The Nottingham Health Profile (NHP) is a widely used 6-dimensional (energy, pain, emotional reactions, sleep, social isolation, and physical mobility) generic health status questionnaire 1. The NHP has undergone extensive evaluation and both strengths and weaknesses have been demonstrated 2. A commonly observed limitation of the NHP has been skewed score distributions with large ceiling and, particularly, floor effects 345. This complicates interpretation of extreme scores and impairs the ability to detect changes and differences. Furthermore, some of the NHP domains have relatively few (3 to 5) dichotomous items. This limits the precision of scores 678.

The NHP index of Distress (NHPD) is a 24-item measure of illness-related distress embedded in the NHP 9. While it has not been extensively used or evaluated, available data have shown promise and suggest that it can provide a unidimensional measure of illness-related distress 4101112. Indeed, the NHPD has the potential, at least in part, to overcome limitations associated with NHP domain scores. The larger number of items should improve reliability and precision of scores. Accordingly, available studies have shown less floor/ceiling effects and indicated better responsiveness and reliability of the NHPD than the six NHP domain scores 49101112. However, its generic properties, i.e. whether scores can be interpreted the same way across different diagnoses, remain to be determined. This is particularly important because a main assumption and theoretical advantage with generic outcome measures is the possibility to make valid comparisons across patient groups. Support for these properties is gained when scales work the same way and have the same meaning in different groups. This can be assessed by analyzing the presence of differential item functioning (DIF) 1314.

Generic outcome measures can be more or less suitable for certain groups of people. As such, the NHP has been found to work best with chronic, disabling conditions, and with elderly populations who are likely to have at least some of the problems represented in each of its six domains 2. Parkinson's disease (PD) and peripheral arterial disease (PAD) exemplify two chronic disabling disorders associated with aging where the NHP has been commonly used 41115161718. PD is a chronic progressive neurodegenerative condition characterized by motor symptoms such as bradykinesia, rigidity and resting tremor. However, non-motor features such as fatigue, depression, sleep disturbances, pain and autonomic dysfunctions are also frequent and a common source of disability 19. PAD is associated with a wide spread arterial disease and significantly increased risk of stroke, myocardial infarction and cardiovascular death. Symptoms range from leg pain while walking to severe pain in the limb also at rest, non-healing ulcers and gangrene 20. Besides pain and restricted mobility, fatigue, emotional distress and sleep disturbances are common problems in PAD 18. PD and PAD therefore appear to represent suitable diagnostic groups for assessing the NHPD and explore its cross-diagnostic validity and comparability.

The Rasch measurement model 21 offers particular advantages over traditional psychometric methods in evaluating measurement scales 82223. The model rests on a mathematical definition of the requirements for linear measurement, which is achieved when data accord with model specifications. Because the model articulates measurement requirements, sources of violations to model assumptions are sought and adjusted for in the data rather than trying to fit another model 24. Rasch analysis thus determines the extent to which observed data conform with model specifications and provides a powerful means of assessing a scale's measurement properties, including DIF 142325262728.

This study assessed the measurement properties and cross-diagnostic validity of the NHPD as a survey instrument among people with PD and PAD.

Raw NHPD scores covered the full range (0–24) in the PAD sample (median, 5; q1–q3, 2–9) and ranged between 0 and 21 (median, 4; q1–q3, 1–7) in the PD sample. The median in the combined sample was 5 (q1–q3, 2–8).

The aim of this study was to evaluate the measurement properties and cross-diagnostic validity of the NHPD as a survey tool among people with PD and PAD. We found that the NHPD displayed generally good measurement properties but signs of DIF by diagnosis for seven items. However, this DIF did not impact the total score, thus supporting the generic measurement properties of the NHPD among people with PD and PAD.

The most important observation from this study is that observed DIF cancelled out and was not found to have any meaningful effects on the total NHPD score. This conclusion is based on the observation that estimated person locations were virtually identical regardless of whether DIF was adjusted for or not, and the linear measures corresponding to different raw total scores were also very similar. The approach employed here to assess the impact and clinical significance of DIF on the total score is reasonable because the total raw score is directly related to, and a sufficient statistic for estimation of, the linear measure of a person 35. These results provide empirical support for the assumed generic properties of the NHPD. However, additional studies in other target populations are needed to generalize these conclusions.

Overall model fit did not improve but showed signs of deterioration following adjustment for cross-diagnostic DIF. This may be considered somewhat surprising given that DIF violates model assumptions 22. However, DIF represents an aspect of model fit additional to that provided by residual based assessments across class intervals. One possible explanation for the significant item-trait interaction statistic following item splits may be that the observed DIF were signs of multidimensionality rather than "true" DIF among these items. This view is supported by the lack of improved overall fit following item split and signs of multidimensionality in the independent t-test protocol (see below). An additional explanation could be that item 24 displayed some signs of misfit in the DIF-adjusted scale, although removing this item did not lead to unequivocal improvements. The statistically significant item-trait interaction statistic also needs to be interpreted in view of the sample size 3540. If, for example, the sample studied here had consisted of ten people less, this statistic would not have been significant. Taken together, we therefore consider the statistically significant item-trait interaction not to be of any greater practical significance. Similarly, it also appears reasonable to retain items 9 and 24 since the observed misfit largely stemmed from one (item 9) or two (item 24) class intervals. Furthermore, these items behaved well otherwise and their removal did not result in unequivocal scale improvements.

The independent t-test protocol 3738 identified signs of multidimensionality in the scale when not adjusted for DIF. However, given that this finding was just marginally significant (lower 95% CI bound, 0.06) it may be argued whether this is of any practical concern. Indeed, this test, as any other statistical test 40, is dependent on sample size. If, for example, half or two thirds of the current sample size had been used instead (with the same proportion of significant individual t-tests), the statistical conclusion would have supported unidimensionality. Therefore, although the independent t-test protocol appears more useful in detecting multidimensionality than residual based fit indices and factor analytic approaches 3738 it must be borne in mind that this, in itself, also is a somewhat arbitrary test. Inferences are dependent on and, therefore, differ according to sample sizes 40. Other aspects of this test also need to be considered. First, although often considered non-problematic with sample sizes above 200 41, methods such as PCA assumes that data are normally distributed. Secondly, the rationale for the suggested loading of 0.3 as a cut-off to define items to be included in the independent t-test protocol 38 is unclear and other criteria could also be conceivable; additional studies regarding the optimal approach to using this test are warranted. Unidimensionality is not an absolute but a relative matter and there is no single agreed-upon method to test unidimensionality. Therefore, the decision whether a scale is sufficiently unidimensional should ultimately come from outside the data and be driven by the purpose of measurement and clinical/theoretical considerations 22.

In accordance with expectations and previous observations 41012 we found the NHPD to display considerably less floor effects than the original NHP dimension scores typically have and that the observed proportion met the suggested 15% criterion 42. This is an important observation because large floor and ceiling effects impact the possibility to differentiate between respondents and detect changes over time 43. However, examination of the distribution of persons and items in this study revealed that a proportion of people exhibited levels of distress that were lower than that covered by the NHPD items. The implication of this observation is that those people are measured with less precision and confidence, which impacts the ability of the scale to reliably detect differences and changes in this region of the outcome space. However, the NHPD was still able to distinguish among three different strata of people, as indicated by reliabilities above 0.8 44, and experiences from clinical trials in PD and post-acute inpatient care 114546 have provided general support for its responsiveness.

Although the NHPD appears more useful than the six-dimensional NHP, our observations are in general agreement with recommendations for the NHP 2 and suggest that the NHPD probably is most suitable for studies of people with chronic, disabling conditions expected to experience relatively high levels of distress. The suitability of a scale relates to the purpose of its use. For example, our observations suggest that the NHPD would not be suitable for a clinical trial targeting people experiencing relatively mild disease impact, whereas there is support for its usefulness in trials aimed at more severely affected individuals. The NHPD also appears useful for survey purposes, where it generally (and arguably) is of greatest concern to identify those who fare least well. Increasing the number of items and/or modifying the response scale from a dichotomous to a polytomous one 47 may provide means of improving and expanding the scale's usefulness.

The sample used here was drawn from earlier studies not designed for the present purpose. However, we do not consider this a major problem since the Rasch model enables scale items to be examined in a way that is freed from the characteristics of the study sample. Another limitation could be the concurrent use of multiple questionnaires in some of the original studies and the fact that people did not respond to the 24-item NHPD but to the 38-item NHP, from which NHPD data were derived. This may, hypothetically, have influenced responses and, hence, psychometric performance. However, this strategy is a common procedure in psychometric studies and has generally not been found problematic 484950. Nevertheless, further studies using only the NHPD and not the full NHP are warranted. Furthermore, our data did not allow us to address some important measurement properties such as test-retest stability and responsiveness. Finally, this study only considered two diagnostic groups. Additional analyses in other patient populations are needed to further determine the generic properties of the NHPD.

The NHPD displayed good measurement properties among people with PD and PAD but exhibited varying degrees of DIF by diagnosis for seven items. Although this DIF may represent some degree of multidimensionality, it did not have a clinically significant impact on the total score. This supports the generic measurement properties of the NHPD as a sufficiently unidimensional survey tool among people with PD and PAD. These results should encourage others to consider using the NHPD as a simple patient-reported index of illness-related distress in chronic disabling disorders and to reassess available NHP data within the NHPD framework to further evaluate its strengths and weaknesses.

The authors declare that they have no competing interests.

CWH conducted literature searches, participated in designing the study, data analyses and interpretation, and drafting of the manuscript. RK participated in designing the study and drafting of the manuscript. PH conceptualized and participated in designing the study, conducted the analyses and drafted the manuscript. All authors collected data and read and approved the final manuscript.