## 4 Discussion  
The present systematic review highlights that very few studies (only six) have examined the efficacy of SGs in improving literacy disorders through a rigorous experimental  
Figure 2: Result of the Cochrane risk-of-bias assessment (RoB 2) [32, 34, 35, 36, 37, 38]. design with pre- and post-tests, as well as randomized control trials. In fact, although several SGs have been developed to improve reading skills among children with learning disorders, controlled studies of efficacy are almost entirely lacking. It should be noted that all the studies included in this review were conducted in recent years. Specifically, one study was conducted in 2016, while others are dated after 2019. All the studies examined the effects of SGs in children with difficulties or deficits in literacy acquisition at a very early stage of schooling. Studies examining the efficacy of SGs in the subsequent phases of literacy acquisition (in older children) are lacking. Moreover, all the studies in this review were performed on pupils using consistent orthographies, and evidence of the effectiveness of SGs in opaque orthographies is lacking. Notably, orthographic opaqueness produces qualitative differences in reading and spelling skills [39; 40; 41; 42; 43; 44; 45; 46; 47] and, therefore, it is not certain whether the results can be generalized to opaque languages. Moreover, the studies examined have some limitations. First, the studies do not allow the characterization of the type of reading deficit as phonological or superficial impairment. Therefore, it is not possible to verify the efficacy of SGs in terms of the function of the type of deficit. It should be noted that the SG included in the review trained only sublexical grapheme-to-phoneme (and vice versa) mapping or phonological awareness, and there are no SGs to improve lexical difficulties. Therefore, it is not possible to verify whether sublexical training was performed on children who suffered from a sublexical deficit. It should be noted that surface deficit is the most frequent disorder among children speaking a language with a consistent orthography [48; 49; 50; 51] and, therefore, that SGs aiming to train lexical procedures might be more effective for these populations. It should be noted that the studies examined in the present review were conducted on languages with consistent orthographies, such as German or Finnish, and that it is possible that literacy gain after SG training would have been even more profitable following a lexical treatment. On the other hand, children mainly benefit from phonological awareness training in the earlier stages of literacy development or, in the case of more severely affected children [52; 53; 54; 55], at the age of the participants in the studies in this systematic review. Furthermore, there are no studies with evidence of the effectiveness of the use SGs to improve the comprehension of written texts, a skill that is very important to rehabilitate due to its major impact on academic and life success [56; 57; 58]. Moreover, none of the studies used reading aloud or spelling as training exercises. The unique exception was the letter painting task in the [PERSON] study (2016), although this task did not examine the spelling process, but only visuo-constructive, perceptive, and motor skills. This is a limitation of these SGs. The practice of spelling could have guaranteed a greater literacy gain [59; 60; 61]. Regarding reading aloud, the availability of an oral word form might support further orthographic learning [62]. Moreover, reading aloud allows researchers to monitor precisely what children read and would, in turn, allow the characterization of reading deficits through the examination of reading profiles [63; 64; 65; 44], as well as monitoring the reasons for eventual reading comprehension deficits.