Table 1 Explanations of concepts used in the analysis

Coefficient alpha

An index of reliability (internal consistency)

Differential Item Functioning (DIF)

Assessment of weather items work invariantly in different subgroups of respondents, e.g., age and gender groups

Fit residual

Quantification of differences between observed and expected item responses. The expected value is 0 (perfect fit) and values between -2.5 and + 2.5 are considered acceptable

Item Characteristic Curve

Displays the relationship between observed and expected item responses at various levels, from less to more dysautonomia. This is one aspect of Rasch model fit (see below)

Local dependency

Occurs when there is item redundancy (different items represent similar content), or can be due multidimensionality (see below)

Multidimensionality

Items do not represent one common construct

Person separation index (PSI)

An index of reliability that is conceptually analogous to coefficient alpha (see above)

Rasch model fit

Different related approaches to assess to what extend empirical data fit the Rasch model

Response category functioning

Weather response categories (for example never [0], sometimes [1], regularly [2], and often [3]) function as intended. Response category thresholds (see below) that are disordered imply that response categories are not functioning as expected from less to more

Response category thresholds

The locations where there is equal probability of responding in either of two adjacent categories

Strata

The number of statistically distinct groups of persons that can be identified. For instance, with two strata, the scale can differentiate between persons with less versus more dysautonomia

Targeting

To what extent items in a scale represents the levels of the construct (in this study dysautonomia) reported by the sample

Unidimensionality

Items represents one common construct (in this study dysautonomia)