The literature search identified 237 articles of which 14 were eligible for this scoping review. 79% (n = 11) of these articles were published and 21% (n = 3) were grey literature. All studies were from developing countries. Most of the studies reported rationales with a few that were vague. The majority of the studies used survey-based techniques, where the limitation and strengths were reported.
Our scoping review showed that very little had been researched into WTP for spectacles despite the global burden of uncorrected refractive errors. Perhaps the fairly recent recognition as a public health issue that included visual impairment caused by uncorrected refractive errors into VISION 2020 global initiative of elimination of avoidable blindness  explains why the scoping review found the earliest published article in 2007. Additionally, all the studies undertaken were mainly from developing countries because low socio-economic status is one of the risk factors of visually impairment - socio economic status that affects 90% of vision impaired people live in developing countries . Although the justification of using certain methods in research is considered to be essential , little is reported on the pros and cons of WTP for spectacles methodology.
The reported rationale of WTP analysis for spectacles in the eligible studies
As mentioned previously, WTP is a cost-and-benefit measure. The rationale for WTP analysis in health care is due to the financial dilemma inherent in all service delivery. In developing countries, health services are paid out of pocket, and WTP assessment ascertains the potential for cost recovery and the demand for service delivery of a specific health issue [40–42]. Accordingly, the assessments are viewed as a premise for financial sustainability and development of cross-subsidisation procedures that allows for equity, affordability, accessibility, availability and quality [43, 44]. Our scoping review found a similar rationale in several of the eligible studies in terms of understanding the demand through participant’s preferences that could facilitate the provision of affordable and accessible spectacle services [26–31, 33, 34].
Similarly, another study defined its rationale in terms of improving the quality of spectacle delivery . Further, in most of the eligible studies in this scoping review, WTP for spectacles was reported as a small component of a bigger study of eye care service delivery. Hence, the rationale for conducting a WTP analysis was not mentioned. It is clear, however, that the rationale to undertake these studies was to improve refraction services, spectacle dispensing and financial sustainability.
The scoping review has demonstrated how the eligible studies reported their methods in terms of study participants, sampling method and sample size calculation in the assessment of WTP for spectacles. Most of the participants in the selected studies were adults due to the 2004 estimated prevalence of uncorrected refractive errors which was 2.43% for people aged 40–49 years and 7.83% for people aged 50 and above .
Regarding sampling methods the majority of studies used probability sampling which comprised of simple random sampling [28, 30, 35], systematic sampling [26, 27] and cluster sampling [32, 33, 36, 45]. All are advantageous as simple random sampling is usually representative s of the general population, systematic sampling spreads the sample evenly over the population and cluster sampling is economical and feasible for larger populations . Although there are various disadvantages, probability sampling is still appropriate as there is a possibility for each individual to be selected in the study.
Conducting sample size determination is a pre-requisite step in the study design to enable researchers to make inferences about the wider study population. In reporting the sample size determination adequately allows for a study to be conducted on a defined sample size that leads to the precision of estimates and a power of the results [47, 48]. It simply means that an appropriate sample size will yield precise and accurate conclusions. Our scoping review found only four studies that reported sample size determination [28, 33, 35, 36]. Since most of the studies did not report the sample size determination, the interpretation of these studies [26, 27, 29–32, 34] needs to be approached with caution. Further, the replication of these studies will be problematic as the methodology is not explicit.
Limitations, strengths and recommendations
Although several studies did not report any WTP technique shortcomings [30, 32, 35, 36], some reported general survey biases such as recall bias and selection bias . In light of these limitations, WTP does not enforce restrictions on the scope of benefits in which participants are allowed to express their preferences . All preferences are observed as appropriate for WTP results, both beneficial and the non-beneficial.
Criticism of stated preferences techniques are mainly based on hypothetical situations where studies are undertaken, leading to a hypothetical bias [4, 12]. It is claimed that the dichotomous nature of the questions, comprising a defined price and choice on a pre-defined set of prices, may lead participant’s to overestimate the responses of WTP depending on the environment or conditions at the time [4, 9, 49]. It is possible, however, to remove hypothetical bias by a follow-up certainty question, which may result in realistic WTP . While 13 studies in this scoping review were conducted in similar hypothetical conditions most did not report this shortcoming while others explicitly stated the overestimation and underestimation of WTP [28, 33, 34, 45, 50]. It is thus unclear whether these flaws were observed in any of the included studies.
Alternatively, WTP studies expected to address hypothetical biases used revealed preferences techniques such as BDM as it estimates WTP in real settings of the phenomenon of study. Elicitation of WTP using BDM is claimed to be incentive-compatible as the distribution of maximum prices is offered by the participants [9, 10]. Participant WTP that is greater than or equal to the randomly determined price must pay for the product. BDM similar to survey techniques is vulnerable to strategic bias, which affects WTP in terms of overestimation or underestimation . Hence, it has been stated that efforts to employ different techniques to correct WTP estimate may reduce bias but not eliminate it . Wertenbroch and Skiera stated that the BDM technique is realistic, transparent to participants, operationally efficient in terms of time, cost and effort to conduct . Only one study reported WTP using BDM technique with follow-up questions . The follow-up questions seem to validate the estimated WTP as participants express their satisfaction with the product . The selected studies of this review concurred with most of the pros of BDM method, such as being realistic and incentivising truthful responses . However, it also claimed that it was too complex to understand during implementation. The incentive of BDM has been seen as irrelevant as the determined sale price is not independent of distribution prices .
This scoping review is instructive for the future of WTP for spectacles as it has highlighted the strengths and weaknesses of reporting research methods which may be deemed necessary in guiding future research. However, studies were searched in the English language only, which might demonstrate its limitation. Further, despite our attempt to search published and grey literature on databases, webpages and others as well as not having contacted researchers for further identification, we might have missed some studies during the process.