Research At COMSATS

Electrical Power and Control Systems (EPCS) Research Group

Dr. Laiq Khan

	Designation:	Professor
	Email:	laiq@cuiatd.edu.pk
	Phone No. :	N/A
	Role:	Faculty
	Research Interests:	Power System Dynamics Stability and Control FACTS and HVDC Hybrid Power System Smart Grid Intelligent Control (Neural Networks and Fuzzy Logic) Adaptive Control Predictive Control Fault Diagnosis and Fault Tolerant Control

Publications/Papers

2021

Journal

L.Khan,S.Mumtaz,S.Ahmed "A Finite-Time Robust Distributed Cooperative Secondary Control Protocol for Droop-Based Islanded AC Microgrids" , Energies, 19 May 2021 ,Volume 14 ,Issue 10 ,Page 2936 , 2021

L.Khan,S.Ahmed,S.Mumtaz "Legendre-Wavelet embedded NeuroFuzzy feedback linearization based control scheme for PHEVs charging station in a microgrid" , Turkish Journal of Electrical Engineering and Computer Sciences, Apr 2021 , 2021

L.Khan,S.Ahmed,S.Mumtaz "RBF neural network based backstepping terminal sliding mode MPPT control technique for PV system" , Plos one, Apr 2021 ,Volume 16 ,Issue 4 ,Page e0249705 , 2021

Muhammad Awais ,Mohsin Jamil,L.Khan,S.Ahmed "Feedback-Linearization-Based Fuel-Cell Adaptive-Control Paradigm in a Microgrid Using a Wavelet-Entrenched NeuroFuzzy Framework" , Energies ,Volume 14 ,Issue 7 ,Page 1850 , 2021

Conference

L.Khan,S.Ahmed,S.Mumtaz "NeuroFuzzy Full-Recurrent Hybrid B-Spline Wavelet Based Feedback Linearization Control for PMSG-WECS in a Grid-connected Hybrid Power System" , In2021 International Bhurban Conference on Applied Sciences and Technologies (IBCAST) 2021 Jan 12 , 2021

2020

Journal

L.Khan,S.Ahmed,Uzair Khan "Neuro-Adaptive Backstepping Integral Sliding Mode Control Design for Nonlinear Wind Energy Conversion System" , Turkish Journal of Electrical Engineering and Computer Sciences ,Volume 2020 ,Issue 01 ,Page 1-10 , 2020

L.Khan,S.Ahmed,S.Mumtaz "Nonlinear adaptive NeuroFuzzy feedback linearization based MPPT control schemes for photovoltaic system in microgrid" , Plos one. 2020 Jun 30 ,Volume 15 ,Issue 6 ,Page e0234992. , 2020

Ameen Ullah,L.Khan,Qudrat Khan,S.Ahmed "Variable gain high order sliding mode control approaches for PMSG based variable speed wind energy conversion system" , Turkish Journal of Electrical Engineering & Computer Sciences ,Volume 28 ,Issue 5 ,Page 2997 , 2020

Conference

L.Khan,S.Mumtaz,S.Ahmed "Wavelet-Hybridized NeuroFuzzy Feedback Linearization based Control Strategy for PHEVs Charging Station in a Smart Microgrid" , In2020 International Symposium on Recent Advances in Electrical Engineering & Computer Sciences (RAEE & CS) 2020 Oct 20 , 2020

2019

Journal

Kamran Ali,L.Khan,S.Ahmed,Qudrat Khan,Shafaat Ullah,S.Mumtaz,Fazal Wahab Karam,Naghmash "Robust Integral Backstepping Based Nonlinear MPPT Control for a PV System" , Energies ,Volume 12 ,Issue 16 ,Page 3180 , 2019

Book/Chapter

S.Mumtaz,L.Khan "An Online Self Recurrent Direct Adaptive Neuro-Fuzzy Wavelet Based Control of Photovoltaic Systems" , In Solar Photovoltaic Power Plants 2019 (pp. 233-250). Springer, , 2019

2018

Journal

S.Mumtaz,S.Ahmed,L.Khan,S.Ali "Adaptive Feedback Linearization Based NeuroFuzzy Maximum Power Point Tracking for a Photovoltaic System" , Energies, 9 March 2018 ,Volume 11 ,Issue 3 ,Page 606 , 2018

Tariq Kamal,Murat Karabacak,Syed Zulqadar Hassan,Luis M. Fernández-Ramírez,Muhammad Hussnain Riaz,Muhammad Tanveer Riaz,Muhammad Abbas Khan,L.Khan "Energy Management and Switching Control of PHEV Charging Stations in a Hybrid Smart Micro-Grid System" , 22 August 2018 ,Volume 7 ,Issue 9 ,Page 156 , 2018

Conference

S.Mumtaz,L.Khan "Dynamic Modeling, Control and Power Management in a Grid-Integrated PV/SOFC/SC based Hybrid Power System" , International Conference on Engineering & Emerging Technologies-ICEET , 2018

2017

Journal

S.Ahmed,L.Khan "A self-tuning NeuroFuzzy feedback linearization-based damping control strategy for multiple HVDC links" , 2017 ,Volume 25 ,Issue 2 ,Page 913-938 , 2017

S.Mumtaz,L.Khan,S.Ahmed "Indirect adaptive soft computing based wavelet-embedded control paradigms for WT/PV/SOFC in a grid/charging station connected hybrid power system" , PLOS ONE , 2017

S.Mumtaz,S.Ahmed,L.Khan "Indirect adaptive NeuroFuzzyHermite- wavelet based control of PV in a grid-connected hybrid power system" , Turkish Journal of Electrical Engineering & Computer Sciences , 2017

S.Mumtaz,L.Khan "Adaptive control paradigm for photovoltaic and solid oxide fuel cell in a grid-integrated hybrid renewable energy system" , PLOS ONE ,Page 1-33 , 2017

S.Mumtaz,L.Khan "Neuro-Fuzzy Wavelet based adaptive MPPT algorithm for photovoltaic systems" , Energies (19961073) ,Page 1-16 , 2017

S.Riaz,L.Khan "Adaptive soft computing paradigm for a full-car active suspension system with driver biodynamic vibration damping control" , Journal of the Brazilian Society of Mechanical Sciences and Engineering , 2017

S.Ahmed,L.Khan "Performances analysis of conjugate gradient algorithms applied to the Neuro-Fuzzy Feedback linearization-based adaptive control paradigm for multiple HVDC links in AC/DC power system" , energies ,Page 1-23 , 2017

S.Mumtaz,S.Ali,S.Ahmed,L.Khan "Energy Management and Control of Plug-In Hybrid Electric Vehicle Charging Stations in a Grid-Connected Hybrid Power System" , Energies, 21 November 2017 ,Volume 10 ,Issue 11 ,Page 1923 , 2017

Conference

Bilal,L.Khan,Shahid Qamar,Muhammad Shafiq "Fuzzy-like adaptive position control of induction motor" , IEEE Connecticut Conference on Industrial Electronics, Technology & Automation, 14-15 Oct. 2016 Bridgeport, CT, USA , 2017

2016

Journal

R.Badar,L.Khan "Legendre wavelet embedded NeuroFuzzy algorithms for multiple FACTS" , International Journal of Electrical Power and Energy Systems (Elsevier) ,Volume 39 ,Issue 02 ,Page 161-165 , 2016

L.Khan,S. Qamar,U,Khan "“Adaptive PID Control Scheme for Full-Car Suspension Control”" , Journal of Chinese Institute of Engineers, (Taylors & Francis) ,Volume 39 ,Issue 02 ,Page 161-165 , 2016

S.Mumtaz,L.Khan "CONTROL AND ENERGY MANAGEMENT SCHEME FOR A PV/SC/BATTERY HYBRID RENEWABLE POWER SYSTEM" , Science International, 01 March 2016 ,Volume 28 ,Issue 2 ,Page 955-64 , 2016

Conference

S.Mumtaz,L.Khan "Energy Management and control of grid-connected wind/fuel cell/electrolyzer/battery hybrid renewable energy system" , ICISE 2016 , 2016

S.Mumtaz,L.Khan "“Fuel cell/electrolyzer/ultra-capacitor hybrid power system: Focus on integration of power control and grid synchronization" , IBCAST 2016 , 2016

S.Riaz,L.Khan "“Adaptive Soft Computing Strategy for ride quality improvement with anti-lock braking system" , IBCAST 2015 , 2016

S.Riaz,L.Khan "“Adaptive Soft Computing Algorithm for Seated pregnant subject biodynamic vibrations damping control" , IBCAST , 2016

S.Ahmed,L.Khan "Adaptive Feedback Linearization Based HVDC Damping Control Paradigm for Power System Stability Enhancement" , 18-19 October 2016 , 2016

S.Ahmed,L.Khan "Adaptive Feedback Linearization Based FACTS Damping Control Paradigm for Power System" , 7-8 April 2016 , 2016

S.Mumtaz,L.Khan "Design of Power Management and Control Strategy for Wind/SOFC/SC in a Grid-Connected HPS" , 31st IEEEP Multi-topic International Symposium 2016 , 2016

2015

Journal

L.Khan,R.Badar "Hybrid adaptive neuro-fuzzy B-spline{based SSSC damping control paradigm using online system identi cation" , Turkish Journal of Electrical Engineering & Computer Sciences ,Volume 23 ,Page 395-420 , 2015

S.Riaz,L.Khan "NeuroFuzzy Adaptive Control for Full-car Nonlinear Active Suspension with Onboard Antilock Braking System" , Arabian Journal for Science and Engineering June 2015 , 2015

Conference

S.Mumtaz,L.Khan "PV based PHEVs/EVs smart charging station facility" , 2nd International Conference on Power Generation Systems and Renewable Energy Technologies, International Islamic University, Islamabad Pakistan , 2015

S.Mumtaz,L.Khan "Performance of Grid-connected Hybrid Power System" , 2nd International Conference on Power Generation Systems and Renewable Energy Technologies, International Islamic University, Islamabad Pakistan , 2015

S.Ahmed,L.Khan "“Intelligent Feedback Linearization Based Adaptive Control Paradigm for Damping Power System Oscillations" , ICET 2015 , 2015

S.Ahmed,L.Khan "Adaptive feedback linearization control for power oscillations damping in AC/DC power system" , FIT 2015 , 2015

S.Mumtaz,L.Khan "A road to wind based PHEVs Smart Charging Station”" , FIT 2015 , 2015

Book/Chapter

L.Khan,R.Badar,S.Ali,U. Farid "Comparison of Uncertainties in Membership Function of Adaptive Lyapunov NeuroFuzzy-2 for Damping Power Oscillations" , Handbook of Research on Artificial Intelligence Techniques and Algorithms (A volume in the Advances in Computational Intelligence and Robotics (ACIR) Book Series) , 2015

2014

Journal

L.Khan,R.Badar "Power system oscillations damping using HABsW based FACTS-SSSC " , Journal of Intelligent and Fuzzy Systems , 2014

L.Khan,S. Qamar,U,Khan "Comparative Analysis of Adaptive NeuroFuzzy Control Techniques for Full Car Active Suspension System" , Arabian Journal for Science and Engineering ,Volume 39 ,Issue 3 ,Page 2045-2069 , 2014

L.Khan,S. Qamar,U,Khan "Adaptive PID Control Scheme for Full Car Suspension Control" , Journal of Chinese Institute of Engineers , 2014

R.Badar,L.Khan "Coordinated Adaptive Control of Multiple Flexible AC Transmission Systems using Multiple-input—Multiple-output Neural-fuzzy Damping Control Paradigms" , Electric Power Components and System ,Volume 42 ,Issue 8 ,Page 818-830 , 2014

L.Khan,R.Badar "Power system oscillations damping using HABsW based FACTS-SSSC" , Journal of engineering and Fuzzy Systems ,Volume 27 ,Issue 3 ,Page 1575 - 1587 , 2014

L.Khan,R.Badar "Coordinated Adaptive Control of Multiple Flexible AC Transmission Systems using Multiple-input-Multiple-output Neuro-fuzzy Damping Control Paradigms" , Electric Power Components and Systems ,Volume 42 ,Issue 8 ,Page 818-830 , 2014

2013

Journal

L.Khan,R.Badar "NeuroFuzzy Legendre based adaptive control algorithm for SSSC" , Electric Power Components and Systems 30, May ,Volume 41 ,Issue 9 ,Page 845-867 , 2013

S. Qamar,L.Khan,S.Ali "Online Adaptive NeuroFuzzy based SVC control strategy for damping low frequency oscillations in muti-machine power system" , International Journal of Energy and Power Engineering ,Volume 2 ,Issue 4 ,Page 172-183 , 2013

S. Qamar,L.Khan,S.Ali "Hybrid Adaptive recurrent NeuroFuzzy based SVC control for damping inter-area oscillations" , Middle-East Journal of Scientific Research ,Volume 16 ,Issue 4 ,Page 536-547 , 2013

L.Khan,S.Ali,S. Qamar "Adaptive B-Spline Based Neuro-Fuzzy Control for Full Car Active Suspension System" , Middle-East Journal of Scientific Research ,Volume 16 ,Issue 10 ,Page 1348-1360 , 2013

Conference

L.Khan,Usman.Khalid,S. Qamar "Level control of coupled three tank system using adaptive neurofuzzy control approach" , International Conference on Modeling and Simulation (ICOMS), Islamabad, Pakistan ,Page 314-318 , 2013

L.Khan,Usman.Khalid,S. Qamar "A nonlinear steam boiler control based on adaptive recurrent neurofuzzy strategy" , International Conference on Modeling and Simulation (ICOMS), Islamabad, Pakistan ,Page 308-313 , 2013

R.Badar,L.Khan "Indirect Adaptive NeuroFuzzy Based MIMO Control for Multi-type FACTS Controllers" , International Conference on Modeling and Simulation (ICOMS), Islamabad, Pakistan ,Page 108-114 , 2013

R.Badar,L.Khan "Adaptive NeuroFuzzy Damping Control for Power System Stability Enhancement " , IEEE international Conference on Emerging Technologies (ICET-2013), Islamabad, Pakistan , 2013

S.Ahmed,R.Badar,L.Khan "Power System Stabilty Enhancement Using Adaptive NeuroFuzzy Control for UPFC" , IEEE international Conference on Emerging Technologies (ICET-2013), Islamabad, Pakistan , 2013

S. Qamar,Usman.Khalid,L.Khan "Wavelets Based Recurrent Adaptive NeuroFuzzy Approach For Active Full Car Suspension Control" , IEEE international Conference on Emerging Technologies (ICET-2013), Islamabad, Pakistan , 2013

R.Badar,L.Khan "Online Adaptive Legendre Wavelet Embedded NeuroFuzzy Damping Control Algorithm" , 16th IEEE international multi topic conference (INMIC) 2013, University of Engineering and Technology Lahore, Pakistan, ,Page 7-12 , 2013

S. Qamar,L.Khan "Online Adaptive Full Car Active Suspension Control Using B-spline Fuzzy-Neural Network" , 2013 11th International Conference on Frontiers of Information Technology (FIT) ,Page 205 , 2013

R.Badar,L.Khan "NeuroFuzzy Based Fully Adaptive Indirect Controls for SSSC: A Comparative Analysis" , 2013 11th International Conference on Frontiers of Information Technology (FIT) ,Page 95 , 2013

Book/Chapter

L.Khan,S. Qamar "Online Adaptive Neuro-Fuzzy Based Full Car Suspension Control Strategy" , Handbook of Research on Novel Soft Computing Intelligent Algorithms: Theory and Practical Applications, 15 August ,Volume 2 ,Page 617-666 , 2013

L.Khan,R.Badar "Hybrid Adaptive NeuroFuzzy Bspline Based SSSC Damping Control Paradigm: Power System Dynamic Stability Enhancement Using Online System Identification" , Handbook of Research on Novel Soft Computing Intelligent Algorithms: Theory and Practical Applications, 15 August ,Volume 2 ,Page 787-828 , 2013

S.Mumtaz,L.Khan "Generators Maintenance Scheduling Using Music-Inspired Harmony Search Algorithm" , Meta-Heuristics Optimization Algorithms in Engineering, Business, Economics, and Finance , 2013

2012

Journal

U,Khan,L.Khan "Adaptive fuzzy control of vehicle suspension system" , Journal of university of science and technology of China,10 September ,Volume 42 ,Issue 9 ,Page 100-105 , 2012

U,Khan,L.Khan "Comparative Study and Analysis of Suspension Systems using Adaptive Fuzzy Control" , Sensors & Transducers Journal, 14, December ,Volume 17 ,Page 81-89 , 2012

L.Khan,R.Badar "Nonlinear Adaptive NeuroFuzzy Wavelet Based Damping Control Paradigm for SSSC" , Advances in Electrical and Computer Engineering (AECE), 31 August ,Volume 12 ,Issue 3 ,Page 97-104 , 2012

Conference

L.Khan,S. Qamar,U,Khan "Adaptive wavelets based fuzzy NN control for active suspension model" , Conference on Emerging trends and applications in information communication technologies,1 March ,Volume 281 ,Page 249-260 , 2012

L.Khan,S. Qamar,U,Khan "Active Suspension Control Based on Adaptive Wavelets Neuro-Fuzzy Strategy" , 15th International Multi Topic Conference (INMIC),15 December , 2012

L.Khan,U,Khan,S. Qamar "Comparative analysis of suspension systems using adaptive fuzzy control" , IEEE 4th international conference on intelligent and advanced systems (ICIAS 2012), 1 June ,Volume 1 ,Page 22-27 , 2012

U,Khan,L.Khan "Neuro-fuzzy wavelets based network for full car active suspension system" , IEEE International Conference on Emerging Technologies, 8 October ,Page 1-6 , 2012

S.Ali,R.Badar,L.Khan "Damping low frequency oscillations using online Adaptive NeuroFuzzy Type-2 based STATCOM" , Multitopic Conference (INMIC), 2012 15th International, 15 December ,Page 185-191 , 2012

S.Mumtaz,Amjad Ullah Khttak,L.Khan "Comparison of Genetic Algorithm and Harmony Search for Generator Maintenance Scheduling”, " , Mehran University Research Journal of Engineering & Technology,12 October ,Volume 31 ,Issue 4 ,Page 932-947 , 2012

L.Khan,R.Badar "Adaptive NeuroFuzzy Legendre based damping control paradigm for SSSC" , IEEE 47th International Universities’ Power Engineering Conference (UPEC), September ,Page 1-6 , 2012

L.Khan,R.Badar "Online adaptive NeuroFuzzy wavelet based SSSC control for damping power system oscillations" , IEEE International Conference on Emerging Technologies (ICET), October ,Page 1-6 , 2012

L.Khan,R.Badar "Hybrid NeuroFuzzy B-spline wavelet based SSSC control for damping power system oscillations" , The 15th International Multi-Topic Conference (INMIC), December ,Page 80-87 , 2012

Book/Chapter

R.Badar,S.Mumtaz,L.Khan "Generators Maintenance Scheduling Using Music-Inspired Harmony Search Algorithm" , Meta-Heuristics Optimization Algorithms in Engineering, Business, Economics, and Finance, 10 Octotber ,Page 448-483 , 2012

L.Khan,R.Badar,S. Qamar "Adaptive Fuzzy Wavelet NN Control Strategy for Full Car Suspension System" , Fuzzy Logic- Emerging Technologies and Applications ,Page 147-174 , 2012

2011

Journal

L.Khan,S.Mumtaz,Siddique Ali,Usman Ali "ANN based water inflow forecasting of Tarbela reservoir, Pakistan" , The Horizon journal of the institution of electrical and electronic engineers Pakistan,27 September ,Volume 73 ,Page 9-16 , 2011

L.Khan,R.Badar,S.Mumtaz "Generator maintenance scheduling using harmony search algorithm" , Journal of Engineering and Applied Sciences, 03 June ,Volume 29 ,Issue 1 ,Page 99-115 , 2011

S.Mumtaz,L.Khan "ANN based water inflow forecasting of Tarbela reservoir, Pakistan" , The Horizon journal of the institution of electrical and electronic engineers Pakistan ,Issue 722 , 2011

2010

Journal

L.Khan,S.Mumtaz,Kamran Javed "Generators Maintenance Schedule for WAPDA System using Meta-Heuristic Paradigms" , Australian Journal of Basic and Applied Sciences, 1 June ,Volume 4 ,Issue 7 ,Page 1656-1667 , 2010

S.Mumtaz,Kamran Javed,L.Khan "ANN based Short Term Load Forecasting Paradigms for WAPDA Pakistan" , Australian Journal of Basic and Applied Sciences, 20 September ,Volume 4 ,Issue 5 ,Page 932-947 , 2010

U,Khan,R.Badar,L.Khan "Soft Computing Techniques for System Identification using Matlab/Simulink" , Australian Journal of Basic and Applied Sciences,10 June ,Volume 4 ,Issue 6 ,Page 1527-1541 , 2010

L.Khan,IkramUllah,Saeed Tariq,K.L.Lo "Virtual Bees Algorithm Based Design of Damping Control System for TCSC" , Australian Journal of Basic and Applied Sciences, 5 August ,Volume 4 ,Issue 1 ,Page 1-18 , 2010

R.Badar,Sadaf Anjum,L.Khan "Standard Fuzzy Model Identification using Gradient Methods" , World Applied Sciences Journal, March ,Volume 8 ,Issue 1 ,Page 01-09 , 2010

L.Khan "Attitude Regulating Control of a rigid Spacecraft Model in actuating failure mode" , International Journal of Modeling and Simulation (ACTA PRESS) ,Volume 30 ,Issue 3 ,Page 277-288 , 2010

Sifat Shah,S.Khattak,L.Khan "Crisis of electrical energy in Pakistan and future guide line for policy makers" , Canadian Journal of Electrical and Electronic Engineering ,Volume 1 ,Issue 3 ,Page 60-67 , 2010

L.Khan "GPS assisted handoff in overlapping IPv6 WLAN with multiple CoA" , Journal of Engineering and Applied Sciences ,Volume 29 ,Issue 1 ,Page 125-129 , 2010

L.Khan "Reliable multicast implementation in JAVA" , Journal of Engineering and Applied Sciences ,Volume 29 ,Issue 2 ,Page 55-59 , 2010

S.Mumtaz,L.Khan "Generators maintenance scheduling using Harmony Search Algorithm" , Journal of Engineering and Applied Sciences ,Volume 29 ,Issue 1 ,Page 99-115 , 2010

S.Mumtaz,L.Khan "Generator maintenance scheduling using harmony search algorithm" , Journal of Engineering and Applied Sciences, June 2010 ,Volume 29 ,Issue 1 ,Page 99-115 , 2010

2009

Journal

IkramUllah,K.L.Lo,L.Khan "Hierarchical VBA based TCSC Robust damping control system design" , Australian Journal of Basic and Applied Sciences ,Volume 3 ,Issue 4 ,Page 4132-4148 , 2009

L.Khan "A novel random walk optimization based technique for robust damping control system design for TCSC" , World Applied Sciences Journal ,Volume 7 ,Issue 9 ,Page 1152-1163 , 2009

L.Khan "Implementation of GA in designing control system for paraplegic walking movement using walker" , ", World Applied Sciences Journal ,Volume 7 ,Issue 9 ,Page 1164-1175 , 2009

L.Khan "Anycast based routing in Vehicular Adhoc Networks (VANEST) USING Vanetmobisim" , World Applied Sciences Journal. ,Volume 7 ,Issue 11 ,Page 1341-1352 , 2009

Conference

L.Khan "Performance Comparison of Two Anycast based Reactive Routing Protocols for Mobile Ad hoc Networks" , 2 nd IEEE International Conference on Computer, Control & Communication (IEEE-IC4 2009). , 2009

2008

Journal

L.Khan "Robust damping control system design for TCSC using particle swarm optimization" , International Journal for Computation and Mathematics in Electrical and Electronic Engineering (COMPEL) ,Volume 27 ,Issue 3 ,Page 593-612 , 2008

L.Khan "Multiobjective PSSs Design for Damping Intera-area Oscillations in Multi-machines Power System using Hierarchical Micro GA" , Journal of Engineering and Applied Sciences ,Volume 27 ,Issue 1 ,Page 1-10 , 2008

2007

Journal

L.Khan "Automatic Weight Selection in H-infinity Loop-shaping using GA" , MUR Journal of Engineering & Technology ,Volume 26 ,Issue 2 ,Page 179-190. , 2007

Conference

L.Khan "Approximation of large scale algebraic equations using implicit restart scheme" , Proceedings of International Conference on Applied Sciences and Technology (IBCAST) ,Volume 1 ,Page 114-119. , 2007

L.Khan "Coordination through multicast messaging in a multiple-mobile agent system (MMAS" , International conference on Electrical Engineering (ICEE) ,Page 1-6. , 2007

2006

Journal

L.Khan "Hybrid GA Neuro-Fuzzy damping control system for UPFC" , International Journal for Computation and Mathematics in Electrical and Electronic Engineering (COMPEL) ,Volume 25 ,Issue 4 ,Page 841-861 , 2006