When the students have no way of ‘mathematically’ solving the question posed, they can substitute another approach that is within their reach. This leads to obvious failure, as in the case of the students at the end of the 1st grade who substituted counting ‘1, 2, 3, 4’ for the estimation task. This may be due to a lack of understanding or confusion between the magnitude and order indicated by the target number. Even some successes may be underpinned by a phenomenon of under-comprehension when the latter is defined as success through partial processing of information associated with a mathematical concept (Pluvinage, 1983). For example, the most successful item for second graders (the target number 97: see Fig. 9) may have been chosen by some students on the basis of an impression of size or proximity to 100, but without considering the existence of numbers between 97 and 100, or that the third mark was not very far from 100 either. The target number 68 could have been chosen at random between the two large numbers by a majority of fourth-graders (see Fig. 11), although these students certainly did not check that an incorrect mark was (very slightly) nearer the correct answer than the incorrect answer overwhelmingly chosen.
Admittedly, in a class, and even more so in a large sample of students, there will always be one or two students who misunderstand a question. But our studies show that the same mistakes are made on a massive scale:
- At the end of the 1st grade, 326 students (out of 2803) incorrectly chose 4 in 4th position, a choice very distant from the correct choice; similarly, 465 students incorrectly chose 40 in 4th position.
- At the start of the second grade, more than 400 students (out of 2433) each time, incorrectly chose 3 in 3rd position and 40 in 4th position, rather than in a position, also incorrect, but closer to their respective correct positions.
- Finally, and most importantly, at the start of 4th grade, over 3300 students (out of 7713) incorrectly placed 485 on a mark that was not the (incorrect) mark nearest the correct mark.
As a result, the educational value of this activity of placing numbers on a number line is questionable for many students, particularly for young students, before the second year of school (see also Fuson, 2009). It is therefore surprising that children, some of whom are not yet 5 years old, are asked to place numbers on a line bounded by 1 and 15 (James-Brabham et al., 2024). Admittedly, the authors avoid 0, a ‘dangerous’ number (Seife, 2002) that can confuse young children, but do they know the number ‘fifteen’? Having tested the numerical skills of over a thousand individual kindergartners, let me relate, from memory, the following anecdote: a boy of 3 or 4, when I asked him to show me ‘three’ with his fingers, replied: “What's three”?
But this question of the relevance of the number placement activity for young children can be raised above all in relation to certain researches on Chinese children. Perhaps encouraged by numerical performance of the latte, which is often superior to that of US children, including in the area of estimation on the number line (Siegler & Mu, 2008), Xu et al. (2013) tested 3- and 4-year-old Chinese children (age range: 3.3-4.7). With a line bounded by 0 and 10, and the 8 target numbers (all one-digit number except 5), they obtained a percentage absolute error (PAE)[2] of .25 with symbolic numbers in this age group. A simple simulation then shows that if the children had provided random responses, based on a prior differentiation between small (1, 2, 3, 4) and large (6, 7, 8, 9) numbers, their performance would have been significantly better (PAE » 0.15). As completely random responses would lead to a PAE of 0.325, the result of Xu et al. is compatible with the hypothesis that half (approximately) of the children respond completely at random and that the other half respond in a restrictively random way. Their random behavior would be restricted by the small/large opposition, that is, they respond at random on the left for small numbers, and on the right for large numbers. This hypothesis implies that the responses are not derived from a logarithmic representation. In fact, Xu et al. observed that a linear representation fits their data as well as, if not better than a logarithmic representation (R2 = .88 vs. .79).
More recently Wu et al. (2024), with 70 participants aged 5 ½ years, mostly from the upper middle class of Wuhan (China), attempted to show that dragging the mark of a number from left to right to its final position on a touch screen generally facilitates performance compared to simply taping to the presumed position. The only condition where the difference in accuracy proved significant was when placing the numbers 11, 12, 13, 14, 16, 17, 18, 19 on a bounded line [0, 20]. However, the demonstration is flawed by an exchange between accuracy and speed, since the participants in the dragging condition made significantly less incorrect placements than those in the taping condition (the distance to the correct answer was on average 1.86 vs. 2.49), but also took much longer to reach their placements (9.43 s vs. 5.89 s). This difference is noteworthy because it allows participants in the dragging group to check, as they drag the mark, whether or not the experimenter's facial expression expresses satisfaction (it is not known whether the experimenter was aware of the authors' hypotheses). In addition, from the point of view of statistical methodology, we can also note that (1) the children who did not understand a preliminary question on the game with iPad/Smartphone were eliminated from the study, (2) of the children retained, the participants in the dragging group already showed, prior to the formal test, marginally superior spatial abilities to those of the participants in the taping group, p = . 066 with a two-tailed t-test (according to our calculation), (3) the significance of the difference between the dragging and taping groups is also due to the poor performance of the latter. A simulation shows, for example, that if children taped randomly in the 3rd quarter for numbers 11 to 14, and in the 4th quarter for numbers 16 to 19, their performance would be much better than that of children in the taping group, with a mean error of 1.50 versus 2.49.
In the numerous researches using a placement task, some have tempered the usefulness of the number line (Ellis et al., 2021; Huber et al., 2014; LeFevre et al., 2013; Ni, 2000). For example, James-Brabham et al. (2024), in a board game with online numbers, played with children from disadvantaged socio-economic backgrounds aged 61 months on average, report significantly lower gains on the numerical line test than on other numerical tests. More specifically, other researchers have highlighted the influence of positional numeration on number placement (Dotan & Dehaene, 2020; Nuerk et al., 2001; Patalano et al., 2023; Williams et al., 2022). The automatic activation (Zanolie & Pecher, 2014) and innateness (Núñez, 2011; Pitt et al., 2023) of the number line have also been challenged. Finally, Nuraydin et al, (2022) found no transfer of a fraction set including the number line on fraction comprehension or arithmetic.
Some teachers, who have learned from their training that mathematics is the science of rigor, may also be troubled by this task, which asks their students to place numbers approximately and, in part, on visual impressions. Furthermore, teachers generally avoid asking questions that students cannot—or cannot yet—answer. For example, in the case of 337 (see Fig. 1 for the item and Fig. 11 for the results), for which we proposed an adult solution in the previous discussion, students at the beginning of their 4th grade were unlikely to arrive at a perfectly assured answer. The reason for this avoidance by teachers is obvious. Such questions lead, if not to random answers, at least to the kind of under-comprehension described at the start of this discussion. Finally, some pedagogues, skeptical of this new placement task, may also argue that, until 1990, students were never trained in such representations and that, without their support, they calculated rather better (cf. Chabanon & Pastor, 2019; Ninnin & Pastor, 2020).
But what is perhaps most disturbing, in France at least, is to observe that the Éducation Nationale (French national education authority) uses in its assessments bounded lines systematically graded (cf. Andreu et al., 2023). We illustrate this, in the Appendix, with the placement items in the 2023 second grade national assessment. In the presentation of this assessment, it is stated that the target skill is ‘Associating a whole number with a position’ and, in the resource sheet for student support, reference is made to estimation games on the Arithm'école ACE site (cf. ev22-ce1-maths-nombres-calculs-representer-ligne-numerique_0). Unfortunately, the graduations do not encourage placement by estimation. Worse still, they make it, if not impossible, at least more risky than counting. So we can see that placement by estimation on the number line can be circumvented and would even be counter-productive in second grade. One might think that, in later grades, we would end up with placements requiring real estimation, that is, where the graduation of the number line no longer enables a choice to be made by simple or complex counting. But this is not the case. At the beginning of fourth grade, the September 2023 assessment continues to evaluate the skill of ‘Placing a number on a graduated line’ (Bourgeois et al., 2024).
This inconsistency, resulting from a misunderstanding between cognitive psychologists and mathematics educators, may have led Vignali (2023) to try to find logarithmic versus linear representations of numbers in students with learning difficulties. However, as figure 12 shows, the logarithmic nature of the representations obtained by Vignali is doubtful. In fact, in her only illustration of a logarithmic representation, the numbers are compressed between 3 and 7 rather than between 7 and 9. Vignali's approach seems to ignore the fact that a model is not reality, and that a model may not be followed by anyone, mainly because of intra-individual variability (cf. Roth et al., 2024). In addition, a pragmatic explanation, probably closer to reality, is that the students (working in groups) who had spaced the first numbers too far apart (in particular 1 and 0), narrowed the following numbers from 3 to 7 when they realized this, and then used the whole space from 7 to 10 to space the last numbers evenly.
When the aim is to assess number knowledge rather than number representation, particularly cerebral representation (e.g., Berteletti et al., 2015; Cattaneo et al., 2009; Lavidor et al., 2004; Liu et al., 2019), the placement task appears to be very rich. To be successful, it requires estimation, number-measure comparisons, reasoning (Ruiz et al., 2024) and the notion of proportionality (Barbieri et al., 2020; Georges & Schiltz, 2021; Gunderson & Hildebrand, 2021; Smaczny et al., 2024). As a result, the number line placement task appears to be a useful measure of children's ability to assemble a collection of relevant knowledge to perform a new and complex numerically relevant task (LeFevre et al., 2013). The results of Schneider et al. (2018) also show that the numerical estimation task is a robust tool for diagnosing and predicting mathematical competence. This leads the latter authors to recommend its use in experimental and developmental learning studies.
Nevertheless, the massive student errors reported in the present study suggest that students need to be provided with aids to avoid under-comprehension. For a boundary [0, 100], for example, the reference numbers (e.g., 25, 50, 75; in connection with lengths: 1/4, 1/2, 3/4]) are certainly a major help. Indeed, (1) the developmental study by Rouder and Geary (2014) shows that improved performance on the placement task is largely the result of segmenting the line in the middle, (2) the fMRI study by Vogel et al. (2013) confirms the significant influence of landmarks (0, 50 and 100) on brain activation in the right intraparietal sulcus[3], (3) the research by Xing et al. (2021) shows that placements relative to a median landmark, inferred in addition to the two ends of the line segment, contribute strongly to the link between estimation on the number line and mathematical skills at the age of 6 to 8 years, and (4) in kindergarten, Xu and LeFevre (2016) have shown that sequential learning (e.g., what comes before and after the number 5) greatly improves performance on a version of the number line task where a reference point is presented in the middle of [0, 10] (i.e., at 5). We therefore suggest that teaching could teach students such benchmark numbers to enable them to compare the relative position of the target number when making placements on a number line (Namkung & Fuchs, 2016). With this in mind, a communication game, as implemented by Saxe et al. (2010) in 5th grade, could be effective. It should be noted, however, that for fractions these cues are of little use (Siegler & Thompson, 2014). But the usefulness of representing fractions on a number line is not obvious.
[2] The PAE formula of Xu et al. (2013, p. 355) is considered to be a simple reproduction error.
[3] Von Aster and Shalev (2007) mention that the intraparietal sulcus (IPS) is “a cortical area that has been proved to be the domicile of the mental number line” (p. 869).