Writing is a complex skill (Dockrell et al., 2014). It requires the production of legible letters following conventional spellings, to produce words that are organized into sentences and form a coherent written text, expressing the writer's ideas (Abbott et al., 2010). Given the complexity of writing, research into the development of this ability throughout schooling is particularly relevant to understand the trajectories of learning and, based on these, to provide educational guidelines to foster writing skills. Much of extant research provides cross-sectional comparisons (Lerkkanen et al., 2004), and a few studies provide a longitudinal analysis of writing development (Berninger et al., 2010; Jagaiah et al., 2020), mainly with long gaps between the measurement points. These may help to better gauge the development of writing, given its long learning curve. This was the goal of the present study in which we examined the development of different levels of written language and their contribution to writing performance in two cohorts of Portuguese students (from Grades 4 to 7 and 6 to 9). These grades were chosen as they represent critical transitions for Portuguese students, from the first cycle of basic education (Grades 1–4) to the second one (Grades 5–6), and from the second cycle to the third cycle (Grades 7–9).

Levels of language, an analytic tool, is used to understand the complexity of oral and written language (Berninger and Garvey, 1982) based on the analysis of words, sentences, and discourse. In the word level, spelling could be defined as the ability to retrieve, assemble, and select orthographic symbols (Abbott and Berninger, 1993). Large-span cross-sectional studies found that spelling errors (an indicator of spelling skill) decreased throughout schooling, for example, from Grades 2 to 5 (Alves and Limpo, 2015), Grades 2 to 6 (Llaurado and Dockrell, 2020; Magalhães et al., 2020), and Grades 1 to 9 (Bahr et al., 2012). However, a few studies examined the type of errors produced, which can inform about the spelling difficulties of students in each language. Portuguese is a romance language with a simple syllabic structure and orthographic complexities and inconsistencies, classified as an intermediate depth orthography (Seymour et al., 2003; see more details about the Portuguese spelling system in Supplementary Material, section 1). Among several error categorization systems (Treiman et al., 2019), phonological, orthographic, and morphological assessment of spelling (POMAS) seems to be particularly useful, given its specificity of analysis and theoretical support. Grounded on the triple word form theory (Bahr et al., 2012), POMAS codes misspellings into three categories: phonological, orthographic, and morphological. Findings from POMAS revealed that from Grades 1 to 9 there was a decrease in phonological errors coupled with an increase in morphological ones, with most errors across grades being orthographic (Bahr et al., 2012). Despite not assessing morphological errors, Magalhães et al. (2020) found a similar pattern in Portuguese children from Grades 2, 4, and 6. The authors also found that stress marks errors—largely underexplored in Portuguese studies—were present equally in the assessed grades.

Writing a text also requires sentence-level abilities as children need to convert their ideas into sentences. Two key sentence-level measures are syntactic complexity and correctness (Dockrell et al., 2014). One of the most frequent measures of syntactic complexity is clause length (Jagaiah et al., 2020), which is the mean number of words per clause (Berman and Slobin, 1994). Syntactic correctness can be measured through the correctness of word sequences, defined as two contiguous syntactically and semantically acceptable writing units (Videen et al., 1982). A systematic review on Grades 1–12 concluded that syntactic complexity increased throughout schooling (Jagaiah et al., 2020). Similar evidence was found for syntactic correctness. In Grades 3–5, Dockrell et al. (2014) found that younger students produced significantly less correct word sequences than older ones. Likewise, similar findings were found by Malecki and Jewell (2003) showed that several indicators of syntactic correctness consistently increased from early elementary (Grades 1–2) to elementary grades (Grades 3–5), and from elementary to middle grades (Grades 6–8).

By serving specific communicative goals and functions, writing a text requires discourse-related knowledge concerning the structural features of each genre (Berman and Nir-sagiv, 2007; Graham et al., 2013; Dockrell et al., 2014). Like word and sentence levels, discourse-level abilities seem to increase throughout schooling, with students progressively producing texts with more and more genre-specific features. Tolchinsky (2019) found that descriptiveness (i.e., degree to which descriptive texts include the representative features of this genre) increased from Grades 1 to 4. Berman and Nir-sagiv (2007) found similar increases across grades in narrative and expository writing in older samples (Grades 4, 7, and 11 and University).

In addition to improvements in word, sentence, and discourse levels throughout schooling, research has shown that mastering these levels is important for writing performance, assessed in terms of the quality and amount of writing (Berninger, 2012). Both in primary and middle grades, a few studies found that better writing performance is predicted by (a) higher spelling skills [Grades 1–3 in Graham et al. (1997); Grades 3–6 in Abbott et al. (2010); and Grades 7–8 in Limpo et al. (2017)], (b) greater sentence-level abilities [Grades 2 and 3 in Arfé et al. (2016); Grades 3–5 and 5–7 in Beers and Nagy (2011); and Grades 7–8 in Limpo et al. (2017)]; and (c) more genre-related knowledge, including descriptive texts (Beers and Nagy, 2011; Tolchinsky, 2019). Providing stronger evidence on these links, meta-analyses showed that interventions promoting the writing levels of students improved the overall writing performance [Grades 1–6 in Graham et al. (2012) and Grades 4–12 in Graham and Perin (2007)].

Excepting one measure, we confirmed that our study revealed no distributional problems, as the absolute values of these indexes of skewness and kurtosis did not exceed 3.0 and 10.0, respectively (Kline, 2005). We found a ceiling effect in the descriptive dimension of aspectualization in the younger cohort at T2. Thus, this variable was not included in the analyses. Descriptive statistics for variables are presented on Table 1.

The first goal of the present study was to trace the developmental path of writing at word, sentence, and discourse levels. The overall results indicated that older students showed better performance at word and sentence levels than younger students, but mixed findings were found for the discourse level.

In line with prior studies (Alves and Limpo, 2015; Llaurado and Dockrell, 2020; Magalhães et al., 2020), we found a general decrease in misspellings from T1 to T2 and more misspellings in the younger than the older cohort. This finding is not surprising as students in higher grades had more years of formal instruction and therefore had more spelling knowledge and writing experience. Moreover, it has been suggested that in free writing older students may be better at selecting words they know how to spell correctly (Graham and Santangelo, 2014). A misspelling analysis revealed four noteworthy findings. First, the pattern of older students producing less misspellings than younger ones was not observed for morphological errors. This finding aligns well with the proposal of Bahr et al. (2012), who suggested that more morphological errors may occur in older students due to the use of more complex vocabulary. This may require advanced morphological (derivational) knowledge that takes time to growth (Nagy et al., 2006; Berninger et al., 2010). In the future, POMAS could be used to examine the development of misspellings beyond Grade 9. Second, phonological misspellings decreased from T1 to T2 in both cohorts, being the least frequent type of misspelling in the older cohort. This is in line with previous research (Bahr et al., 2012) and suggests that sound-based spellings are learned in the earliest phases of spelling development (Treiman and Bourassa, 2000). Third, stress mark errors were the most frequent misspelling in both timepoints and cohorts, also corroborating past findings with younger Portuguese students (Magalhães et al., 2020). Stress mark errors indicate poor lexical knowledge of stress and difficulties in prosodic and orthographic mapping (Defior et al., 2012). From an applied standpoint, this means that spelling instruction is not being entirely successful in fostering this kind of knowledge. Finally, after stress mark errors, orthographic misspellings were the most predominant errors in Grades 4, 6, and 7. This is a common finding in the field (Bahr et al., 2012; Magalhães et al., 2020; Mesquita et al., 2020), suggesting that the Portuguese orthographic complexities and inconsistencies take several years to be mastered. Future research seems to be needed for developing evidence-based practices for improving orthographic knowledge beyond primary grades.

In line with previous meta-analytic findings (Jagaiah et al., 2020), the sentence-level results showed longer clauses in the older than the younger cohort. Moreover, we found stronger time-related increases in the ability of students to craft complex sentences in the older cohort. These results indicate that improvements in syntactic complexity are more salient in older writers, as it has been proposed by scholars in the field (Hunt, 1970; Berninger et al., 2011). This may be related to teacher practices, with a sentence-related explicit instruction, including vocabulary and sentence expansion exercises only at later stages when students are more familiarized with complex genres (Connors, 2000). Our results showed a growth in syntactic correctness from Grades 4 to 7 and 6 to 9, with better performances in the older than the younger cohort. Similar findings have been reported in the field (Malecki and Jewell, 2003; Dockrell et al., 2014), indicating that the ability to craft syntactically correct sentences progresses throughout the primary and middle school. The measure of incorrect word sequences seems particularly sensitive to gauge that progress, including in older students (Espin et al., 2000; Weissenburger and Espin, 2005).

The analyses examining cohort and grade differences in the discourse level revealed three main findings. First, the younger cohort performed better than the older one in terms of anchoring. This unexpected finding may be related to the Portuguese curricula, which, in the initial grades, emphasize the use of titles and introductory sentences to contextualize the theme to readers (Buescu et al., 2015). Though vivid in late primary and early middle grades, these recommendations may be lost over the years. Second, we observed the anticipated increase from T1 to T2 in the ability of students to connect the prompt with other topics, with older students performing better than younger ones. Tolchinsky (2019) already suggested that older students tend to provide elaborated descriptions of topic-related aspects, whereas younger ones usually present lists of attributes, with a few efforts to articulate content. This progressive increase in ideas elaboration over time is common to other genres (Berman and Nir-sagiv, 2007; Beers and Nagy, 2009) and may be linked to progressive mastery of writing of students. Third, relationship was the most absent descriptive feature (either alone in Grade 4, or together with anchoring in Grades 6, 7, and 9). Clearly, ability of the students to relate and compare concepts was poor in all grades assessed, which is alarming, given the importance of this feature. Descriptive texts should establish links between sub-topics through comparisons or metaphors, which allow readers to form a picture in their minds (Adam, 2001). More research is needed to understand which factors underlie this poor performance and which strategies may be used to foster it.

The second goal of this study was to examine the contribution of word, sentence, and discourse levels to writing performance. However, considering the participants/predictors ratio, some caution is needed when interpreting these findings, which should be replicated in future studies with larger samples. Concerning word-level predictors, we found that more stress mark errors were associated with poorer texts at T1. This finding aligns with those of Magalhães et al. (2020) showing that in Grade 4, stress mark errors were reliable predictors of text quality. Regarding sentence-level predictors, we found the overall contribution of syntactic complexity to the amount and quality of students writing, confirming the importance of producing complex and good sentences to perform well in writing (Beers and Nagy, 2011; Limpo et al., 2017). Interestingly, the contribution of word- and sentence-level predictors to writing performance was only observed in the younger cohort, indicating that once students master a writing level, its role in writing performance diminishes (Graham, 2006). A striking finding involving both word- and sentence-level predictors at T2 in the younger cohort was that more stress mark errors, and more incorrect word sequences were associated with longer texts. Although this finding may be an artifact of the current study, it may also hint that by ignoring some aspects of writing, such as word stress and sentence correctness, students may be able to write more. Additional research is needed to replicate and explore these results.

Discourse-level variables were the most salient predictors of writing performance in both cohorts, confirming that the amount and quality of the writing of students is heavily dependent on their ability to follow genre-specific structures (Graham, 2006). This means that a powerful way to increase writing performance in a given genre is to improve student's knowledge about its underlying. Previous meta-analyses on the best methods to develop writing are in line with this conclusion (Graham and Perin, 2007; Graham et al., 2012). It should, however, be noted that our findings showed that not all descriptive categories contributed equally to writing performance. The more relevant category seems to be subthematization, that is, the ability of students to elaborate on the content. Experimental research is needed to examine the degree to which teaching of each of the descriptive categories results in better writing performance.

In sum, this study showed the overall growth in word and sentence levels and an increase in the discourse level only for sub-thematization. Moreover, whereas word and sentence level predicted writing performance only in the younger cohort, the discourse level was a relevant predictor in both cohorts. By helping us to understand the long-term curve of writing development, these findings provide hints for researchers to develop evidence-based practices tailored to the writing needs of students.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Ethical review and approval was not required for the study on human participants in accordance with the local legislation and institutional requirements. Written informed consent from the participants' legal guardian/next of kin was not required to participate in this study in accordance with the national legislation and the institutional requirements.

LP was responsible for initial study design and data collection. LC coordinated data coding and the manuscript preparation. TL analyzed the data. All authors contributed to the design and conceptualization of the study, literature review, discussion, and wrote and reviewed the manuscript and approved its final version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.