Ethics approval and consent

Ethical approval for the study was obtained from the Ethics Review Committee, Faculty of Medicine, University of Peradeniya, Sri Lanka (Ref No. 2023/EC/48), and all methods were performed in accordance with the Declaration of Helsinki. Informed written consent was obtained from each participant after giving them adequate time to ask questions and clarify doubts about the research.

Recruitment

Panellists (n = 10) were selected using purposive sampling to assess content validity. These panellists had qualifications and expertise in human nutrition, dietetics, and sports nutrition (Step I). For face validity, elite and highly trained level [22] track and field athletes were recruited through purposive sampling (n = 16) (Step II).

To assess construct validity, voluntary participants were recruited from three distinct groups, each with 18 members, using purposive sampling: NTG (Nutrition-trained group): Participants in this group had at least a postgraduate qualification in human nutrition and prior training in sports nutrition. NNTG (No nutrition-trained group): This group comprised banking professionals without prior SNK. AG (Athletes’ Group): This is a separate group of athletes consisting of male and female track and field athletes at the elite and highly trained level [22] (Step III).

The SLn-SNKQ was then administered online to the NTG and NNTG groups from Step III, twice, with a three-week interval between administrations (Step IV). Additionally, the AG group (n = 18) from Step III assessed the questionnaire’s duration (Step V; Fig. 1).

Five-step validation process of the tool

The SLn-SNKQ tool is an online questionnaire developed using Google (Google LLC, California, USA, version 2016) [23] and is available in all three official languages of Sri Lanka: English, Sinhala, and Tamil, ensuring inclusivity for participants from different linguistic backgrounds. The procedures followed during the development have been published elsewhere [21]. To establish validity and reliability, five steps were employed based on established protocols outlined in the literature [4]. These steps included: (I) Assessment of Content Validity, (II) Assessment of Face Validity, (III) Assessment of Construct Validity, (IV) Assessment of Reproducibility and Internal Consistency, and (V) Assessment of Questionnaire Duration (Fig. 1).

Fig. 1

Flow chart of 5-step validation and assessment of the reliability of the SLn-SNKQ. AG: Athletes’ Group, SLn-SNKQ: Sri Lankan-Sports Nutrition Knowledge Questionnaire, CVI: Content Validity Index, NNTG: No Nutrition-Trained Group, NTG: Nutrition-Trained Group

Sample size calculation

Content validity was assessed based on feedback from a panel of experts related to sports nutrition recruited via purposive sampling. Calculating a Content Validity Index (CVI) requires the involvement of three to ten experts [24]. In the context of telephonic in-depth interviews to assess face validity, a recommended range of six to ten participants is often cited to capture a diverse set of perspectives effectively [25]. However, based on the research team’s prior experience and understanding of cultural nuances, it was anticipated that Sri Lankan athletes might express their opinions with restraint. Therefore, to enhance the robustness of the data collected and ensure a wider representation of views, the sample size was increased, resulting in a target of 16 elite or highly trained athletes. To determine the appropriate sample size for assessing construct validity, a power analysis for an independent sample t-test was conducted using G-POWER [26]. This analysis considered an alpha level of 0.05, a power of 0.80, a substantial effect size (d = 0.8), and adopted a two-tailed approach. Based on the above-mentioned criteria, the recommended sample size for each group—NTG, NNTG, and AG—was determined to be 18 [27].

Content validity

Upon the formulation of individual statements or prompts (items) of the questionnaire and determination of suitable response formats, invitations were sent to ten potential panellists who were recognized experts in the field of nutrition. All panellists possessed advanced qualifications, including a minimum of a Master’s degree in Human Nutrition or a related field. Additionally, several panellists had completed specialized certifications in sports nutrition, and each panellist also had substantial experience working with athletes, which included roles such as sports dietitians or nutrition consultants for professional sports teams. Those who agreed to participate engaged in individualized telephonic conversations during which relevant materials were shared via email, and the content validation form specifically developed for this study is given as Supplementary Material 1.

Accompanying the validation materials was an information sheet providing comprehensive instructions to guide the experts, including the most recent version of the questionnaire, with the correct response marked. Each expert was tasked with evaluating the questions within each subsection of the questionnaire and their assessment encompassed considerations of relevance, appropriateness, accuracy, and clarity. The evaluation employed a 4-point Likert scale, ranging from 1 (‘Not relevant, Not appropriate, Not accurate, Not clear’) to 4 (‘Highly relevant, Very appropriate, Very accurate, Very clear’), with accompanying written comments for each sub-Sect. [12].

The CVI for each sub-section was calculated by dividing the cumulative score assigned to that sub-section by all raters, and then dividing by the total number of raters. Index scores exceeding 3.2/4.0 (≥ 0.8) were considered adequate for ratings related to relevance, accuracy, clarity, and appropriateness [24]. Sub-sections with a mean score below 3.2 were precisely reviewed, and the individual comments provided by experts were incorporated to modify each question qualitatively. Following these revisions, experts who had initially assigned lower scores (1 or 2 out of 4) were contacted and asked to review and provide new ratings on the revised questions. This iterative Delphi process was repeated multiple times until the mean scores for each sub-section reached ≥ 3.2 for the categories of relevance, appropriateness, accuracy, and clarity.

Face validity

To evaluate face validity, feedback was solicited from a separate cohort of 16 male and female track and field athletes who were recruited using the purposive sampling technique. The recruited athletes represent the following tiers according to McKay et al. [22]: ‘Elite athletes (Tier 4)’ are defined as those competing at the international level, such as members of national teams; ‘Highly trained athletes (Tier 3)’ are defined as those competing at the national level. A retrospective think-out-loud protocol was used. The athletes were provided with the questionnaire ahead of time, but its precise nature was not disclosed. Subsequently, an in-depth telephone interview was conducted with the research team using a guided questionnaire specifically developed for this study (Supplementary Material 2). This interactive session aimed to address inquiries such as the purpose behind specific sections, the appropriateness of questions for evaluating the SNK of Sri Lankan athletes, and the relevance of sub-sections. Participants were also asked about their satisfaction with the clarity and formulation of the questions. In cases where dissatisfaction arose, alternate approaches to phrasing were explored. Additionally, participants were questioned about the comprehensibility of terms, the clarity of images, and whether any terms appeared unfamiliar. Further, the discussion addressed the potential confusion or misleading nature of any question.

These sessions were audio recorded. While we did not use a formal analytical approach, feedback was collected through a separate questionnaire, and the responses were carefully noted. Adjustments to the questionnaire were made based on this feedback. A table summarizing the feedback and the corresponding revisions is available if needed.

Construct validity

To assess construct validity, a comparative analysis was undertaken by administering the questionnaire to three distinct groups NTG (n = 18), NNTG (n = 18), and AG (n = 18). Participants were contacted via WhatsApp messages from Meta Platforms, Inc., to facilitate communication and ensure engagement. The use of WhatsApp was solely for logistical purposes.

The administration of the questionnaire to an equivalent number (n = 18) of elite-level [22] track and field athletes served two purposes: to evaluate the questionnaire’s effectiveness in measuring the intended construct, specifically SNK, and to ensure precision.

Internal consistency and reproducibility

To assess internal consistency, Cronbach’s alpha was applied to each of the five sub-sections separately, as each sub-section addressed a different area of knowledge. A minimum Cronbach’s alpha requirement of α > 0.7 was accepted to demonstrate sound internal consistency [28].

To assess reproducibility, the questionnaire was administered online to the NTG (n = 18) and NNTG (n = 18) groups twice, at a three-week interval, to evaluate test-retest reliability. This interval was selected to ensure an adequate time lapse for participants’ recall of their previous responses to diminish, while also minimizing the likelihood of significant changes in their nutrition knowledge during the period [29]. Both data sets were collected during different phases to evaluate the questionnaire’s reliability over time.

Assessment of the duration of the questionnaire

The revised version of the SLn-SNKQ was administered to a new cohort of participants, specifically the AG group from Step III, which consisted of 18 elite-level [22] track and field athletes recruited through purposive sampling. These participants conscientiously completed the questionnaire, noting the time taken for its completion and providing insights into the duration of the questionnaire.

Statistical analysis

Data was assessed for normality using the Kolmogorov-Smirnov Test and the significance level was set at P < 0.05. Data were considered non-parametric. A Kruskal-Wallis test was conducted to determine if there were differences in total and subsection nutrition knowledge scores between three groups (NTG, NNTG, and AG) that varied in their expected nutrition knowledge levels. Where differences were statistically significant, pairwise comparisons were performed using the Mann-Whitney test with a Bonferroni correction for multiple comparisons. A Spearman’s correlation was conducted to examine the correlation between nutrition knowledge scores of NTG and NNTG groups at two distinct time points.