis, red blood cells, plasma, and whole blood. The samples were retrieved from five major breeding areas in Turkey. Significant correlations were observed between curved carapace length and the δ13C and δ15N values of the green turtles in all the breeding populations except for Alata. The bimodal distribution of δ13C and two clusters based on the δ13C and δ15N values indicated a dichotomy concerning the foraging habitats of the green turtles nesting on the Eastern Mediterranean. The body sizes of the females in the first cluster (with depleted values) were significantly larger than the second cluster (with enriched values). The total niche area of the females in the first cluster was larger than that in the second cluster, which may correspond to the generalist/specialist feeding strategy. The present study gives some novel results about the foraging ecology of green turtles in the Mediterranean. For effective and viable conservation actions, further studies are needed to identify the foraging areas of green turtles in the Mediterranean.
Ambient Air Quality and General Health Outcomes in Nigde (Turkey) between 2011 and 2017
(Tehran University of Medical Sciences, 2021) Ertan, Kara; Özdilek, Hasan Göksel; Kara, Emine Erman; Balcı, Fatih; Mestav, Burcu
Background: We aimed to provide information for health practitioners and other related people about the association between ambient air quality and adverse health outcomes in the general population of Nigde, a cen-tral Turkish city, within the context of current health data epidemiological evidence. Methods: The present study highlights the connection between health problems and time series of particulate matter (PM10) and sulphur dioxide (SO2) in Nigde, Turkey between 2011 and 2017. Significant morbidity is linked to ambient air pollution, resulting in a significant economic cost to society. Results: We found that the required funds to treat cancers and chronic obstructive pulmonary disease triggered by ambient air pollution in Nigde, exceed 9 million US dollars per year, even when only the city center is taken into account. Conclusion: As Turkish cities grow and urban population density increases, air pollution issues need to be given priority in order to protect the health of the public and support sustainable development for future generations. It is recommended that particulate matter concentration in this urban center should be significantly reduced to minimize health problems.

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