This study employed a random stratified cluster sampling technique. In theory, clusters of children should be as heterogeneous as possible and each cluster of children should be, to some extent, representative of the general child population. In this survey, the study children were chosen based on kindergartens, and the use of this sampling technique helped to reduce the time and cost associated with the survey. However, the background of the children within a cluster could be fairly homogeneous while the clusters might vary from one another. Therefore, the variance of this cluster sample with a defined sample size may be larger than that of a simple random sample, and hence the estimates are less precise. Despite this, this study achieved a very high response rate from the children and their parents. This is partly because initial contacts were made three months in advance to allow ample time for the selected kindergartens to prepare. There were pre-survey visits to the kindergartens and invitation letters, consent forms, and questionnaires were delivered to parents through the kindergartens and sufficient time was provided for collection. In addition, there was sufficient training of and calibration exercises for the examiners before the survey, leading to a very good inter-examiner agreement.
The mean dmft score of the children surveyed in this study, 2.2, is similar to those of the 5-year-old children in previous surveys conducted in 1997 and 2001, which are 1.8 and 2.3, respectively [8, 12]. There is no clear trend of change in the dental caries status of the preschool children in Hong Kong over the past decade. The findings in all the surveys that over 90% of the decayed primary teeth in the Hong Kong preschool children were untreated indicate that there has been no significant improvement in their access to and use of proper dental care services.
The dental caries experience of the preschool children in this survey is higher than that of the preschool children in a recent survey conducted in Singapore which found a mean dmft score of 1.5 . It should be noted that Hong Kong and Singapore are similar in term of economic development and that both cities have implemented water fluoridation. The mean dmft score of the 5-year-old children in recent survey conducted in Taiwan was 7.0  and that found in the second national oral health survey conducted in Mainland China was 4.5 . When compared to these figures, the dental caries experience of the Hong Kong preschool children is much lower.
In the United Kingdom where the National Health Service provides free comprehensive dental care for preschool children, a mean dmft score of 1.6 for 5-year-old children was reported in 2003 . In New South Wales in Australia where there is a health program for preschool children which incorporates components of oral health education, risk assessment and clinical care, a mean dmft score of 1.5 was reported .
There are a number of meaningful observations in this study. Dental caries starts at an early age among the preschool children in Hong Kong, as shown in the finding that two fifths of the surveyed children at age four had dental caries, and becomes more severe over time. There is a need to implement dental caries prevention measures at early age and to reinforce them continuously. Different oral health promotion measures may be needed because early childhood dental caries is a multi-factorial disease .
The observed dental caries pattern in this study sample agrees with the common observations and general theories . The highest prevalence of caries was found in the maxillary incisors. This can be explained by the longer duration of exposure to cariogenic challenge according to the chronological tooth eruption sequence. It has been shown that the prevalence of caries in posterior primary teeth is higher in children who have caries in their maxillary anterior teeth . This was also found in this study among the preschool children in Hong Kong. Presence of caries in maxillary primary anterior teeth should be noted during dental examination of young children and used as an indicator for high caries risk in the posterior teeth, and these children should receive more preventive dental care.
Results of this study show a correlation between dental caries of the Hong Kong preschool children and their snacking habit which is in agreement with those of other studies [8, 17]. A high frequency of sugar intake prolongs the duration of lowered pH in the mouth which leads to a higher demineralization rate resulting in dental caries . Thus, the control of frequent sweet snack taking in young children is important in preventing dental caries.
Findings of this study show that toothbrushing is another important factor affecting dental caries in young children. The surveyed children who had their teeth brushed at an early age had less dental caries. This has also been found in earlier studies conducted in Ireland  and in China . It should be noted that eruption of primary teeth into the mouth starts before 12 months of age and continues till 2–3 years. Practice of toothbrushing can help to keep the erupted teeth clean as well as to deliver fluoride onto the tooth surfaces if fluoridated toothpaste is used. Promotion of parental and self toothbrushing in young children should be carried out in the maternity and child health centres in Hong Kong where health care services for infants and health education for parents are delivered.
The finding that among the preschool children surveyed in this study, those who had visited a dentist had more dental caries experience needs some explanation. A possible reason is that for most of the young children Hong Kong, they are only brought to see a dentist when they have dental problems such as tooth decay. This may be related to the prevalent problem-oriented dental care seeking behavior among the adults in Hong Kong . To change this pattern of dental care seeking behavior, more effort should be made to promote early and regular preventive dental visits among young children. The establishment of a public or a subsidized dental service for the preschool children in Hong Kong would probably help to improve the situation.
Another group of factors which have significant associations with the study children’s dental caries experience is their demographic and socio-economic background. Similar to the findings of previous survey of preschool children in Hong Kong , children born in Mainland China had more dental caries than those born in Hong Kong. Since there has been a recent sharp increase in the number of new Chinese immigrant children in Hong Kong (Census and Statistics Department, ), it is likely that the dental caries situation of the preschool children in Hong Kong will become worse if no actions are taken.
Similar to other studies on young Chinese children [24, 25], this study found the parents’ dental knowledge to have a significant association with their child’s dental caries experience. In this study, less than 40% of the parents could correctly answer two thirds or more of the dental knowledge questions in the questionnaire. Quite a large proportion of the parents in Hong Kong may have a variety of misconceptions on matters relating to the dental health of their young children. Dental preventive measures for preschool children would probably be more successful when all three parties concerned, namely children, teachers and parents, cooperate with one another. Oral health promotion programs should include educating the parents so as to improve their dental health knowledge. Hence, besides involving the kindergarten teachers, more effort should be paid to involve the parents as well in oral health programs for preschool children in Hong Kong.
Social inequality in dental health among children exists in many countries [26–28]. This was also found in this study among the preschool children in Hong Kong. Children from the lower socio-economic classes as indicated by a lower family income and lower parental education level had more dental caries. The finding that the study children who were taken care of by domestic helpers had less dental caries may be because these children came from wealthier families that could afford to employ domestic helpers. To improve social equality in oral health, the Hong Kong government working with the dental profession should allocate more resources for providing oral health promotion activities and services for the young children who are less privileged so as to improve their dental health-related behaviours and conditions.