An observational, international and cross-sectional study (one point-in-time evaluation, with retest) was carried out. This study is part of a macro research project with the general aim to achieve a unique Spanish-speaking scale to evaluate the process of living with one or more than one long term condition (LTC), as T2DM, osteoarthritis, chronic obstructive pulmonary disease, chronic heart failure, high blood pressure, and Parkinson’s disease [17,18]. In particular, the contribution of this manuscript in this field is to present the psychometric properties of the LW-CI-T2DM scale in a Spanish-speaking population.
Sample, sampling and sample size
A consecutive cases sampling [19,20] was applied to participant identification.
The sample was composed by people living with T2DM from different primary and secondary healthcare centres and community groups from Spain and Colombia. Inclusion criteria were a) patients with T2DM diagnosis made by a endocrinology or General Practitioner (GP), in any stage of the disease; b) Colombian or Spanish nationality; c) able to read and understand properly the questionnaires; and d) non-hospitalized patient at the moment of the study. Exclusion criteria were a) patients with cognitive deterioration, acute disorder and/or pharmacological effects that potentially could distort the objective of the study; b) refusal to participate in the study and c) not meeting established inclusion criteria.
Sample size was calculated according to the model proposed by MacCallum-Browne-Sugawara . To test a five-factor model, assuming the null hypothesis of a mean square error of approximation (RMSEA) from 0.05 to 0.08, a statistical power of 0.80, an alpha value of 0.05, with 205 df, a minimum sample size of 225 was required. This sample was over-estimated by 20% to cover possible losses. These calculations were carried out using STATISTICA 12 (Dell Software, Tulsa, OK).
A sociodemographic questionnaire was used to collect personal data of the patient living with T2DM related to gender, age, marital status, educational level and employment situation. Besides, T2DM related questionnaire was used in order to know age of diagnosis, disease duration, and type and duration of treatment for T2DM. As in other validation studies carried out in Spain and South America  in addition to sociodemographic and disease related data, the following Spanish versions of self-reported scales were also collected:
- LW-CI-T2DM  scale focused to measure living with LTC. It is 26 items distributed in the following 5 dimensions: acceptance (4 items), coping (7 items), self-management (4 items), integration (5 items) and adjustment (6 items). It is a five-point Likert-scale ranging from 0 (nothing/never) to 4 (much/always) (except for the "acceptance” dimension, in which the score is reversed). The final score ranges from 0 and 104 (higher scores better living with the LTC).
- The Duke-UNC Functional Social Support Questionnaire (DUFSS) [22,23] was used to evaluate social support of the patients’ from their perspective. It is an 8-items that evaluates different dimensions of social support as confidant, affective and instrumental support. The score for each item varies from 1 (much less than I would like) to 5 (as much as I would like).
- The World Health Organization Quality of Life Instrument-Brief (WHOQOL-BREF)  was used to measure the quality of life of people living with T2DM. The WHOQOL-BREF is comprised by 24-items that evaluates physical health, psychological health, social relationships, and environment. Item response options range from 1 (very dissatisfied) to 5 (very satisfied/very good quality of life).
- The modified version of the Satisfaction with Life Scale  is a to evaluate satisfaction overall with life (item 1) and in regard to other five areas: physical, psychological wellbeing, social relations, leisure, and financial situation. Each item scores from 0 (unsatisfied) to 10 (totally satisfied).
- The Patient Based Global Impression of Severity Scale (PGIS)  was used to evaluate the patient global impression of severity of the T2DM. It is a six-point Likert-scale ranging from 0 (not ill at all) to 5 (extremely ill) according to the patient.
For this validation study, the Spanish version of the scales was used.
Data collection was carried out between May 2018 and June 2019. The potential participants (people living with T2DM) filled in the scales during the consult with the endocrinology, GP, nurse specialist or primary care nurse. To ensure homogeneity and reproducibility of the procedure of data collection a standardized protocol was established with the following steps: explaining the research study; asking about doubts; reading out load instructions of the scales and its answer options; writing a check marc in the answer chosen by the patient; reading out load instructions of self-reported scales and giving participants time to complete it. The median time to complete all the measures was approximately 30-40 minutes.
Data collection related to test-retest was also protocolized to minimize potential random errors.
Patients were asked to answer a second time to the LW-CI-T2DM at home. The LW-CI-T2DM was in an envelope with seal and the research postal direction in order to complete the questionnaire and send it in an easy and free way to the researchers. A minimum sample of 50 subjects and a time span of 7 to 10 days for the retest was planned.
Descriptive statistics (central tendency measures, proportions) were used to determine the sociodemographic and T2DM characteristics. Main data were ordinal or did not fit normal distribution. Therefore, non-parametric statistics were used.
For the following psychometric properties were tested in this LW-CI-T2DM validation study:
Feasibility and acceptability. Quality of data was considered satisfactory if 95% of the data were computable. The limit for missing data was <5% . Floor and ceiling effect were deemed acceptable if they were < 15%  and the skewness was expected between -1 to +1 .
Internal consistency was tested by Cronbach’s alpha coefficient (criterion value >0.70) , item-total correlation (corrected for overlap; criterion value, rs ≥0.30) , inter-item correlation (criterion value, r≥ 0.20 and ≤ 0.75) , and item homogeneity (criterion value > 0.30) .
Reproducibility (test-retest) was determined using weighted kappa (with quadratic weights) for items (standard: >0.41 moderate)  and intraclass correlation coefficient (one way, random effect; ICC) for domains and total score. Values ≥ 0.60 were considered acceptable .
Precision was estimated by means of the standard error of measurement (SEM = SDpooled *√ [1–rxx]), where was the ICC of the test-retest. A SEM value < ½SDpooled was used as criterion of acceptable precision [36,37].
Construct validity. Confirmatory factor analysis (CFA) was applied to corroborate the original 5-factor matrix of the instrument, using the following parameters of indices of good fit: (CMIN/df) less than 5 (preferably less than 3), RMSEA less than 0.08 with the respective 90% CI, comparative fit index (CFI) of more than 0.90, and goodness-of-fit index (GFI) of more than 0.90 . CFA estimations were all standardized, which allowed that factor loading and covariance matrix to vary. For convergent validity, and according to a previous study of the scale in patients with Parkinson’s disease , a moderate (rs ≥ 0.35-0.50) or strong relationship (rs > 0.50)  was hypothesized between LW-CI-T2DM and DUFSS, SLS-6, and WHOQOL-BREF, and a weak/moderate association with other variables of the study, as age, T2DM duration or treatment. Spearman rank correlation coefficients were obtained to this purpose.
Internal validity, defined as the inter-correlations between the LW-CI-T2DM dimensions (standard, rs = 0.30-0.70)  and known-groups validity for gender, treatment and PGIS scores were determined. Mann-Whitney and Kruskal-Wallis tests were used for groups comparison.
The COSMIN Checklist (COnsensus-based Standars for the selection of health Measurement INstruments) and its extension for content validity, were used for assessing the final measurement properties of the instrument and the methodology [40,41].