Design

This study was an online survey analysed using standard questionnaire validation methods, such as principal components analysis. People with CB were involved in the design and implementation of this study.

This first part of this paper describes the development of the questionnaire, and the second part describes the validation of the questionnaire. In the second part, the scores of people with CB are compared with the scores of people with normal colour vision.

Setting

The setting of this study was online.

Participants

Participants were recruited from various sources, mostly online (details in the section Initial Validation of Final CB Questionnaire, below). They were categorised as being CB or not, as described in the Materials section below. The authors use the common term ‘colour blindness’ (CB) for the sake of clarity, but recognize that participants in fact had varying degrees of colour vision deficiency (CVD). Characteristics of the sample are shown in Table 1 below.

Table 1 Description of the colour vision and social background of the participants by sex (128 men and 291 women) Full size table

Ethical approval was granted by the University College London Research Ethics Committee (Project ID: 4075/004). Informed consent was given by the participants before filling in the questionnaire.

Phase 1: development of CB questionnaires

Items for the questionnaire were developed through (a) a literature search, (b) an online lay focus group consisting of six individuals with CB and two normally-sighted people, recruited via Psychology on The Net and snowball sampling, and (c) an expert focus group consisting of three consultant ophthalmologists (SM, MAB, AKD), and a psychologist (JB) who has published several papers on questionnaire development in health and psychological conditions. JB has congenital deuteranopia (‘red-green’ CB). At these meetings, discussion focused on the types of experiences that were important to people with CB. Three main themes emerged: the career impact of being CB, the emotions associated with being CB, and the lifestyle impact of being CB. These various experiences were listed and then phrased as questions, with appropriate Likert scales added. Through this process, the 36 items that formed the basis of the questionnaire were derived.

Six-point Likert scales were used, with lower scores indicating worse QoL. Responses were on a 6-point Likert scale from 1 = A severe problem, to 6 = No problem, with an option for ‘not applicable’. In this development phase, the questionnaire was administered to six CB people and two normal vision (NV) controls. The survey was completed online. All suggestions for revisions to the survey were recorded and changes to the questionnaires were made based on these suggestions. For example, an item was added regarding judging by colour whether food is sufficiently cooked. Also, the background colour of the survey was changed so that the questionnaire was easier for CB people to use.

A principal components analysis (PCA) was conducted to examine the factor structure of the questionnaire. The PCA used Varimax rotation and Kaiser normalization. Missing data were deleted pairwise, so that where a participant gave some answers but had not completed the questionnaire, the responses they gave could be included in the analysis. Extraction and retention of factors was based on visual examination of the scree plot [9] and eigenvalues of >1.0 were retained [10]. The threshold for the Kaiser-Meyer-Olkin (KMO) Measure of Sampling Adequacy was 0.6 [11]. Cronbach’s α coefficient values were assessed [12] in order to measure the internal reliability of a questionnaire. The usual threshold for acceptability for Cronbach’s α is 0.7 [13]. A factor loading threshold of .60 was applied to enhance the strength of factors, so only items of this strength, or with loadings that could be rounded up to .60, were retained. For factor loadings of .6 to be significant, a minimum of 85 participants are required to allow sufficient statistical power [14].

Phase 2: initial validation

Initial validation of final CB questionnaire

For the initial validation of the questionnaire, an online survey invited individuals with CB and NV controls to fill in the questionnaire. Participants were recruited between Sept 2014 and Sept 2015 from relevant websites and social media sources, including Colour Blind Awareness, the Men’s Health Forum, Psychology on The Net, Online Psychology Research and Birmingham University’s Medical School Newssheet.

Materials

Ishihara colour test

Colour blindness was assessed using the Ishihara Colour Test [15]. This is a set of up to 24 coloured plates [16] in which the visibility of numbers or letters will depend upon the colour vision of the viewer. The plates are scored by giving one point for each plate correctly identified. This study used 15 plates, the number required to identify colour deficiency. Those who scored up to 9 plates correctly were categorized as colour blind; those scoring 13 or more were considered to have normal colour vision. For the purposes of our study, those who scored between 10 and 12 were categorized as borderline colour blind.

Health-related Quality of Life (HRQoL)

To assess the degree to which CB might have an impact on health-related quality of life, the participants filled in the Short Form 36 (SF-36) [17]. This 36-item health questionnaire is widely used and assesses QoL for eight dimensions of health. The subscales are rated on Likert scales, with lower scores indicating worse health. The response format of the scales varies, for example, ‘excellent’ to ‘poor’ for one item, and ‘not at all’ to ‘all the time’ for another. For the purpose of the present study, only the mental health (SF-36 MH) subscale was used. The Cronbach’s alpha for this subscale is 0.926.

Positive state of mind

To assess the degree to which CB might impact mental health, The Positive Mindset Index (PMI) [18] was administered to participants. This scale consists of six items (happiness, confidence, being in control, emotional stability, motivation and optimism) on a 5-point Likert scale. This scale shows good internal reliability (Cronbach’s alpha = 0.926) and shows good concurrent validity with measures of mental health [18, 19].

Characteristics of the sample

Table 1 shows the characteristics of the sample.

As expected, there were significantly more men than women with colour blindness (χ 2 = 118.98, df = 2, p = 1.5 × 10−26). Of the 65 CB participants, four (6%) reported that their CB was acquired rather than congenital. There was no significant difference in the socioeconomic background distributions in the three vision groups, with 56–68% coming from a professional background, 23–44% from a manual background, and 0–9% from an intermediate background.

The mean (SD) age of the CB group (44.8 ± 15.6 years) was significantly older than that of the NV group (30.1 ± 15.1 years) (p = 9 × 10−11). In order to reduce any effect of age on outcomes, matching of participants in each group by age was carried out.

Of the 56 CB participants aged 18 to 65 years old, it was possible to match 30 of them by age to the nearest year to one or more NV community controls. This age limit of 65 years was chosen both because it spans the average working age in the UK and avoids the general declining of vision which is common with older age. Where there was more than one person in a group of the same age, the mean of their scores was used. For example, for participants aged 30 years old, there was one CB and three NV controls, thus the sole CB score was compared to the mean of the three NV scores. In some cases there was more than one CB participant of the same age e.g. at age 45 the mean of four CB participants was paired with the mean of six controls. Using this process, 30 age-matched pairs were possible from 46 CB and 199 control participants. Because men and women typically have different rates and types of colour blindness, the groups were further subdivided by sex. Fewer age-matched pairings were possible within each sex: 11 pairings of men (16 CB and 18 controls) and five pairings of women (five CB and 20 controls).

Initial validation analysis

As a first step in validating a newly developed questionnaire, the construct validity of the questionnaire can be tested by assessing differences in scores between groups who are known to be different in relevant ways. In the present study, the two groups were the CB and normal sighted controls, matched for age. The mean scores on the Ishihara test, CBQoL, PMI and SF-36 MH were compared using matched t-tests. In the QoL questionnaire subscales, a higher score indicated a better QoL. All statistical analyses were carried out using SPSS statistical software for Windows, Version 22 (Armonk, NY: IBM Corp). The significance threshold was set at .05, and all P values were 2-tailed.

A further step in validating new questionnaires is testing how much they are in agreement with existing validated questionnaires measuring similar constructs. This is known as concurrent validity, and acceptable concurrent validity is indicated by a Pearson’s correlation coefficient (r) of 0.5 or more [20]. The criterion by which the new questionnaire subscales were measured was the Ishihara Colour Blindness Test [15], which assesses colour vision acuity. Thus concurrent validity between the CBQoL questionnaire and the Ishihara was assessed in this way.

To assess concurrent validity with health and psychological aspects of QoL, two other questionnaires were used. Firstly, the QoL mental health subscale of the SF-36 [17], on which lower scores indicate worse mental health. Secondly, the Positive Mindset Index (PMI) [18] was used, on which higher scores represent a more positive state of mind.