People’s Willingness to Pay for Dental Checkups and the Associated Individual Characteristics: A Nationwide Web-Based Survey among Japanese Adults

This study aimed to determine the willingness-to-pay (WTP) values for dental checkups and analyze the association between the values and individual characteristics. This cross-sectional study was conducted using a nationwide web-based survey, and 3336 participants were allocated into groups that received regular dental checkups (RDC; n = 1785) and those who did not (non-RDC; n = 1551). There was a statistically significant difference in the WTP value for dental checkups between the RDC (median: 3000 yen [22.51 USD]) and non-RDC groups (2000 yen [15.01 USD]). In the RDC group, age 50–59 years, household income <2 million yen, homemaker and part-time worker employment status, and having children were significantly associated with decreased WTP values; male sex, household incomes ≥8 million yen, and tooth brushing ≥3 times daily were associated with increased WTP values. In the non-RDC group, age ≥30 years, household incomes <4 million yen, and having ≥28 teeth were significantly associated with decreased WTP values; household income ≥8 million yen was associated with increased WTP values. Conclusively, WTP values for dental checkups were lower in the non-RDC group than in the RDC group; in the non-RDC group, those with lower household income aged ≥30 years were more likely to propose lower WTP values, suggesting the need for policy intervention to improve access to RDC.


Introduction
Maintaining good oral health hygiene entails routine dental checkups and consulting dentists to diagnose underlying dental diseases. Apart from maintaining masticatory function, good oral health conditions contribute to an enhanced quality of life [1,2] and reduce the impact of systemic non-communicable diseases [3,4]. Therefore, creating health policy plans to promote the importance of regular dental checkups among the population is essential.
However, access to dental services such as routine checkups is affected by the health insurance system in each country. In countries with extensive dental insurance policies, the use of insurance coverage contributes to an increase in dental service utilization [5][6][7]. Moreover, studies in several countries have shown that an individual's economic situation is one of the factors associated with access to dental services [8][9][10][11]. That is, even if they perceive the need for a specific dental service, their decision depends on their own income limitations and whether they are willing to spend resources for that dental healthcare service.
The contingent valuation method (CVM) is used for measuring the benefits of healthcare services. This method evaluates the Willingness to Pay (WTP), which is "the maximum amount of monetary value that an individual would be willing to sacrifice to obtain the benefit of that healthcare service," through questionnaires or face-to-face interviews based on a hypothetical scenario regarding the healthcare service [12][13][14]. Several studies 3336 participants were randomly selected from the research company's database of registrants using a quota sampling method based on the Japanese national census population [28]. The distribution of the study participants was divided according to gender (men: 50.3%, women: 49.7%), age group (20-29 years: 15.7%, 30-39 years: 18.3%, 40-49 years: 23.8%, 50-59 years: 21.7%, and 60-69 years: 20.6%), and regional category (Hokkaido region: 4.2%, Tohoku region: 6.8%, Kanto region: 35.9%, Chubu region: 18.0%, Kinki region: 15.9%, Chugoku region: 5.4%, Shikoku region: 2.8%, and Kyushu region: 11.0%), which reflects the representation of the Japanese population [28].
As this study used a web-based survey, all study participants had to answer each question before they could proceed to the next question. Additionally, all participants completed the survey; thus, no missing values were obtained. All questions were asked in the Japanese language. The web-based survey was conducted over a 3-day period, from 12-14 October 2022.
All participants agreed to participate in the study and answered the survey questions. Participants' personal information was protected by Macromill, Inc. [29]. The participants were given points that could be converted into cash. This study was approved by the Research Ethics Committee of Nippon Dental University College in Tokyo before the web-based survey was conducted (9 August 2022, approval No. 293).

Outcome Variable (WTP Values for Dental Checkups)
The outcome variable in this study was the WTP value for dental checkups. WTP values were obtained from the study participants based on the payment card method [12,30]. (The questionnaire is provided in a Supplementary File.) The participants were asked about the maximum amount they would be willing to pay to receive one dental checkup. As a proviso to this question, the following description was provided to the study participants: (1) "Under the Japanese medical insurance system, healthcare services for disease prevention are not covered by insurance. Please answer this question by assuming full payment at your own expense." (2) "'Dental checkups' in this survey refers to a checkup by a dentist to assess the condition of the teeth for the purpose of early detection of dental caries and periodontal disease (radiographs are obtained, if necessary). It does not include scaling of calculus or polishing of tooth surfaces." The study participants were presented with the following range of amounts for their responses: 0 yen, 1000 yen, 2000 yen, 3000 yen, 4000 yen, 5000 yen, 6000 yen, 7000 yen, 8000 yen, 9000 yen, 10,000 yen, 11,000 yen, 12,000 yen, 13,000 yen, 14,000 yen, 15,000 yen, 16,000 yen, 17,000 yen, 18,000 yen, 19,000 yen, and 20,000 yen or more (as of February 2023, 1000 yen = 7.5 USD). These ranges were set based on a previous study [22]. The questionnaire on WTP values for dental checkups was pretested before the actual survey was administered to the study participants.
Participants who responded "0 yen" were given additional questions to determine whether their reason was "true zeros" or "protest zeros" [14]. If the participant answered "The cost of dental checkups should be fully paid by the government, insurers, or other parties" as the reason for choosing 0 yen, this response was defined as a "protest zeros" response because it does not reflect an economic evaluation of healthcare services such as dental checkups [14]. Hence, these "protest zeros" responses were excluded when conducting the statistical analysis.

Explanatory Variables
The explanatory variables were set according to the individual characteristics of the study participants, which consisted of socioeconomic factors and oral health status. Socioeconomic factors included gender, age, household income, employment status, marital status, presence of children, and the municipality of residence. Variables related to oral health status included the number of teeth and frequency of tooth brushing.
The participants' ages were categorized into the following five groups: 20-29 years, 30-39 years, 40-49 years, 50-59 years, and 60-69 years. Household income was categorized into six groups: <2 million yen, 2-4 million yen, 4-6 million yen, 6-8 million yen, ≥8 million yen, and unknown (As of 2020, the average household income of Japanese people was 5.64 million yen, and the median was 4.4 million yen [31].) Employment status was categorized into four groups: regular workers, homemakers, part-time workers, and not working and others. Marital status was categorized as married or single. The presence of children variable was categorized as having children or no children. The municipalities in which study participants resided were categorized into four groups based on the Japanese municipality system: metropolises (ordinance-designated cities with populations of ≥500,000 and the 23 wards of Tokyo), core cities (ordinance-designated cities with populations of ≥200,000), other cities (cities with populations of ≥50,000 excluding metropolises and core cities), and towns/villages (small municipalities that do not meet the specifications of cities).
The number of teeth in the study participants was categorized into three groups: <20, 20-27, and ≥28 teeth. The frequency of tooth brushing was categorized into four groups: ≥3 times daily, twice daily, once daily, and occasional/no brushing.

Statistical Analysis
First, descriptive statistics were calculated for each variable. The outcome variable (WTP values for dental checkups) was used as quantitative data, and the explanatory variables were used as categorical data. In addition, study participants were divided into two groups based on whether they received regular dental checkups (those who received regular dental checkups: RDC group; those who did not receive regular dental checkups: non-RDC group). The criterion for whether or not the participants received regular dental checkups was "whether or not they received dental checkups at least once a year," based on a survey by the Ministry of Health, Labour, and Welfare [32].
Second, to understand the distribution of WTP values for dental checkups, graphs were created for the RDC and non-RDC groups. In addition, the descriptive statistics of the WTP values for both the RDC and non-RDC groups were calculated, and the two groups were compared after excluding the "protest zeros" responses, using the Mann-Whitney U test; this test was used because the WTP values did not follow the normal distribution.
Third, the association between the outcome variable (WTP values for dental checkups) and the explanatory variables (gender, age, household income, employment status, marital status, presence of children, municipality of residence, number of teeth, and frequency of tooth brushing) was evaluated using the Tobit regression model for the RDC and non-RDC groups. The Tobit regression model was used because the WTP values for dental checkups were either zero or more amounts but not negative amounts and because they exhibit characteristics as censored data [33]. In addition, Tobit regression was calculated using robust standard errors. With regard to the inclusion of explanatory variables, univariate and multivariate analyses were conducted after adjusting for all variables. In all analyses, the "protest zeros" responses were excluded.
In this study, Stata version 17 (StataCorp LLC, College Station, TX, USA) was used for statistical analysis. Statistical significance was set at p < 0.05. Table 1 shows the demographic characteristics of the study participants (n = 3336) and the number and proportion of each when divided into the RDC group (n = 1785; 53.5%) and non-RDC group (n = 1551; 46.5%). Note: RDC = regular dental checkups; RDC group = group of participants who received regular dental checkups; non-RDC group = group of participants who did not receive regular dental checkups; * Chi-square test.

Demographic Characteristics of the Study Participants and the Number and Proportion of the RDC and Non-RDC Groups
The Chi-squared test revealed statistically significant differences between the two groups in the following variables: gender, household income, employment status, marital status, municipalities, number of teeth, frequency of tooth brushing (p < 0.001), and presence of children (p = 0.004).

Distribution and Comparison of WTP Values for Dental Checkups between the RDC and Non-RDC Groups
The distribution of WTP values for dental checkups is shown in Figure 1 for the RDC group and Figure 2 for the non-RDC group. Table 2 shows the comparison of the descriptive statistics of WTP values for dental checkups in the RDC and non-RDC groups, excluding responses with protest zeros (RDC group: 22, non-RDC group: 49). In the RDC group (1763 participants), the median was 3000 yen (22.51 USD), the interquartile range was 2000-4000 yen (15.01-30.02 USD), and mean was 3439.6 yen (25.81 USD). In the non-RDC group (1502 participants), the median was 2000 yen (15.01 USD), the interquartile range was 1000-3000 yen (7.50-22.51 USD), and the mean was 2713.0 (20.36 USD) yen. The Mann-Whitney U test revealed a statistically significant difference between the RDC and non-RDC groups (p < 0.001).

Distribution and Comparison of WTP Values for Dental Checkups between the RDC a Non-RDC Groups
The distribution of WTP values for dental checkups is shown in Figure 1 for the group and Figure 2 for the non-RDC group. Table 2 shows the comparison of the de tive statistics of WTP values for dental checkups in the RDC and non-RDC group cluding responses with protest zeros (RDC group: 22, non-RDC group: 49). In the group (1763 participants), the median was 3000 yen (22.51 USD), the interquartile was 2000-4000 yen (15.01-30.02 USD), and mean was 3439.6 yen (25.81 USD). In th RDC group (1502 participants), the median was 2000 yen (15.01 USD), the interqu range was 1000-3000 yen (7.50-22.51 USD), and the mean was 2713.0 (20.36 USD) ye Mann−Whitney U test revealed a statistically significant difference between the RD non-RDC groups (p < 0.001).   Note: RDC = regular dental checkups; RDC group = group of participants who received r dental checkups; non-RDC group = group of participants who did not receive regular dental ups; Calculated excluding answers with protest zeros; * Mann−Whitney U test.

Distribution and Comparison of WTP Values for Dental Checkups between the RDC a Non-RDC Groups
The distribution of WTP values for dental checkups is shown in Figure 1 for the group and Figure 2 for the non-RDC group. Table 2 shows the comparison of the de tive statistics of WTP values for dental checkups in the RDC and non-RDC group cluding responses with protest zeros (RDC group: 22, non-RDC group: 49). In the group (1763 participants), the median was 3000 yen (22.51 USD), the interquartile was 2000-4000 yen (15.01-30.02 USD), and mean was 3439.6 yen (25.81 USD). In th RDC group (1502 participants), the median was 2000 yen (15.01 USD), the interqu range was 1000-3000 yen (7.50-22.51 USD), and the mean was 2713.0 (20.36 USD) ye Mann−Whitney U test revealed a statistically significant difference between the RD non-RDC groups (p < 0.001).   Note: RDC = regular dental checkups; RDC group = group of participants who received r dental checkups; non-RDC group = group of participants who did not receive regular dental ups; Calculated excluding answers with protest zeros; * Mann−Whitney U test.  Note: RDC = regular dental checkups; RDC group = group of participants who received regular dental checkups; non-RDC group = group of participants who did not receive regular dental checkups; Calculated excluding answers with protest zeros; * Mann-Whitney U test.

Association between WTP Values for Dental Checkups and Study Participants' Individual Characteristics in the RDC and Non-RDC Groups
The multivariate Tobit regression model demonstrated the association between WTP values for dental checkups and characteristics of the study participants for both the RDC and non-RDC groups (Tables 3 and 4) (The results of the univariate Tobit analysis are shown in Supplementary Tables S1 and S2).    Regarding the WTP values for dental checkups in the non-RDC group (Table 4)

Major Findings of This Study
Using a nationwide web-based survey, the WTP values for dental checkups in the RDC and non-RDC groups were ascertained and analyzed to assess their association with the study participants' individual characteristics. As a result, two major points were revealed.
First, the median WTP value for dental checkups was 3000 yen ( [20.36 USD]) in the non-RDC group, which was a statistically significant difference between the two groups.
Second, age 50-59 years, lower household income, homemaker and part-time worker employment status, and having children were significantly associated with lower WTP values, while male gender, higher household income, and tooth brushing ≥3 times daily were associated with higher WTP values. In the non-RDC group (Table 4), age ≥30 years, lower household income, and presence of ≥28 teeth were significantly associated with lower WTP values, while higher household incomes were associated with higher WTP values.
Therefore, the results of this study suggest that the WTP values for dental checkups were lower in the non-RDC group than in the RDC group, and socioeconomic factors were associated with WTP values in both groups.

WTP Values for Dental Checkups in the RDC and Non-RDC Groups
Although many WTP studies have reported on dental treatment [15][16][17][18][19][20], few have focused on receiving dental checkups [21,22]. A previous study [22] related to the results of this study showed that the WTP values for dental checkups were evaluated by targeting patients in dental clinics, with a median WTP value of 2000 yen (mean: 2252.6 yen) for regular visitors and a median WTP value of 2000 yen (mean: 2124.9 yen) for infrequent visitors. The results of this study were different from the findings of the previous study; however, they are not simply comparable because the previous study surveyed patients visiting dental clinics, whereas, in this study, the sample was recruited from the general population that approximates the Japanese population, using the quota sampling method and conducting a web-based survey. Therefore, the results of this study are the first to determine WTP values for dental checkups in the general population nationwide and can be expected to contribute to health policy planning.
This study found that the RDC group responded with a higher value than the non-RDC group regarding the maximum amount they could pay for a dental checkup (RDC group: median 3000 yen, mean 3439.6 yen; non-RDC group: median 2000 yen, mean 2713.0 yen). Several previous studies have suggested that those who habitually receive regular dental checkups have an increased awareness of oral health [34,35]. Therefore, the results of this study may also have been influenced by the fact that the RDC group gave more importance to receiving dental checkups to maintain their oral health than the non-RDC group. Another possible factor affecting the WTP value is its association with household income, as described in Section 4.3.

Association between WTP Values for Dental Checkups and Individual Characteristics in the RDC and Non-RDC Groups
There was a positive correlation between WTP values and household income in both the RDC and non-RDC groups. Those with lower household incomes were likelier to report lower WTP values for dental checkups. Of particular note is that in the non-RDC group, associations were observed in a wide range of age groups over 30 years. In addition, the non-RDC group responded with significantly lower WTP values for dental checkups than the RDC group. Several previous studies have shown that income limitations are a barrier to regular dental attendance [8][9][10][11], and the results of this study support these previous findings from the perspective of economic evaluation of dental checkups. Based on these findings, it can be implied that compared to the RDC group, there is a limitation to the maximum amount that can be paid for dental checkups within a wide age range in the non-RDC group and that this may be associated with economic background factors; hence, this suggests the need for policy interventions.
Further, homemakers, part-time workers, and those with children responded with lower WTP values only in the RDC group. Even if these participants were in the habit of receiving regular dental checkups, they might be limited in the amount of cost they can spend on dental checkups; therefore, it is possible that they reported low WTP values. Moreover, the results of this study showed that men had higher WTP values for dental checkups than women. Previous studies have shown that women are more likely to have an increased awareness of oral health than men [11,36]. However, according to a report by Japan's Ministry of Health, Labour and Welfare, the income of working men is about 2.1 times that of working women [31], and the reason for this is reportedly due to the differences in employment positions and length of service between men and women [37]. Therefore, it is possible that men in the RDC group answered that they could afford to spend on dental checkups because of their financial stability rather than because of their awareness of oral health.
Regarding oral health conditions, in the RDC group, those who brushed their teeth ≥3 times daily reported higher WTP values. This result suggests that participants in the RDC group had a high awareness of oral health [34,35] and identified the importance of dental checkups. In contrast, in the non-RDC group, those with ≥28 teeth had lower WTP values. The reason for this may be that they have a full set of teeth and have no trouble chewing; therefore, they have little awareness of the need to protect their oral health. However, this causal relationship remains unclear and requires further investigation.
Regarding the municipality in which the study participants resided, there was no statistically significant association in either the RDC or non-RDC group. Generally, there are reportedly more barriers in rural areas than in urban areas with regard to access to dental services [10,38]. However, there is reportedly little inequality in the geographic distribution of the number of dental clinics in Japan [39]. That is, there are fewer differences in barriers to access to dental services between rural and urban areas; as a result, there may have been less impact on WTP values for both groups in this study.

Implications for Health Policy of This Study
Under the Japanese medical insurance system, most dental treatments, such as caries treatment, endodontic treatment, periodontal disease treatment, and prosthetic treatment, are covered by insurance. However, preventive practices such as dental checkups are not covered by insurance [23]. The results of this study showed that in both the RDC and non-RDC groups, those with a lower household income were more likely to report a lower maximum amount they could pay for dental checkups. This result raises concerns, particularly among the non-RDC group, who may face barriers to accessing dental services due to economic reasons.
Universal Health Coverage (UHC) is one of the Sustainable Development Goals (SDGs) advocated by the United Nations in 2015 [40], and the Japanese medical insurance system may be considered to be achieving UHC. However, several Japanese studies have suggested that income limitations may affect access to dental services [8,11]. Therefore, the establishment of a system that allows people to receive dental checkups without copayment, using public funds and other financial resources, may result in improved access to dental services [41,42]. It is necessary for policymakers to plan health policies that consider socioeconomic factors, such as people's incomes, to ensure equality of oral health status.

Limitations of This Study
This study has several limitations. First, in WTP studies, there are several methods to obtain WTP values from study participants, and each method has its advantages and disadvantages [12,14]. This study used a payment card as the appropriate method because a web-based survey was used to obtain the WTP values for dental checkups from study participants on a nationwide scale. Therefore, this method may have had range bias since the study participants' choices were bound by the amount in the payment card they were presented with [30]. Second, although the study sample approximated the Japanese population using a quota sampling method, sampling bias cannot be completely ruled out because the study participants were selected from among those registered with a web-based survey company. Internet usage among the Japanese is increasing [43]; however, the possibility of sampling bias remains a concern in web-based surveys [44]. Third, this was a cross-sectional study conducted using a web-based survey for the study participants. Although this study revealed the individual characteristics of the study participants associated with high/low WTP values in both the RDC and non-RDC groups, it was not possible to determine a causal relationship between their factors due to the cross-sectional design of the study.

Conclusions
Based on the results of this study, the null hypothesis stated in the objective was rejected, and the following conclusions were obtained: (1) the WTP values for dental checkups were lower in the non-RDC group than in the RDC group, and (2) there was a significant association between high/low WTP values and socioeconomic factors in both groups; in particular, in the non-RDC group, those with a lower household income aged ≥30 years were more likely to propose lower WTP values for dental checkups.
Hence, this result suggests the need for policy intervention to improve access to regular dental checkups.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/ijerph20054145/s1. Supplementary File S1: Questionnaire provided to the study participants; Table S1: Association between WTP values for dental checkups and study participants' characteristics in the RDC group (univariate tobit regression analysis); Table S2: Association between WTP values for dental checkups and study participants' characteristics in the non-RDC group (univariate tobit regression analysis; Supplementary File S2: STROBE Statement-Checklist of items that should be included in reports of cross-sectional studies.