Effects of Problem-based English Writing Instruction on Thai Upper Secondary School Students' Critical Thinking Abilities and Argumentative Writing Skills

The objective of the study is to examine the effects of Problem based English writing instruction on Thai Upper secondary school students' critical thinking abilities and argumentative writing skill.In this study,the researcher constructed 18weeks' training in a prestigious secondary school M6 level.The instruments used in this research include lesson plans,writing assignments,instructional material and topics.Critical thinking pre- test and post test were used to measure students' critical thinking development.Scoring rubric was designed to measure students' argumentative writing development.The data were analyzed using mean,t-test,correlation through SPSS system.The results of the analyses reveal that 1) students who learn through Problem based English Writing Instruction gained significantly higher average scores on the critical thinking post test than the critical thinking pre-test at the significance level of 0.05 and the mean difference is 0.5 which is referred to large effect;2) students who learn through Problem based English Writing Instruction gained significantly higher average score on their fifth argumentative writing assignment than the average score of their first argumentative writing assignment at the significance level of 0.05,with the mean of the effect size at 0.84 which referred to large effect.Problem-based English writing instruction was proved to be an effective way in improving students' Critical thinking and argumentative writing.