This study investigated the caries prevalence and severity trends in Nevada youth using both the mean DMFT index and the SiC Index. The use of DMFT has been an accepted practice for assessing the prevalence and severity of caries in a population . However the epidemiologic changes in the dental caries picture during the last 2-3 decades, have made it increasingly evident that mean DMFT values do not capture the polarized caries development with a more skewed distribution of caries . Mean DMFT values are an average of all members of the population, irrespective of the distribution of the severity of the disease within the population.
Mean DMFT Trends
Eight years of oral health screening data were collected through the Oral Health Surveillance Initiative across the state of Nevada, with over 62,000 adolescents participating. The mean DMFT from these data, compared to national NHANES data confirmed that dental caries remains a common chronic disease among Nevada youth, and that Nevada youth present with higher prevalence rates and greater mean DMFT indices than the national average (Table 2) . Where the NHANES study reported a prevalence of around 51% among the younger group and around 67% in the older group  we found 65% and 77% respectively (Figure 2). Furthermore, because this sample was assessed using a modified protocol, data from this study may be an underestimate of caries prevalence compared to NHANES data. Improvements were found in mean DMFT scores across all demographics compared (age, sex, race group, whether residing in fluoridated area, and dental insurance status) between year 1 and year 6 with the exception ofyear 3. However since year six, there has been a trend towards more caries and less cariesfree individuals in all demographics (Figures 1, 2, 3). Although dental caries is largely preventable, it remains the most common chronic disease of children aged 6 to 11 years (25%) and adolescents aged 12 to 19 years (59%) [4, 18, 19]. Additionally, certain segments of the population (e.g., members of racial or ethnic minority groups, sex, and older children) have more dental decay, much of which remains untreated [18, 20, 21].
SiC Index Trends
The same eight year data set was used to compute the SiC Index. As expected, the SiC Index was significantly higher in all comparisons with DMFT (p < 0.001). A comparison between the two indices, indicated there is a large Nevada youth population subgroup that presented with a significantly higher caries rate than the targeted mean DMFT score of 3.0 . The Mean DMFT score for Nevada youth, although higher than the national average and in some of the subgroups higher than the targeted 3.0 mean DMFT, demonstrates how skewed the present caries problem is and does not reflect the true extent of caries prevalence or severity in all subgroups of the population. In fact, the mean of the highest caries scorers is close to three times the population mean. Recently, Sheiham et al.  have pointed out that caries patterns seem to exist in subgroups of a population with those groups remaining at a stable relative position to each other even when prevalence changes over time. This pattern seems to be reflected in this study. It is of note that the present population is in fact 8 populations of the same age studied over eight years sequentially. In no instance is any specific sub-group found to be veering away from its pattern of dental caries or crosses into the pattern of another group. For instance, the younger group remains at the lower level of dental caries and even improves through increasing rates of freedom of caries. With regard to racial background, the white examinees remain at the lowest caries level of the racial groups.
The use of the SiC Index that includes DMFT can elucidate interpretations of findings, especially in situations where resources are limited for interventions. While mean scores provide a good measure of population disease levels, it is important to also look at those who might be carrying a significant burden of the dental disease experience in the population. To help in identifying high risk groups, it is recommended to calculate the SiC Index at several levels. It has been suggested that a SiC goal be established so that public health professionals can have 2 goals: mean DMFT and SiC Index. One  suggested a SiC of less than 3.0 in 12-year-old children as a global oral health goal to be achieved by the year 2015, while another suggested that a SiC global goal of less than 5.0 be set for 15-year olds rather than the WHO targeted age of 12-year olds .
Interpretation of Trends
Downward trends were found in both Mean DMFT scores and SiC Indices across all demographics compared (age, sex, race group, whether residing in fluoridated area, and dental insurance status) between year 1 and year 6 with the exception of year 3. Althought these trends were parallel, the SiC Indicies were significantly higher than the mean DMFT Index across all years. Since year six, there has been an upward trend in all demographics (Figures 1, 2, 3). Data showed that minority children had higher prevalence of caries. Research demonstrates that minority children are more likely to experience tooth decay and have their cavities untreated . Because children of color are the fastest growing subpopulation of children in the U.S., their higher caries experience predicts an upturn in disease prevalence over the coming years unless special efforts are made to address their oral health needs .
Community water fluoridation has been ranked as one of the ten great public health achievements of the 20th century [18, 24, 25]. Healthy People 2010 objectives seek to eliminate health disparities ensuring that all Americans receive the benefits of good oral health. Community-based programs, such as community water fluoridation are cost-effective ways to achieve this goal [24, 25]. This study found that those children living in communities with fluoridated municipal water supplies experience substantially lower mean DMFT scores. This has special importance in Nevada where attempts to expand the fluoridation program to counties other than Clark County have met with considerable resistance.
Inadequate access to dental care for children of low-income families may be largely due to lack of dental insurance . In one study, subjects without dental insurance were 20-40% more likely to present with higher mean DMFT indices than those with insurance . Despite improvements in children's oral health through prevention, dental caries remained the most common chronic childhood disease in the US during the twentieth century .
A report released by the CDC  in 2002 reported a 15.2% increase in disease among the nation's youngest children ages 2 through 5 years. Because tooth decay in the primary teeth predicts future tooth decay in permanent teeth, the upturn in caries experience in preschoolers could be expected to continue in permanent teeth. A recent epidemiologic review found that in many countries there is a marked increase in dental caries prevalence that affects both children and adults . This could partially explain the upward trend in caries prevalence observed in this study from early 2000. Overall this study found that older adolescents, those of racial groups, those who live in non-fluroidated areas, and those without dental insurance all experienced higher mean DMFT scores in all years included in this report. The trend line for each of the comparison groups was similar (Figures 2, 3, 4). With reference to the previous part of the discussion we would expect that the relative stability of the subgroups in a special caries pattern could be used to conduct an early identification of the children with the highest caries prevalence and incidence, because they would be considered a special risk group for ending up in late adolescence with considerably higher caries rates than the remainder of the population.
Limitations and Future Recommendations
Self-reports warrant some caution in interpreting those data. However, data collection and entry protocols were well documented and quality control guidelines were implemented during the oral health screening process throughout the time span reported. Due to confidentiality issues, students could not be tracked over time preventing longitudinal data collection, therefore to help strengthen the data; analysis of cross sectional data across all years by school was examined to reduce the likelihood of repeat students. Since the sample was so large, assumptions could be made that all would have access to similar secondary fluoride influence, for example all would have access to fluoride toothpaste, as well as water that contains fluoride. Inclusion of these other potential sources of fluoride supplementation may influence these results. While this study focused on comparisons of results using traditional calculations of mean DMFT and the Significant Caries Index, it doesn't make comparisons between those who are cariesfree and those with the highest reported mean DMFT. Future reports may examine these differences for more in-depth interpretations.