The children participating in pilot test I had a mean age of 10 with slightly less girls (47.2%) compared to boys (Table 1). Based on YSQ-1 a high proportion of the children reported that they had experienced low back pain (47.2%) or neck pain (41.5%) while slightly less had experienced thoracic pain (28.3%) at least once in their life. The 18 children participating in pilot test II also had a mean age of 10 and there were 11 girls (61%). The focus of pilot test II was on the pain drawings for which reason no spinal pain data is available.
Testing of the YSQ
Pilot Test I. During the interview the children were tested for agreement of the upper and lower boundaries of the cervical, thoracic and lumbar spine between the drawings included in the questionnaire (see Additional file 1) and the interview findings. Using the YSQ-1 the children were good at identifying the borders of the cervical spine (both boundaries correctly identified: 91.8%) but the lumbar (67.4%) and thoracic spines (63.3%) were somewhat less clearly recognised (Table 2). Consequently, we added several easily recognisable bony landmarks such as an outline of scapula, lines for the 12 ribs and gluteal folds to aid the children in recognizing the regions of the spine properly. Furthermore, the interviews revealed that the children included pain from the coccyx (most often due to falls) as low back pain (YSQ-2). As a result we modified the adult drawings to not include the gluteal areas and move the lower border of the lumbar spine up to midway on the sacrum.
Agreement between the questionnaire prevalence estimates (YSQ-1) and the interviews was also tested (Table 3). This showed agreement estimates ranging between 83.7% (cervical pain today) to 97.9% (thoracic pain today). We also scrutinised the response options during the interviews. This resulted in changing the response categories of several questions as the children had difficulty understanding their meaning.
The score of the rFPS (YSQ-1) was compared to the NRS score from the interviews (Table 4). Correlations ranged from 0.67 for the cervical spine to 0.79 in the lumbar spine.
Lastly, section 5 (the child's perception and consequences of parental back trouble) and section 6 (pain drawings of additional sites of pain) were changed fundamentally during pilot test 1 (YSQ-1). Due to many unanswered questions it was decided to split question 5 into 2 sections; one relating to the mother's back pain and consequences thereof and another to the father's. Section 6 asked the child to circle other areas of pain on a pain drawing. This gave rise to a variety of very different responses ranging from no other pain to pain almost everywhere. The interviews disclosed widespread misinterpretation of the pain drawing as many children included small injuries as scraping their knee during a break or a small knock to the arm during sports. Consequently, it was decided to completely remove section 6 from the questionnaire and the YSQ-3 and onwards only consisted of 5 sections.
Pilot Test II. The drawings from YSQ-4 (pilot test I) were tested in pilot test II. The percentage of correctly identified boundaries for each region ranged from 72.2% - 100% (Table 2). Compared to pilot test I there was a significant improvement in the amount of children being able to demarcate the thoraco-lumbar junction (lower border of the thoracic spine and upper border of the lumbar spine) whereas the lower boundary of the neck was slightly less well delineated.
Cross-cultural Translation. The rFPS was translated into Danish with no notable issues. Subsequently, it was adapted to fit the structure of the full questionnaire 1) by modifying the vignette to be patient/respondent driven and not interviewer driven and 2) by adding the verbal descriptors (anchors) from the vignette above the upper and lower end pictures.
The translation of the YSQ-final revealed only a few noteworthy issues. Firstly, the semantics for each question or answer category was chosen to match the ability of the age group. Consensus was reached by scrutinising common language and conceptual equivalence. Secondly, the translators discussed how to sensibly translate question 5a (father) and 5c (mother). The direct translation of question 5a reads: "Has your father (him you live with) ever had back or neck pain?". The Danish wording was chosen as a small but important minority of children left the questions unanswered due to problems interpreting 'father' when living in a divorced family. Consensus was agreed on the wording "Has your father or stepfather ever had neck or back pain?", as it was felt that the children would intuitively relate to the person with the largest influence on their behaviour. Similar considerations were made for the mother in question 5c.