The Rasch measurement models
The Rasch model (RM) is part of the family of Item Response Theory (IRT) models and its development aimed to provide scientific measurement in social sciences according to the fundamental measurement properties such as As abovementioned, two main requirements for items fitting a RM are local independence and the absence of differential item functioning (DIF). Regarding local independence, psychological instruments are developed to measure a latent trait (e.g. an unobservable psychosocial process such as social support) and the latent trait should be the common cause generating the item responses. Similarly, items responses are influenced by respondents’ characteristics through the latent trait. For example, it is expected that respondents with higher educational attainment (i.e. sociodemographic characteristic) will experience higher social support (i.e. latent trait) and, consequently, will have higher scores on the SSS items (e.g. higher scores on the instrumental support item). Hence, the item responses will be (marginally) correlated with respondents’ characteristics (e.g. scores on the instrumental support item will be positively correlated with educational attainment). However, once the influence of the latent trait is accounted for, there should be no more correlation between the item responses and respondents’ characteristics. For example, for respondents with the In summary, the RM implies that items are conditionally independent to other items (i.e. local independence) and exogenous variables (i.e. absence of differential item functioning (DIF)) given the latent trait. However, these mathematical requirements of the model are strict, and it is common for items of rating scales applied in health sciences to exhibit LD and/or DIF. In this case, items with LD and/or DIF would not fit a RM and would need to be removed from the questionnaire for the remaining items to obtain fit to the RM. However, this practice is problematic since the removed items can be relevant in terms of their content and they might cover important aspects of the construct being measured [