Table 1 Measurement Properties Tested

Distribution Characteristics

Floor and Ceiling effect

The distribution of frequency of dimension scores across condition groups was used to determine the distribution characteristics in terms of ceiling and floor effects (i.e large numbers of respondents reporting no problems or severe problems respectively, in each dimensions). The individual dimensions were considered, by the authors, to have a floor or ceiling effect if reporting exceeded 70%.

Measurement Structure

Factor Analysis

Rotated Factor Analysis was used to examine the structure of the questionnaire and determine the variance each of the factors contributed to the scale.

Reliability

Internal Consistency

Reliability and internal consistency of the dimensions was established through Cronbach’s Alpha. A Cronbach’s Alpha value of greater than 0.7 is considered acceptable [38].

Test-retest reliability

Test-retest reliability was calculated, in a sub-set of GP children, for dimensions with percentage agreement and intraclass correlation coefficient (ICC) for the VAS score. An ICC of > 0.7 was considered reliable [39].

Validity

Known-group validity

The known group validity was established from the significance of chi-square results of dimension scores across AI, CI and GP children and one-way ANOVA and post hoc analysis of the VAS scores between AI, CI, and GP children. The dimensions were assessed for their equivalence across the age groups through the proportion of no problems which were reported and the 95% confidence intervals (CIs). It was hypothesised that the general health VAS score would distinguish between healthy toddlers and infants from the general population and those with acute and chronic health conditions based on previous findings from the EQ-5D-Y on older children [40, 41] and testing of the PedsQL Infant Scales [42]. It was anticipated that dimension scores would be similar to that reported for the EQ-5D-Y by Scott et al. (2017) and AI children would report the most problems across dimensions of movement, play, pain and eating; CI children would report more problems on the on dimensions of relationships and communication and some problems with movement; GP children would have ceiling effects for all dimensions with similar reporting of pain in GP and CI children [40].

Concurrent Validity

The concurrent validity of the TANDI dimensions and the associated domain scores on the ASQ, FLACC scale or NIPS and Dietary Information was calculated by Sperman’s Rho. Correlation values were interpreted according to Dancey and Reidy guidelines with correlations from 0.1 to 0.3 were considered weak, 0.4 to 0.6 moderate and correlations of 0.7 or above as strong [43]. The performance of the dimensions across age groups was assessed to ensure that the measure was valid across the age range and different versions would not be needed for each age group.

Performance of the TANDI General Health Question measured on the VAS

Regression Analysis of the VAS score was used to determine the effect that the dimension scores had on the general health. Outliers with residual scores > 2 Standard Deviations from the mean were excluded for regression analysis.