Design
This study included data from two cross-sectional anonymous surveys: one was conducted in Japan to develop the final format of the KDI (11 items and one dummy item) from a provisional KDI (30 items and one dummy item)9, and the other was conducted for this study for people in the US as an English-speaking country with a Western background and Singapore as an English-speaking country with an Asian background. The latter two countries were selected by considering cost for the survey.
This study was granted by an institutional research ethics committee (Saitama Prefectural University, #24015), and all methods were performed in accordance with the Declaration of Helsinki. Informed consent was obtained from all participants. Data were collected by survey companies via recruitment on the web among its monitors (the Japanese group was conducted by NEO MARKETING, INC., Tokyo, Japan, and the US and Singapore group was conducted by Cross-Marketing, Inc., Tokyo, Japan).
Participants
For the three groups, the same inclusion criteria were used: (1) those with subjective Katakori, and (2) age of 20–69 years.
For the US and Singapore groups, an online survey that was composed of 2 parts was conducted over 2 weeks in 2024. The first part of the survey asked for a Yes/No answer to the following question: “Do you have a nonspecific symptom including discomfort or dull pain around the occiput through the cervical spine to the acromion and scapular area?” In the second part, those who answered “yes” to the first part were invited to respond to the final version of the KDI. Data in the second part were not eligible to this study when (1) those who did not complete the survey, (2) those who provided an inadequate response to the dummy item in the KDI, and (3) those who have not done any activities in the KDI, resulting in no KDI score. The minimum sample size of valid data in the second part was estimated to be 100 from each country to satisfy the “very good” criterion in the COSMIN guideline (i.e., 7 times number of items and ≥ 100 participants per each group) for factor analysis. Therefore, they tried to collect eligible data of more than 10 male and female individuals of each age group (20’s, 30’s, 40’s, 50’s, and 60’s) for each country in the second part of the survey.
For the Japan group, data of the provisional KDI were collected for 2 weeks in 2022. As the same as the US and Singapore groups, data were not eligible to this study when (1) those who did not complete the survey, (2) those who provided an inadequate response to the dummy item in the KDI, and (3) those who have not done any activities in the KDI, resulting in no KDI score. The survey company tried to collect valid data from approximately 50 male and female individuals of each age group (20’s, 30’s, 40’s, 50’s, and 60’s)9. Finally, valid data from 515 participants (244 men and 271 women, mean [SD] age = 45.8 [13.9] years) were analyzed to create the final format of the KDI9, which was shared in the current study.
KDI
The KDI uses a semi-individualized format designed to allow comparison of conditions across individuals and to reflect the individuality of them. In this format, participants were asked to rate both the importance and their perception of the bothersomeness caused by Katakori on 11 items using a 6-point scale. In addition, to ensure that respondents are responding mindfully, one dummy item is included with instructions to put a specific number for importance and bothersomeness.
Participants are asked to recall their experience with Katakori over the past week and respond accordingly. The Importance scale consists of six categories: “not important at all” or “have not done at all over the past week”; 1, “not important”; 2, “not important so much”; 3, “slightly important”; 4, “important”; and 5, “extremely important.” The Bothersomeness due to Katakori is also rated on six categories: 0, “not bothersome at all”; 1, “not bothersome”; 2, “not bothersome so much”; 3, “slightly bothersome”; 4, “bothersome”; and 5, “extremely bothersome.” Items rated as “not important at all” or “have not done at all over the past week” in the Importance scale are excluded from further score calculations and treated as missing values.
The product of the Importance and Bothersomeness scores is calculated (i.e., 0, 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 16, 20, and 25). These products are then transformed into a 15-point scale (0–14) in decreasing order (e.g., a product of 0 = 0, a product of 25 = 14). As a result, each item score is either a missing value or falls within the range of 0–14. Finally, the following formula is used to calculate a percentage score, where a higher score indicates a greater disability of Katakori on daily life.
$$KDI~\% score = \frac{{Mean~score~across~all~items~except~with~the~missing~value}}{{14}} \times 100$$
Analysis
Cross-cultural validity was assessed using the MGCFA as suggested by the COSMIN guideline. The analysis was conducted using SPSS (Amos™ 20.0, NY, US). Data of the participant with a missing value of the KDI were excluded from the MGCFA because the MGCFA requires no missing data. Although the COSMIN guideline does not provide clear criteria for cross-cultural validity, since the KDI is a unidimensional PROM, the model fit of a configuration with no constraints on factor loadings or intercepts must first be acceptable (i.e., confirmation of configural invariance). Model fit was considered acceptable if the CFI was greater than 0.95 or the RMSEA was less than 0.08, as outlined by COSMIN. Once configural invariance was confirmed, cross-cultural validity was determined if there were no statistically significant differences when transitioning from the unconstrained model to the models shown in Table 317. In this study, considering criticisms that the evaluation by the MGCF is too strict, cross-cultural validity was deemed to exist if no statistically significant difference was found after changing to the partial metric invariance16. Since confirmatory factor analysis does not accommodate missing data, samples with missing values were excluded from the assessment of cross-cultural validity.
For the US and Singapore groups, the presence of Katakori was investigated from all data completed in the first part of the survey. Additionally, the internal consistency of the KDI in each country was assessed using Cronbach’s alpha by excluding data of the participant with missing values from valid data in the second part of the survey. The values were interpreted as follows: acceptable, ≥ 0.7; and not acceptable, < 0.718.
To investigate the scoring characteristics of the KDI, multiple comparisons of % KDI scores were performed using the Steel-Dwass method for: (1) the three countries, and (2) 30 groups divided by five age groups and gender. The analysis was conducted using JMP version 18 (SAS Institute Inc., SAS Campus Drive, Cary, North Carolina 27513, US) for the Steel-Dwass method and IBM SPSS Statistics for Windows version 28.0 (IBM Corp., Armonk, NY, US) for the other analyses, with a significance level set at 5%.
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