Thematic Structure in Students' Writings: Implications on their Ideas Organization and Development

Thematic structure is an important concept in organizing ideas in academic writing. Although numerous studies have focused on investigating how it is realized in students' writing, only a few focus on investigating thematic structure in students' text across different proficiency levels. This article is based on a study analyzing idea organization and development in 24 high and low-scoring essays through Systemic Functional Linguistics (SFL). It was a qualitative content analysis with complementary quantitative data. The results show significant differences in how ideas are organized and developed in high and low-scoring essays. Even though all types of Themes (topical, interpersonal, textual Theme) were observed in both groups, the high-scoring essays elaborated more themes than the low ones indicating that the high-scoring essays comprised more clauses. As for Thematic Progression, all the patterns were present in the essays. The Linear Theme pattern was primarily applied in high-scoring essays, while the Constant Theme pattern was applied frequently in the low-scoring ones. The Split Rheme pattern was less observed in all the students' essays. These findings indicate that the high achievers provided more expounded ideas through linear theme patterns, whereas their counterparts tended to focus on a subject reflected through Theme reiteration. Thus, this implies that proficiency characterizes writers in organizing and developing ideas in writing as reflected through thematic Choice and Progression.


INTRODUCTION
Idea organization and development have long played a significant role in writing quality text as they determine how ideas are understood (Ruegg & Sugiyama, 2013;Ping, 2007). They are directly related to text presentation for their role as coherence and cohesion properties to make ideas connected (Halliday & Hasan, 1976). Their importance can also be seen from the fact that coherence and cohesion always serve as the main criteria in writing assessments, such as IELTS and SAT analytic rating scales (Lumley, 2002). Therefore, every writer should be aware of any property and strategy of idea organization and development for successful writing.
From Hallidayan Systemic Functional Linguistics (SFL) perspective, how language is structured in communication becomes one of the main concerns (Bloor & Bloor, 2004). It is related to the textual metafunction of language (Halliday & Matthiessen, 2014), which connects what is being said or spoken to other texts and contexts. It includes the function of language to shape and structure itself (Bloor & Bloor, 2004). When expressing an idea, whether spoken or written, an author instinctively tries to organize it smoothly to facilitate readers in understanding what the expression means. This information structuring process then forms a Thematic Structure.
It has two main constituents in a clause, Theme and Rheme. Both elements work simultaneously to the stream of information in the discourse. Halliday & Matthiessen (2014) stated that the Theme is "the point of departure of the message or point of departure" (p.89). The clause's initial element in English is the starting point for developing the messages (Wiratno, 2018). As an expansion of Theme, Rheme is "the rest of the message" and always comes after Theme (Halliday & Matthiessen, 2014, p.89). Both Theme and Rheme contribute significantly to the readability and appearance of text and everyday communication and influence how a message is interpreted and understood clearly (Matthiessen & Halliday, 1997).
In a communication process, whether speaking or writing, the Theme and Rheme of a particular clause have connections schematically to the Theme and Rheme of other preceding and following clauses, making a smooth flow/wave of information called Thematic Progression (Hawes, 2015). It is a pattern of continuously repeated use of Themes and Rhemes and their development intra-clauses. Understanding idea organization and development in the clause and how to combine all these elements to form a unity of text are essential in creating coherence and cohesion in writing. Theme and Rheme become cohesion-building bricks, and their role in thematic Progression contributes to text coherence. As they promote cohesion and coherence, their role in creating texture is indisputably significant (Hawes, 2015).
Furthermore, thematic structure contributes significantly to text development (Halliday & Matthiessen, 2004). Its concept of Theme-Rheme and Thematic Progression is essential in understanding how texts are created and interpreted (Seyed & Mohsen, 2012). The thematic structure also demonstrates information development in the text. Moreover, its analysis can significantly diagnose a writer's difficulties in organizing ideas (Wang, 2007). For this reason, numerous studies on thematic structure have investigated how successfully writers organize and present their ideas in particular text types and what difficulties they face.
Most of the recent studies have fruitfully investigated thematic structure in descriptive ways. However, there is no detailed investigation on Thematic structure in EFL student essays according to proficiency levels. As stated by Jing (2014), proficiency level is one of the influencing factors characterizing students in their thematic Choice and Progression. This study would significantly reveal how students' thematic Choices and Progression are associated with their proficiency levels. Therefore, this study aims at analyzing thematic Choice and Progression in student writing based on proficiency variables which were done by comparing the thematic Choice and Progression in high and low-scoring essays. The two main questions to be answered in this study are 1) what are thematic choices made by the high and low achievers in their essays? 2) what are the thematic progression patterns found in both the high and low achievers' essays?

Thematic Structure: Theme and Rheme
The thematic structure is a linguistic concept in SFL as a part of textual metafunction, which has two constituents, Theme and Rheme. The Theme is a clause starting point that serves as a point of departure for a message locating and orienting a clause within its context (Halliday & Matthiessen, 2004, p.64;Bloor & Bloor, 2004, p.71). It contains familiar information already mentioned somewhere in the text or context (Eggins, 2004, p.299). Meanwhile, Rheme is the rest of the clause, in which Theme is developed. Everything which is not Theme is Rheme, which always follows the Theme. These two elements play a major role in organizing message and enabling it to be communicated and understood clearly (Halliday & Matthiessen, 2014). The hearer or reader's interpretation of everything coming next in the discourse is influenced by whatever is chosen as the Theme.

Theme Types
In detail, there are three types of Themes in English, as stated by Halliday and Matthiessen (2004). The Topical, Interpersonal, and Textual Themes reflect the three language meanings. First, the Topical Theme relates to language ideational meaning. It mainly represents the focus or topic of the clause (Bloor & Bloor, 2004, p.79-81). Whatever comes as a clause-initial element to which Transitivity function can be assigned is categorized into topical Theme. This constituent includes participants, process, and circumstance (Butt et al., 2001, p.136;Gerot & Wignell, 1995, p.104;Halliday & Matthiessen, 2004, p.73). Among these three, the most typical (unmarked) Theme of everyday discourse is participant (subject in traditional grammar terms), while the other two are atypical (marked).
Second, the Interpersonal Theme relates to language's interpersonal meaning. Eggins (2004) explains that a constituent to which a Mood label occurs at the beginning of the clause is called an Interpersonal Theme, which is usually placed before the Topical Theme. It includes Vocative, Modal Adjuncts, Finite Verbal Operators, and mental clauses, which express the speaker or writer's opinion or belief. Third, Textual Theme includes any category of textual elements, which do not express any interpersonal or experiential meaning. It is a cohesive tie relating clause to its context (Eggins, 2004, p.305). It includes Continuative, Structural Conjunction, and Conjunctive adjuncts (Halliday & Matthiessen, 2004).

Thematic Progression (TP)
The relationship of the Theme of a clause to other Themes of preceding or following clauses creates Thematic Progression. It is "the choice and ordering of utterance themes, their mutual concatenation, and hierarchy, as well as their relationship to the hyperthemes of the superior text units (such as paragraph, chapter...) to the whole of the text and the situation" (Danes, 1974, p. 114). This structure and Progression create a method of development and contribute to cohesion and coherence (texture) (Halliday, 1976). Bad control of old and new information results in less readable text (Downing, 2015).

TP Patterns
Following Bloor & Bloor's (2004, p.88-93)  In the Constant Theme pattern, the writer reiterates the Theme of the previous clause as the Theme in the following clause. This pattern is commonly called Theme reiteration (Bloor & Bloor, 2004, p.88). As seen in the figure, the writer applies this pattern to focus more on specific subjects introduced in the previous clauses.
Consequently, what has been presented in Rhemes needs to be explored and developed in the following clauses. This pattern is highly applicable in descriptive or narrative texts.

Figure 2. The Linear Theme Pattern
The linear Theme Pattern is commonly known as a zigzag pattern. In applying this pattern, the writer selects what has been introduced in the first Rheme as a Theme in the second clause (Bloor & Bloor, 2004, p.89). Therefore, she/he can explore any potential information provided and develop it within the text. This pattern is significant in argumentative writing.

Figure 3. The Split Rheme Pattern
In the Split Rheme pattern, a writer provides double information in the Rheme of the first clause. Then, she/he selects the two pieces of information as Themes in the second and the third clauses. This pattern is likely similar to the Linear Theme pattern. The difference is that in Linear Theme, the Theme in the third clause is not selected from the Rheme of the first but the second (Bloor & Bloor, 2004, p.89). This pattern is commonly known as Multiple-Rheme (Eggins, 2004, p.85 Cl. 5 Theme 3 Rheme 5

Figure 4. Derived Themes
The last pattern is Derived Theme, commonly present in longer texts. If in the Split Rheme the writer expands points introduced in the Rheme of the first clause, the writer, in applying the Derived Theme expands points selected as Theme in the first clause as Themes in the subsequent clauses (Bloor & Bloor, 2004, p.91). The Themes in the second, third, and fourth clauses are derived from Theme of the first clause. The constituents' relationship is mainly semantic, as no concrete grammatical relationship is observed.

Studies on TP in EFL Writing
There have been a number of studies examining the thematic structure: Theme-Rheme in EFL writing. Some researchers conducting these studies aimed at identifying types of TP in the writings either based on Danes' (1974) TP or Halliday & Matthiessen's (2014) thematic structure concept. The studies revealed that constant or reiterated Theme, linear or Zig-zag Theme, and split Rheme were the predominantly occurred in EFL writings (Alyousef, 2021: Chen, 2019Hendrawan et al., 2021;Leong et al., 2018;Njobvu & Simwinga, 2022;Pavavijarn, 2022;Yunita, 2018). For instance, Leong (2018) examined TP in 30 research articles and found that linear theme patterns were mostly applied by the writers in the article introduction sections. Alyousef (2019) analyzed TP patterns in 119 research article abstracts. His findings showed that most of the patterns used are the combination of constant and linear Theme patterns. They implied that specific use of TP pattern correlated with particular writing genres.
Besides identifying TP realized in the student writings, other researchers conducting these studies also examined the influence of TP on writing coherence and cohesion (Hendrawan et al., 2021;Njobvu & Simwinga, 2022;Pavavijarn, 2022). Hendrawan et al. (2021) analyzed the TP realized in six final reports of academy of secretary students. They reported that linear theme patterns were mostly employed by the students in maintaining coherence. Njobvu and Simwinga (2022) examined the relationship between TP patterns and discourse coherence. Analyzing 100 students writing pieces, they found a relationship between the two variables. Based on their study, the constant and the linear Theme patterns were mostly applied by the students. Meanwhile, Pavavijarn (2022) examined the TP in EFL writings proficiency variable. He analyzed 21 essays grouped into high-score and low-score essays. He found that there is a significant difference of the TP patterns employed by the two groups of the students. The high-score essays comprised multiple TP patterns, which increase textual coherence, while the lowscore ones lacked coherence thematic progression.

RESEARCH METHOD
This study attempted to investigate the thematic structure in students' expository essays based on Halliday and Matthiessens' (2014) framework of Thematic structure and Bloor and Bloor's (2004) Thematic Progression patterns. It was conducted through Qualitative Content Analysis (QCA) as this method has successfully facilitated linguists exploring the organizational processes in text and interpreting the data (Mayring, 2003;Bryman, 2004). Twenty-two exposition texts (writing task 2) were written by students taking the IELTS prediction test at a university language training center in Yogyakarta, Indonesia. The texts were then divided into two groups, high-scoring and low scoring. The grouping of these texts was based on the texts' scores given by the test raters. The texts scoring 5 and 5.5 were categorized low, while the texts scoring 6 and 6.5 were categorized high.
The data were collected through a test, which was conducted at the end of the course session when the students completed the course. In analyzing the data, the researcher was assisted by analysis guidance using Theme-Rheme and Thematic Progression indicators consisting of detailed and brief definitions and practical examples that the expert had validated. In analyzing the data, first, all students' writings were retyped. All clauses in each essay were identified and then separated (e.g., Clause 1, Clause 2, Clause 3). Then, the Theme and Rheme of each clause were identified and classified into a table based on its types (Textual Theme, Interpersonal Theme, unmarked and marked Topical Theme).
The Theme and Rheme occurrences were then counted. Once the analysis was done, the researcher identified and mapped each text's thematic progression patterns (constant, linear, split, and derived progressions). Then, the occurrences were counted. The occurrences of Theme and Rheme summarizing Thematic Choice and Thematic Progression patterns in both the high and low-scoring essays were compared, analyzed, and interpreted to conclude finally. To maintain the validity and reliability of this study, the researcher ensured that the analysis guidance was constructed carefully and validated by the experts. The data analysis was also checked and rechecked.

RESULTS
Two main findings are discussed briefly in this section, namely thematic Choice representing the writers' selection of a particular Theme and Rheme and Thematic Progression showing how ideas are developed within the text through Thematic Choice. First, this section will show thematic Choice by displaying the research findings on selecting particular Theme types. Second, thematic Progression will be displayed according to the patterns found in the essays. The order is set because the display of thematic Choice followed by thematic Progression will lead to a better understanding of how ideas are organized and developed in the essays.

Thematic Choice
All Theme types were present in both the high and low-scoring essays. In line with the number of clauses in the texts, the high-scoring essays contained more Themes than the low ones. Topical Themes (unmarked and marked) are higher in the low-scoring essays. Besides, the occurrence of interpersonal and textual Themes is higher in the high-scoring essays. Details of the occurrences of these Themes are shown below:  In terms of Theme Markedness, a noun phrase is the most typical unmarked Topical Theme both in high and low-scoring essays, followed by a pronoun and extended noun phrase. The occurrences of these units were slightly higher in the high-scoring essays, even though the low ones comprised higher occurrences of Topical Themes. In terms of marked Topical, dependant clauses, prepositional phrases, and adverb groups are found in both essay groups. Details on topical unmarked and marked Topical Theme choices are observed below: Besides the Topical Theme, which shows a higher number of occurrences, the textual Themes are also high in both the high and low-scoring essays. Specifically, the high-scoring essays contain more textual Themes than the low ones do. Conjunctive adjuncts are found more in the low-scoring essays, while structural conjunction and relatives are found more in the low ones. Details of occurrences are shown in the table below. The last Theme type found in the students' essays is the Interpersonal Theme. As seen in Table 3 below, the high-scoring essays contained more Interpersonal Theme than their counterparts. Specifically, mental clauses are found more in highscoring essays. Meanwhile, modal adjuncts are found more in the low ones. Besides Theme types, simple and multiple Themes were also analyzed in this study. Detail description of this finding is observed below: As displayed in the table, both simple and multiple Themes are found in the student essays. The percentage of multiple Themes in both students' essays is significantly higher than in simple Themes. Simple Themes are found more in the low-scoring essays, while multiple Themes are found more in the high ones. Meanwhile, the high-scoring essays contain more multiple Themes than the low ones. Based on Theme combination, both the high and low-scoring essays contain a high percentage of Textual-Topical Theme followed by Interpersonal-Topical and Textual-Interpersonal-Topical Themes.

Thematic Progression
Following thematic choice analysis, the second analysis in this study is Thematic Progression analysis trying to show any TP patterns found in the students'' essays, including The Constant Theme, The Linear Theme, The Split Rheme, Derived Theme. Detailed findings of this analysis are shown in the following table: From the above table, all TP patterns were present in the students' essays, except Derived Theme, which is not found in the low-scoring ones. More specifically, the commonly used TP patterns in the students' essays are The Constant Theme and The Linear Theme, while the Split and the Derived Theme are less common. Several various TP patterns are found in the high-scoring essays. The top achievers employed more of the Linear Theme than The Constant Theme. Besides, they rarely used the Split Rheme and Derived Theme. On the other hand, the low achievers used the Constant Theme more than the Linear Theme. They also rarely used the Split Rheme.

DISCUSSION
This study has revealed two main findings, Thematic Choice represented by Theme selections and Thematic Progression portrayed through TP patterns. Both are useful in showing how ideas are organized and developed in academic writing. This section will briefly discuss these findings, divided into two sub-sections.

Thematic Choice
Findings have revealed that all Theme types functioning to share experiences, maintain relationships, and organize themselves are present in the students' essays and unconditionally contribute to the message construction. Explicitly, the high-scoring essays encompass more Themes than the low ones do, as the high achievers employed more clauses in their texts. It is not surprising since the utilization of various Theme types correlates to writers' proficiency level. Ping (2014) argues that good essays consist of more Themes than bad ones do. This claim is supported by Choung (2017), stating that writers' Thematic choices are influenced by their proficiency level.
Topical Theme, divided into unmarked and marked Topical, is the most frequently used Theme found in the students' essays, followed by Textual Theme and interpersonal Theme. Based on its type, the Unmarked Topical Theme, constituted by subjects dominated by a noun phrase and pronoun, is mostly applied by the students. It is comparable to studies conducted by Bangga (2013), Februana (2014), Jalilifar (20092010), Safitra (2013), Ping (2007), Thalib (2014), Liu & Tucker (2015), Marfuaty andWahyudi (2015), Sianipar (2015), Narita (2016), and Jing (2015) which found that subject is typical unmarked Topical Theme in students' writings. Meanwhile, only very limited numbers of marked Topical Themes constituted by dependent clauses, prepositional phrases, and adverbial groups existed as thematic heads observed the essays. This case is observed in both the high and low-scoring essays.
This high and low-frequency use of unmarked and marked Topical Themes indicates students' tendency to choose simple structures (noun phrase -verb phrase pattern) in their writings. By following common and normal sentence structures in English, they tried to ensure that their sentences were correct and avoid grammatical mistakes (Jalilifar, 2010a;Ebrahimi & Ebrahimi, 2012). The testing situation also influences the tendency to be more concerned about grammatical aspects. In this case, to finish their writing for the IELTS test, the students should finish writing at least 250 words in 45 minutes without revision. Curry (2004) as cited in Khuder & Harwood (2015) argues that writing products are truly influenced by test situations in which students focus more on grammatical and lexical aspects rather than the substance of their writings (ideas development and order).
Following the Topical Theme, the Textual Theme constituted by structural conjunctions, conjunctive adjuncts, and relatives is the second most applied Theme in the students' essays. Structural conjunctions maintain semantic and grammatical relationships and link ideas within clauses or clause complexes, while conjunctive adjuncts connect ideas inter clauses and only maintain a semantic relationship (Bloor & Bloor, 2004;Eggins, 2004;Halliday, 1994, p.49-50). The highfrequency use of this Theme plays a vital role in building cohesion in students' writing and thus impacts writing quality (McCabe, 1999;Lee, 2000ac cited in Mellos, 2011). Based on the frequency level, the high scorers employed more Textual Themes than the low ones. It implies that the high scorers have been able to utilize cohesive ties in their writings. Mellows (2011) observed that Textual Theme was found more in the high-scoring essays than in the low ones. The high achievers tended to employ more conjunctive adjuncts, while the low ones use structural conjunctions as the thematic head. This utilization is correlated with their proficiency levels (Jalilifar, 2010b;North, 2005). Furthermore, the enormous use of structural conjunctions as Textual Theme in the students' essays correlates to the massive use of multiple Theme patterns. It was observed that the textual-topical Theme pattern is the most applied pattern, indicating that the students have been able to write complex clauses in their essays, which is also correlated to their proficiency levels. As stated previously, the high achievers employed more multiple Theme patterns than the low achievers, who mostly applied the simple Theme pattern. It is supported by Jalilifar (2010a), claiming that once the proficiency level increase, the number of simple Themes decreases while multiple Themes increase.
The least applied Theme in the students' essays is the Interpersonal Theme constituted by modal adjuncts and mental clauses. This Theme reflects writers' comments and assessments or attitudes towards the statements (Eggins, 1997: 2;Bloor & Bloor, 2004). The Choice of Interpersonal Theme cannot be separated from the nature of exposition text as the main goal of expository text is to argue through some viewpoints of issue and state judgments (Knap & Watkins, 2005;Schleppegrell, 2004). Some common forms of the Interpersonal Theme are I believe that …, It is true that, and Most people believe that which reflects the writers' judgment. The limited use of the Interpersonal Theme is related to the nature of the text. Ping (2007), Sianipar (2015), and Undayasari & Saleh's (2015) found that this Theme occurred in exposition texts, while its occurrence is rarely observed in scientific books (Jalilifar, 2009) and English news texts (Liu & Tucker, 2015) and advertorials (Thalib, 2014). In other words, the occurrence of interpersonal Theme is determined by the nature of the texts. This analysis has shown that the two groups of students truly differed in their thematic Choice, particularly in writing through arguments. The high scorers employed more Themes than the low scorers did. They were able to elaborate more ideas by providing more Topical Themes, maintaining a relationship with readers, and cohesion through the Interpersonal and textual Themes. Thus, this finding supports the claim that proficiency levels influence students' thematic Choices. In order to find out how students develop and organize their ideas through Theme and Rheme selection, a Thematic progression analysis has been done. The following section discusses the findings of this TP analysis.

Thematic Progression
The previous section discussed the thematic choices in students' expository essays, namely Topical, Textual, and Interpersonal Themes. In this section, the patterns formed by those interrelated Themes among clauses are then called Thematic Progression, which will be discussed. Four patterns of Thematic Progression investigated in this study, namely Constant Theme, Linear Theme, Split Rheme, and Derived Themes are present in the students' essays. Of these patterns, Linear Theme and Constant Theme are the most used pattern by the students. Meanwhile, Split Rheme and Derived Theme are rarely observed.
The students mostly employed linear and constant Theme patterns because they are simple writing strategies applied by the students, particularly to maintain cohesion. Danes (1974as cited in McCabe, 1999 argues that Linear Theme as the most typical of Thematic patterns contributes to the text's cohesion by introducing new information in Rheme. It also provides cumulative development to the text (Eggins, 2004). Meanwhile, the Constant Theme contributes to the text's cohesion through the ongoing use of the introduced Theme, which creates a stagnant sensation and static style (Enkvist, 1974as cited in Jing, 2015Eggins, 1994). As it focuses on a specific subject, it is a technique of simple description and explanation (Abed, 2010).
The frequent use of the Linear Theme indicates that the students maintained relationships implying cause and effects, developed ideas, and improved cohesion between sentences in paragraphs (McCabe, 1999). This pattern is highly required in academic writing, particularly argumentative, in which the students are demanded to provide more complex arguments (Eggins, 2004). On the other hand, the frequent use of the constant Theme in argumentative writing affects students' idea organization and development. McCabe & Belmonte (1998) and Jalilifar (2010b) stated that by applying this pattern, students do not deeply explore their potential ideas presented in Rhemes as they tend to focus on the specific things or participants stated in Themes. Therefore, their texts are full of lists of arguments and need more expanding ideas. However, it is reasonable as the students were given limited time to provide their ideas in the IELTS test.
Furthermore, Split Rheme and Derived Theme are the least applied by the students. In many studies, these two TP patterns were rarely observed. Thomas and Hawes (1997) as cited in Crompton (2004) claim that the application of these patterns is mostly associated with students' proficiency level rather than text genres. They stated that the Derived Theme was mostly found in advanced students' writings. Meanwhile, in this study, notably, the students were still on an intermediate level.
Comparing the two groups of students' essays, Linear Theme is mostly found in the high-scoring essays, while Constant Theme is mostly observed in the low ones. This finding indicates that the high achievers were able to provide more elaborate ideas by expanding the messages they presented in Rhemes. On the other hand, low achievers need more elaborate ideas as they tend to focus on specific subjects of their Themes. As a result, their writings tend to be monotonous. The high occurrence of Linear Theme patterns is observed to correlate with text genre. As observed, this pattern is mostly applied by the students in this study. This finding is comparable to the findings of Adawiyah (2017), Leong (2018), Pavavijarn's (2022)) studies investigating Thematic Progressions in students' argumentative texts. They found that these two TP patterns are typical of argumentative writing. The high use of these patterns is also observed in articles in journals (Jalilifar, 2010a;Ping et al., 2018) and academic texts (Jalillifar, 2009). On the other hand, Constant Theme is dominantly used in narratives (Februana, 2014;Francis, 1990;Jalilifar, 2010b;Narita, 2016;Safitra, 2013), recount texts (Noviyanty, 2015;Yunita, 2018), descriptive (McCabe & Belmonte, 1998), and news report (Liu & Tucker, 2015;Sharndarma, 2013;Varela, 2014).
All of these show that TP patterns realized in students' essays are related to the student's proficiency level. The high occurrence of the Linear pattern in the high-scoring essays indicates that they were able to elaborate more ideas as required in expository writing. Meanwhile, the high occurrence of the Constant Theme pattern in the low-scoring essays indicates that they need more ideas development as they tend to focus on a specific subject. As a consequence, they should have provided more arguments in their texts.

CONCLUSION
The findings have shown that the high and low achievers are expressively different in organizing and developing their ideas in writing through Thematic Choice and Thematic Progression pattern selections. Although all Theme types emerged in the students' essays contributing to message construction, the high achievers employed more Themes in various types than the lower ones did. The high occurrence of unmarked topical Themes indicates a high tendency to choose subjects as the starting points of the messages in the essays. This selection is an ordinary way of structuring sentences in English to avoid grammatical mistakes. Besides, the frequent use of textual Themes contributes to the connection of ideas, while the use of interpersonal Themes allows the students to maintain a relationship with readers. Regarding Textual Theme selection, the high achievers have more potential sources to maintain cohesion and coherence as their essays contain more Textual Themes than those of low achievers do. Furthermore, they also build a stronger relationship with readers by selecting more interpersonal Themes.
As for Thematic Progression, the linear Theme pattern is mostly applied by the students in their essays, followed by the Constant Theme pattern. Meanwhile, the Split Rheme and Derived Theme are rarely observed. Comparing the two groups, the high scorers applied more Linear patterns, in which they developed ideas from Rheme they introduced. Hence, they successfully provided more elaborated arguments as demanded in argumentative writing. Meanwhile, the low scorers applied more Constant Theme patterns in which they repeatedly put the specific subject or thing introduced in the previous clause.
Consequently, what has been introduced in the Rhemes needs to be successfully explored for effective argumentation. To successfully write expository text in which writers should provide and develop arguments on a given issue, the students are highly recommended to apply the Linear Theme pattern as it significantly supports the nature of this text. For that reason, Thematic structurebased writing instruction can be designed to help students in writing courses, particularly the IELTS test preparation course.
Thus, this study suggests that students should be explicitly taught the knowledge of Thematic structure (Theme-Rheme and Thematic progression) as it is significant for their writing development. Language instructors could incorporate this knowledge in their writing instruction. Further researchers may also conduct empirical research to examine the effectiveness of TP in improving student writing quality.