Table 1 Measurement properties analysed and criteria for assessment

Unidimensionality

Whether or not each of the nine HLQ scales measures a single health literacy construct [18].

% of significant t-tests from the Principal Components Analysis (PCA) of the standardised residuals <5% indicates unidimensionality. Where >5% significant t-tests, if lower bounds of CI < 0.05, unidimensionality is supported [16, 33].

Local independence is an element of unidimensionality. This occurs where the response to one item is not dependent on the response to another item [18, 26].

Person-item residual correlation value <0.2 indicates local independence [34].

Internal consistency reliability

The degree to which items in each scale measure the same construct [16].

Person Separation Index (PSI) > 0.7 indicates good internal consistency reliability [15, 28, 34].

Response format

Whether or not participants are able to consistently choose a response category appropriate for their level of health literacy. The point between two response categories (such as strongly agree and agree) where either response is equally probable is known as a ‘threshold’ [28].

The absence of disordered thresholds on the category probability curve graphs indicates appropriate response format [34].

Item bias

Whether or not different subgroups within the sample respond differently to an item, despite having equal levels of health literacy [16, 18]. This is measured using differential item functioning (DIF). Item bias for gender (male or female) and age group (60–75 and 76–90) were analysed.

A Bonferroni adjusted p value for significance was used for the DIF analysis [16]: p > 0.006 for 4 item scales (1 and 2); p > 0.005 for five item scales (3, 4, 5, 6, 8 and 9); and p > 0.004 for the six item scale (7) indicating no item bias.

Targeting

The degree to which the HLQ was appropriately targeted to the RESPOND cohort [16].

Targeting was evaluated through analysis of person-item distribution graphs [35]. The mean person location should approximate zero for a well targeted tool [16]. A positive person mean suggests that on the whole respondents found the scales easy to endorse. A negative person mean suggests that respondents found the scales difficult to endorse. A well targeted scale should see items spanning across the full range of individual person scores.