The Six Thinking Hats Technique (STHT) is a popular teaching method that aims to enhance critical thinking, creativity, and problem-solving skills among students. In recent years, there has been a growing interest in examining the effectiveness of STHT on academic achievement. Since ancient times, the most basic feature that distinguishes individuals from other living species is their ability to think. People can perform mental processes such as understanding, decisionmaking, and problem-solving. The foundation of science and technology is the thinking ability of human beings. In this age of rapid advancement in science and technology, even higher-level thinking, has become a necessity. This fact makes educational approaches based on mental skills important in the field of education (Güneş, 2012). Education is the only way for individuals to keep up with the developing and changing world. Among the techniques that reveal mental processes and thinking skills in education, STHT is at the forefront (Aslan, 2017).

STHT is a technique used in education that enables individuals to look at a subject from different perspectives and facilitate decision-making processes, as revealed by Edward De Bono (Can & Semerci, 2007; Hightower, 2019). Decision-making, which is a skill indicator, is acquired through education (Bolat, 2021). With this thinking technique, individuals can see different aspects of events and situations by going beyond the perspective they are used to. At the same time, STHT helps develop people's creative thinking, decision-making, and communication skills (De Bono, 1999). With the six thinking hats, students can produce different ideas, and they also have the opportunity to see how their peers generate different thoughts. This technique aims to develop people's multidimensional thinking skills by using logical, emotional, positive, negative, and creative thinking (Altikulaç & Akhan, 2010). Undoubtedly, creativity, critical thinking, and communication skills form the basis of this versatile thinking skill that underpins STHT. Creative thinking can be briefly defined as the mental process that helps individuals go beyond what is offered to them by using their imagination to think beyond the given constraints and obligations. On the other hand, critical thinking involves examining acquired knowledge or ideas objectively and questioning them in the light of one's own values, attitudes, and personal philosophy (Bolat, 2021).

STHT is composed of six hats, each representing a different perspective or role that students can adopt. These hats are commonly referred to by their associated colors, namely white, red, black, yellow, green, and blue. Each color symbolizes a specific viewpoint and approach to the topic at hand. Table 1 shows the meanings and characteristics of the six hats.

Table 1. Characteristics of Six Thinking Hats

The color of each hat is different, and each color represents different characteristics. The white hat symbolizes impartiality, objectivity, and the purest form of knowledge. There is no place for feelings, thoughts, comments, personal opinions, and judgments in the white hat. The red hat expresses emotions and intuition. The reflection of emotions such as happiness, sadness, and anger into thoughts belongs to the red hat. There is no room for rational thinking in the red hat. The black hat represents pessimism and judgment. Like the red hat, the black hat is the voice of reason, not emotion. It means looking at the negative side of things to think logically about why things won't work.

The yellow hat represents logical optimism. It expresses the positive aspects and benefits of events logically. The green hat is the hat of creativity. With this hat, efficiency, productivity, and generating new ideas are expressed. Finally, the blue hat means control. It deals with the use of other hats. The six hats in STHT represent six different ways of thinking. As the color of the hat worn by the students changes, their way of thinking also changes (Yavuz, 2005). One of the most important issues in STHT is that each hat represents a different character. It is essential that the student who wears the hat choose to play the role appropriate to that hat's character. The students who can play this role expand and enrich their mindsets over time (Akkılıç, 2018; Akkılıç & Koçyiğit, 2019).

Academic success can be expressed as the level of students reaching the desired goal within the scope of educational institutions. In the school environment, grading and evaluation are often used as a concrete measure of this. In other words, the grade a student gets in a course shows the level of their academic success. Diplomas and degrees obtained by students from formal education institutions represent that they have passed that level academically. All behavioral changes, except for the motor and affective development levels of learners, include academic success. In order for academic success to occur, an effective learning process must first be established (Ahmann & Glock, 1971).

Effective learning is closely related to the student's positive attitudes towards the lesson, motivation, and readiness level. It is considered necessary to use methods that will make the student active and increase their attitude and motivation towards the lesson, rather than the traditional methods in which the student is passive.

The extent to which STHT applications affect the academic success of students and whether permanent learning is provided or not is examined by educators in different course branches and at different levels. The aim of this research is to examine the studies on the effect of the Six Thinking Hats technique on students' academic achievement levels. It is aimed at preparing the study in a way that will shed light on educators who want to apply STHT. The studies will be examined for this purpose with the mixed-meta method. In the mixed-meta method, qualitative and quantitative data can be examined together. This method also provides convenience in terms of time.

The primary objective of this research is to answer the questions,

• How does STHT affect learners' academic achievement levels?
• What are the positive aspects of STHT-based applications?

The research employed a mixed-meta method to investigate the impact of STHT application on students' academic achievement levels. This method includes meta-analysis in the quantitative dimension and metathematic analysis in the qualitative dimension, allowing comprehensive analysis of both types of data. Quantitative data analysis was performed using programs such as CMA and MetaWin, while qualitative data analysis was conducted using programs such as Nvivo and Maxqda. This allowed for the data to be combined and analyzed using these programs (Batdı, 2019). The mixed-meta method can be briefly described as meta-analysis based on document analysis and meta-thematic analysis (Batdı, 2019).

4.1 Meta-Analysis

The research aims to use the meta-analysis process in the quantitative part to analyze the data. Meta-analysis is a statistical method that integrates the results of quantitative studies on the same or related subject to achieve highly generalizable results (Büyüköztürk et al., 2018). Meta-analysis is often described as the analysis of multiple analysis, as each research study can be seen as an analysis in its own right (Glass, 1976). The meta-analysis method combines the results of various studies to obtain quantitative data and reached a common judgment (Sağlam & Yuksel, 2007).

It is important to consider objectivity while conducting a meta-analysis study. To ensure objectivity, the inclusion criteria for studies to be analyzed were determined beforehand, and studies were selected according to these criteria (Oral & Çoban, 2020). Different stages were taken into account during the meta-analysis process. The first step is to identify the research problem. Then, criteria related to the subject to be included in the research are determined, and studies that meet these criteria are selected through a literature review. Finally, the data from the selected studies are collected, and the evaluation stage begins.

In this research, studies on the implementation of STHT were searched nationally and internationally through various databases, including YÖK, Google, ProQuest Dissertations & Theses Global, Web of Science, ScienceDirect, Taylor & Francis Online, EbscoHost, and Emerald Insight. The studies were selected based on certain criteria, such as examining the effect on academic achievement levels, and containing the necessary data (x, n, sd) for meta-analysis. In total, 30 sets of data were meta-analyzed in accordance with these criteria.

4.2 Meta-Thematic Analysis

The meta-thematic analysis used in the qualitative dimension of the research conducted with the mixedmeta method can be explained as the process of creating common themes and codes by combining studies containing qualitative findings. It aims to bring together studies of textual content by creating themes and codes within the scope of document review. Metathematic analysis can be thought of as the thematicization of themes (Batdı, 2019).

One of the most distinctive features of meta-thematic analysis is that the data is unprocessed and raw i.e., primary data that includes the views of the participants. In the meta-thematic analysis process, the raw data is reanalyzed, and themes and codes are extracted. The interpretation is made in this direction rather than just combining the findings (Batdı, 2019). In addition, inductive understanding is dominant in meta-thematic analysis. In other words, it is essential to reach all the information by proceeding systematically with the data obtained during the research process (Batdı, 2019). In the meta-thematic analysis dimension of this research, attempts were made to find studies that examined the effect of thinking with six hats on academic achievement and included direct statements of the participants. Ten studies were included in the research by creating themes and codes.

4.3 Data Analysis

Meta-analysis is conducted by grouping similar studies on a subject within certain criteria and combining and interpreting the quantitative findings of these studies (Dinçer, 2021). For this purpose, CMA software was used to calculate the overall effect size by reanalyzing the quantitative data in the meta-analysis section of the research. In the process of interpreting the effect size of the study, Thalheimer and Cook's (2002) level classification was taken into account. In addition, the Random Effects Model (REM) was used to analyze the studies included in the meta-analysis. This is a metaanalysis model that accepts that the actual effect sizes of all studies are different from each other, assuming that each study has a real effect in REM (Oral & Çoban, 2020).

In the meta-thematic part of the study, a document review was conducted for qualitative studies. Additionally, the qualitative data of the study was analyzed through content analysis. Document review refers to the examination and analysis of written materials that contain information about the subject being investigated (Yildirim & Şimşek, 2013; Yılmaz et al., 2017). On the other hand, content analysis is a systematic technique used to determine the existence, meanings, and relationships of certain words in a text and to make inferences about the message in the text (Büyüköztürk et al., 2018). In the meta-thematic part, some themes and codes were obtained as a result of analyzing the studies included in the research. Figure 1 shows the process of including studies in the research for meta-analysis and meta-thematic analysis.

Figure 1. PRISMA Flowchart (Process of Including Studies in the Research for Meta-Analysis and Meta-Thematic Analysis)

The research was conducted by scanning databases, resulting in 520 studies for meta-analysis and 230 studies for meta-thematic analysis. The number of studies were excluded due to duplications, i.e., publications in more than one place, was 35 studies for meta-analysis and 20 studies for meta-thematic analysis. Afterwards, 220 studies were excluded for meta-analysis and 85 studies for meta-thematic analysis due to unrelated topics. Abstracts that did not meet the inclusion criteria established in the current study, including publication between 2005 and 2022, inclusion of both experimental and control groups, application of the STHT to students in the experimental group, and provision of necessary data for analysis, resulted in the exclusion of 115 studies for meta-analysis and 45 studies for meta-thematic analysis. Finally, 70 studies were excluded from 120 metathematic analyses for meta-analysis due to exclusion criteria, including unsuitability and non-compliance, insufficient digital data, and scientific and contentrelated studies that were found to be insufficient after quality assessment. As a result of the exclusions, 30 studies for meta-analysis and 10 studies for meta-thematic analysis were included in the analysis.

The findings on the learning environments in which STHT was utilized were analyzed using a mixed-meta method approach. The first phase covers the data from the quantitative meta-analysis, or analytical phase, in two stages for the studies obtained. The second phase covers the data of qualitative content derived from participant opinions obtained through meta-thematic analysis.

5.1 Quantitative Findings

The lower and upper limits of the confidence intervals, fixed error, and effect size values of the effect size levels of STHT applications on academic achievement according to FEM and REM were calculated. Table 2 shows the metaanalysis results, including the confidence intervals, fixed error, and effect size values for the different levels of STHT applications on academic achievement according to both FEM and REM.

Table 2. Meta Analysis Results

After conducting the REM analysis, the standard error was found to be 0.165, with the upper limit of the 95% confidence interval at 1.395 and the lower limit at 0.747. The average effect size was calculated as 1.071, indicating that learner success was better with STHT-based applications compared to traditional teaching methods. The effect size obtained falls under the very Thalheimer and Cook's (2002) effect size classification. The z-test calculated to determine statistical significance yielded a value of z = 6.475, with a corresponding p-value of 0.000, indicating that the analysis is statistically significant. The homogeneity test revealed a statistical value of Q = 38.898, but since the meta-analysis included the average effect size levels of 30 data obtained from 8 studies, it can be concluded that the effect of these applications on learner success is 0.950 in the Fixed Effect Model (FEM) and 1.071 in the REM, supporting the use of STHT applications.

The funnel plot reflects the possibility of publication bias (Cooper et al., 2009). After analyzing 1937 studies on the effect of STHT based on achievement scores, it was found that the significant effect can be reduced to zero. In the context of the research, these values are numerically high. Additionally, considering the number of studies included, it was evident that a significant amount of work was required to obtain this number, and there was no publication bias effect (Cheung & Slavin, 2011), making the procedures reliable. However, upon examining the Egger test, it was noted that the double-tailed p value was not significant (p >.05). This situation can be interpreted that there is no publication bias in the analysis. Figure 2 shows the normal quantile plot.

Figure 2. Normal Quantile Plot

5.2 Qualitative Findings

The themes and codes obtained from the studies examined as a result of the meta-thematic analysis were examined. The analysis resulted in the creation of the positive aspects of the STHT theme and various codes related to this theme. Table 3 shows the positive aspects of STHT.

Table 3. Positive Aspects of STHT

Some of the codes related to the theme of positive aspects of STHT applications are providing question-answer opportunities, enabling group work, developing a speaking attitude suitable for the environment, facilitating the development of vocabulary, creating environments where feedback is provided effectively, and promoting the development of communication skills. The direct statements of the participants were used as references in creating the codes.

The reference sentences are "We benefit from group work and individual work in this lesson" (477325, p. 54) and "In the environment designed in accordance with STHT, the students sat on cushions suitable for their height. We gave the students the right to speak, directed them to ask questions, and corrected their mistakes. We provided feedback" (M-16, p. 400). When these referenced sentences are examined, it can be concluded that the use of STHT helps students to engage in group work, developed a speaking attitude appropriate to the environment, improve their communication skills, and expand their vocabulary. For teachers, it can be stated that it contributed to the creation of environments where feedback is presented effectively.

Effective communication that appeals to the affective field creates happiness. "Correspondence is easy, but speaking is more difficult. I was happy when I did the communication activity by speaking. When I answered my friends' questions, I was happy to see that I could talk" (M-16, p. 399). The statement "I prefer to teach with STHT" (477325-p. 55) directly indicates that students prefer teaching with STHT over traditional methods. Additionally, STHT applications have effects such as enabling students to produce or defend different ideas, providing a comfortable environment, promoting active participation of all students in the process, being perceived as interesting by students, enabling animations, effective use of body language, and providing the opportunity to use discussion techniques such as debate. "We debated using STHT" (M-16), "The Six Thinking Hats activity was very interesting to me. I had never heard of such a thing before" (M-16), "I found it interesting that everyone defended something different from different angles, and it was an activity that I really liked" (M-16), and "Because some of the students were excited and some of them had technical problems, they could not achieve the expected success, but in the end, all of them talked, they all tried to answer" (M-16) are the statements that form the basis of the codes. It can be concluded that STHT has positive aspects for students ability to act, use body language, use methods such as debate, produce and defend different ideas, create a comfortable classroom environment, and actively participate in the process. Furthermore, STHT attracts students' attention.

Based on a document analysis that examined the effect of STHT applications on academic achievement, a mixed-meta study was conducted. The study analyzed quantitative data from studies examining the effect of STHT on academic success, which were obtained by scanning databases, using the meta-analysis process. Additionally, a meta-thematic analysis was conducted, creating a theme of positive aspects of STHT, with 31 codes created based on this theme, and 10 studies were analyzed. A total of 30 studies were included in the metaanalysis. In this section, the findings obtained from the studies are interpreted in the context of literature, and the results are discussed. As a result of the meta-analysis of the quantitative data in this study, an effect size value of ES = 1.071 was determined. According to Thalheimer and Cook's classification, this value falls within the wide effect size range, indicating a positive effect of STHT applications on academic success. This result is supported by previous studies in the literature that also suggest a positive effect of STHT on academic achievement. For instance, it is reported that STHT is more effective than traditional teaching methods in increasing academic achievement levels. Yılmazer and Kocakaya (2018) also found that STHT th application enhances academic success among 5th grade students in an information technologies course. Similarly, Güneş and Demir (2013) concluded that STHT has a positive contribution to academic success based on their examination on academic achievement scores. In the meta-thematic analysis dimension which includes the qualitative findings of the research, the data obtained from the studies determined the effect of STHT on academic achievement show that the positive aspects of the STHT theme and various codes related to this theme were created. Some of the codes created for the theme in question are the development of expressive skills, providing question-answer opportunities, allowing individual studies, supporting students wanting to learn with STHT, ensuring effective use of body language, providing speaking skills, and creating environments where feedback is effectively presented. It is possible to say that STHT improves students' speaking and expression skills by encouraging them to look at the subject from different perspectives according to the color of the hat they wear and express their thoughts accordingly. Yılmazer and Kocakaya (2018, p.59) questioned the effect of STHT on the academic success of 5th grade students in the information technologies course in his thesis and concluded that students gain the ability to express themselves with STHT, which supports the current study. In the study of Orhan et al. (2012), where they examined the effect of STHT supported with visual content on the speaking skills of learners, they stated that the speaking studies with STHT yielded positive results. Another code obtained by students shows that they want to apply STHT in different courses, which is supported by the conclusion in Kocakaya's study in the context of teaching a lesson with STHT. Based on the codes obtained, it can be concluded that STHT improves students' speaking and expression skills, as well as their ability to use the Turkish language properly, speak with courtesy, and adapt their speaking to the context. The study conducted by Bezir and Baran (2014), which explored the contribution of STHT to language learning in the second life virtual environment, also supports the present study by indicating that STHT provided opportunities for students to answer questions and receive feedback and reinforcement, thereby increasing their interest in the learning process. As a matter of fact, Orhan (2010, p. 214), in a study examined the effect of STHT on the speaking skills of learners, emphasized that the STHT application is effective in enabling students to speak Turkish in accordance with the rules, obey the rules of courtesy while speaking, and develop a speaking style that fits the environment, according to the classical teaching method. In addition, some of the codes obtained regarding the theme of the positive aspects of the practices related to this technique include establishing a cause-and-effect relationship in the speaking process, using body language effectively while speaking, creating happiness in students, and perceiving it as interesting. Can and Semerci (2007, p. 79), in a thesis study examined effect of STHT on the th success of the 6 grade students in the social studies course and stated that teaching with STHT was different and enjoyable for the students, and this situation increased the interest and success level of the learners in the course. Orhan's (2010, p. 215) finding in the study shows that STHT has a positive effect on increasing the ability to establish cause-effect relationships in speaking and using gestures and mimics effectively in conversations according to the classical teaching method.

The application of the Six Thinking Hats technique can have a positive impact on academic achievement, especially in the development of critical thinking skills, problem-solving ability, and creativity. The findings of this research are supported by previous studies that have also suggested the effectiveness of the Six Thinking Hats technique in education. Moreover, the results also showed that the technique can be implemented in various educational settings, such as primary schools, secondary schools, and higher education. In this mixed-method study, the effect of STHT applications on academic success was examined and it was found that thinking with six hats had a positive effect on academic achievement in the meta-analysis dimension. Reaching the conclusion that it had positive aspects in the meta-thematic analysis dimension. It is suggested that the study had consistency in both quantitative and qualitative dimensions. Based on the results of the study, several suggestions were made. Teachers who intend to benefit from STHT should be trained on the subject in order to provide effective education in accordance with their purpose and become competent. Additionally, teachers who will apply STHT should inform students about the technique beforehand. During the application process of this technique, care should be taken to ensure that students choose the hats according to their preferences to avoid boredom and enable them to express themselves comfortably. Educators who intend to apply the technique should manage the course time effectively. The study showed that implementing the relevant technique had a positive effect on the academic achievement levels of the learners. Therefore, teachers can use the Six Thinking Hats Technique in different grade levels and courses. Researchers can also investigate the effect of STHT practices on student attitudes. Class size selection should be based on the technique, as implementing the technique may become more challenging with larger classes, hindering its effectiveness. If STHT is to be applied, it is recommended to prepare the hats together with the students to make the process more enjoyable and to ensure their engagement with the technique.