Table 1

Overview of planned methods and analyses for the development of the HWISE Scale*

Scale development activity	Procedures
Phase 1: item development
1.1 Domain specification	Literature review.
1.2 Item generation	Literature review and Delphi methodology.
1.3 Content validity	By target population: two styles of cognitive interviews were used in the first eight sites, building on Delphi methodology.
1.4 Face validity	Pretesting and debriefing with enumerators at each site.
Phase 2: scale development
2.1 Data collection	Enumerator training and survey implementation.
2.2 Item reduction	We will drop items with cumulative missing cases >30% (ie, ‘don’t know’, ‘non-applicable’ or true missing responses) in any one site.
	We will assess the performance of each item’s variation with other items in the scale using a correlation matrix; items with very low (<0.30) interitem correlation coefficients and very low (<0.30) item-total correlation coefficients across multiple sites will be considered for deletion, as will items that misfit the model, that is, with residual correlations >0.20.
	Item reduction in Rasch paradigm: item severity and item discrimination test.
2.3 Identify factor structure	We will use factor analysis across multiple sites to test for factor structure; items with very low factor loadings (<0.30), split factor loadings (high factor loadings (>0.50) in two domains) and high residual variances will be considered for deletion.
2.4 Assess measurement equivalence	We will use multigroup confirmatory analysis (a form of measurement invariance) on data from multiple sites to test for exact invariance in the hypothetical scale; invariance will be assessed in terms of factor structure (configural model), factor loadings (matric model), mean intercepts (scalar model) and factor means (strict model).
2.4 Assess measurement equivalence	We will use confirmatory factor analysis alignment optimisation to estimate the group-specific factor means and variances of scale items across all sites; it assesses approximate invariance of scale items across multiple sites.
Phase 3: scale evaluation
3.1 Score scale items	Finalised scale items will be used in their unweighted form as sum scores or in weighted form as factor scores.
3.2 Assess reliability (internal consistency) of scale items	We will use Cronbach’s alpha and the Rasch reliability statistic to test the internal consistency of the scale items within each site and aggregated across sites.
3.3 Assess scale validity	We will measure predictive, convergent and discriminant validity of the final scale items using criteria that were selected based on their strong theoretical relevance in the water insecurity literature; tests of water insecurity differences between ‘known groups’ will also be performed.

*Adapted from ref 34.