70 ‘health status’. Here, Cronbach’s alpha was 0.890 (Bootstrap 95% CI= [0.844, 0.919]), indicating very good reliability of IROS. To evaluate the information content of the IROS instrument, we plotted the item information curves for each of the effect indicators and sum scores and the overall test information curve. The IROS scale primarily provides information from medium to high health burden levels: information in (−4, 0) = 1.14%; in (0, 4) = 96.94%. We then obtained an estimate for the health state of each individual based on his/her response pattern, i.e. the factor score z01. There were in total 168 unique response patterns in our sample. To illustrate this further: a patient with a response pattern of 0000000000000/00000000000 (none to mild for all 13 effect and 11 causal indicators) will obtain a factor score z01 of 0.41; on the contrary a patient with a response pattern of 2212221213333/ 12020000200 on the 24 items will have a factor score close to 1 (0.967). Importantly, we observed no consid- erable changes in factor scores before versus after accounting for the causal effects. Finally, based on the factor scores, we investigated the effect of several collected variables with simple linear regression. In Table 4 we present F test-derived p values for the impact of the selected covariate on the latent trait ‘health status’. Significant impact could only be proven for State at discharge (POPC), although there was a clear tendency towards worse factor scores for children that were older, had a higher ISS or after traffic injury. Table 4 Covariate analysis F test-derived p values (* if <0.05), based on simple linear regression, for the impact of selected variables on the health status’ estimate based on the factor scores from the IRT model Variable Age Gender Proxy education Proxy marital state Main diagnosis ISS Length of stay State at discharge Trauma circumstances Circumstances: traffic Circumstances: burn Diagnosis: abdomen Diagnosis: spine Diagnosis: brain Diagnosis: thorax Diagnosis: extremities Participating centre Eur J Pediatr (2011) 170:65–73 Discussion ‘Outcome’ is a multidimensional (biopsychosocial), con- textual (person, environment) and dynamic (function of the a priori health state) concept. It is inherently subjective in concept and measurement. Instruments capturing outcome should equally be multidimensional and contextual and describe health from a patient’s perspective. When evalu- ating paediatric populations, they should be relevant for children, including infants and their families. As IROS is developed out of the ICF-CY, its underlying construct is well-defined and its content validity very high [39]. To evaluate health status, one could make comparison with an a priori state. Yet most often, this has to be scored in retrospect with a high risk of recall bias and halo effects. A good alternative is to use a representative population sample or a matched-control group. Still, appreciating differences between groups is not easy. Existing differences might be obscured by, e.g. intervening diseases, develop- mental changes, adaptation and response shift [33]. We observed a significant difference between the trauma and matched-control group, especially in items that were expected to be impaired: mobility, social life, family, emotional etc. [32, 37]. Contrary to expected and available adult evidence, this was not the case for chronic pain [28]. Trauma cases had higher healthcare needs than controls, as illustrated by the number of healthcare visits. Importantly, children in the trauma group were not all that severely injured and ‘control’ children were not all 100% healthy, they just did not have a recent injury. In fact, the trauma categories 0–10y; 11–18y male; female Primary/secondary school; higher education/university Married/living together; single/divorced/Widow(er) Brain; abdomen; spine; extremities/pelvis; facial/neck; thorax; vascular; other 1–43 2–124 days POPC [42]: normal, mild, moderate, severe Traffic–sports–burn–fall–other Traffic; not traffic Burn; not burn Any AIS Abdomen 3–5; not AIS abdomen 3–5 Any AIS Spine 3–5; not AIS Spine 3–5 Any AIS Brain 3–5; not AIS Brain 3–5 Any AIS Thorax 3–5; not AIS Thorax 3–5 Any AIS extremities 3–5; not AIS extremities 3–5 18 different hospitals F test: p value 0.103 0.359 0.358 0.697 0.565 0.16 0.39 0.015* 0.304 0.091 0.894 0.301 0.471 0.932 0.497 0.86 0.384