THE NEWBURGH STUDYThe fluoridation trial conducted in Newburgh differs from the other studies in two important ways:
1. In almost all the comparisons made, the data obtained were compared with those from Kingston, the "fluoride-free" control city, instead of the method used in the other trials, by which most comparisons were made between the initial and the latest observations in the test city.
2. The caries attack rates were stated per 100 erupted teeth, instead of per 100 children or per child. The Evanston study was the only other one in which the caries rate per 100 erupted teeth was published; Hill et al. in 1955 and 1957a showed this rate, but only for children aged twelve to fourteen years.
The control city. Kingston was used as the control area. "Both cities are situated on the Hudson River about 30 miles apart. Each has a population of approximately 30,000. The climate of both cities is also similar, and their water supplies at the outset of this study were comparable and have remained so, except for the addition of sodium fluoride to Newburgh's supply" (Ast et al., 1950). Ast and Chase (1953) added the information that the two cities had a "comparable age, sex, and color distribution"; and Schlesinger, Overton and Chase (1950) mentioned that they "bore a close resemblance to each other in respect to size and socio-economic conditions".
Late examination of control city. In Kingston, as in the other "fluoride-free" control cities that have been considered, the basic examinations were not made until after the fluoridation of the water supply of the test city. Fluoridation was started in Newburgh on 2 May 1945 (Ast et al., 1950), but the examinations in Kingston were not conducted until "Sept., 1945 - Feb., 1946" (Ast et al, 1950).
Considerably different composition of waters. In 1950 Ast et al., stated that the water supplies of Newburgh and Kingston "at the outset of this study were comparable and have remained so, except for the addition of sodium fluoride to Newburgh's supply." However, both the source and the composition of the water supplies of these two cities are different. The United Kingdom Mission (1953) stated that the source of Newburgh's water is from "surface water. Algae growths in spring and summer checked by copper sulphate blown on the surface of the water as a powder." The source of Kingston's supply was described as "Mountain spring impounded. Auxiliary supply, small spring reservoir" (Lohr and Love, 1954).
In regard to the composition and other characteristics of these waters, according to analyses of the finished waters made in February 1952 by the U.S. Geological Survey (Lohr and Love, 1954), in each of the ten items - magnesium, sodium, potassium, bicarbonate, sulphate, chloride, dissolved solids, specific conductance, hardness and alkalinity - the values for the Newburgh water were at least four times as great as those obtained from analysis of the Kingston supply. In the very important matter of the calcium content, the Newburgh value of 35 ppm (Ca) was more than five times as large as that of the Kingston one of 6.6 ppm (Ca). Changes in the supplies during the period of the trial, owing to natural or to treatment-chemical variations, are unlikely to have affected these gross differences more than slightly.
The considerably different calcium and magnesium content and hardness of the water supplies of Newburgh and its control city of Kingston, February 1952.
Eight other characteristics of the Newburgh water were at least four times as large as they were in Kingston.
The authors of this study stated that these waters "at the outset of this study were comparable and have remained so" (Ast et al., 1950).
An unsatisfactory control. In proposing this study, Ast (1943) said: "Much care must be exercised in the selection of study areas which should be comparable in as many essential factors as possible." The first of these factors which he mentioned was the "chemical composition of past and present water supply". Therefore it is surprising that Kingston was selected as the control city for Newburgh, for it is clear that in this very important matter the two cities showed considerably different values. The importance of the close comparability of the water supplies was emphasized by the statement of the American Water Works Association (1949) that the experimental verification of the fluoride-dental caries hypothesis "obviously necessitates the use of a nearby "control" city with a water supply comparable in all respects to that to which fluoride is being added."
Variations in methods used. An outstanding characteristic of this study is the variation in the methods used, both in gathering the data and in the presentation of the results. There were changes in the examiners; on some occasions clinical examinations only were made and on others X-rays were also used. The statisticians changed, as did their presentation of the data in age groups. The sampling method varied in regard to residence qualifications, and changes occurred in the age range of the children who were examined. In one report data was obtained from selected schools only. In some examinations the sampling method was different in the control city from that used in the test one, All these matters will now be considered more fully.
The dental findings. These were published in five papers. Ast, Smith, Wachs and Cantwell, in 1956, said: "Progress reports were published after three, four, six and eight years of fluoride experience in Newburgh" (Ast et al., 1950, 1951; Ast and Chase, 1953; Ast et al., 1955). The last-mentioned report (Ast et al., 1955) "after eight years of fluoride experience" gave the results obtained during the examinations of 1953-4. The final report, giving the results for 1954-5, apparently one year later than those in the fourth dental report, was said to show the "dental findings after ten years of fluoride experience" (Ast et al., 1956). However, as fluoridation in Newburgh commenced on "May 2, 1945", and as the examinations given in the final report were made "between October 1954 and June 1955" (Ast et al., 1956), it would appear that, at the most, only a small part of the data of the final examination was obtained "after ten years of fluoride experience."
Different examiners used. The initial examinations in both cities were made by Finn. "The subsequent examinations in Kingston using the same technic were made by two dental hygienists" (Ast et al., 1950). The examinations in 1951-2 were conducted by two examiners, but "Due to loss of one of the examiners during the examination year, it was deemed advisable to use only those examinations made by the remaining examiner in both cities" (Ast and Chase, 1953). The clinical examinations in 1953-4, and the final ones, were made by Wachs (Ast et al., 1955, 1956). These changes were made despite the fact that in 1943 Ast said that "the examinations throughout the study should be made by the same dentist because of the marked variation in diagnosis of small carious lesions, pits, and fissures by different dentists."
The clinical examinations were supplemented by the use of X-rays in the years 1949-50, 1953-4 and 1954-5 (Ast et al., 1956). In the first of these, which was confined to children aged seven, nine and eleven years, the X-rays were taken by a staff dentist and were read by Ast and Finn (Ast et al., 1951). The next series was taken by Wachs and was read by Bushel (Ast et al., 1955); the final X-rays were taken by Wachs and a staff hygienist, and they were read by Wachs and Smith (Ast et al., 1956).
Non-comparability of data. In the last two reports (Ast et al., 1955, 1956), the carious cavities that were detected by the X-ray were added to those found in the clinical examinations. Ast et al. in 1955 said that "the data in this report cannot be compared directly to those earlier data based on clinical examinations alone." However, in Table 3 of the 1956 report, the results of the clinical examination are shown separately, but a satisfactory comparison with those obtained in the earlier years is prevented by the fact that in this report the data were not published for yearly age groups, but for the age ranges six to nine and ten to twelve years. Data for the other two age groups which were shown in the final report, thirteen to fourteen and sixteen years, were not published in the previous ones.
The rates for the deciduous teeth were given in only one report (Ast et al., 1951).
Examiner variability. The between and within-examiner variability was not investigated, although, early in the study, the importance of this matter was recognized by Ast et al. (1950) when they stated: "We cannot entirely rule out the possibility of variation in the interpretations of the examiners. The fact that more than one examiner was used might alter the differences between Newburgh and Kingston to some extent." In the following year (Ast et al., 1951) it was stated: "In the present report an attempt is made to demonstrate that through an objective roentgenographic examination of the teeth of selected age groups, the question of examiner bias in this study is not likely to account for the differences noted." However, the only data published were those of the first permanent molars; and the finding that "the DMF roentgenographic findings of the first permanent molars only" in selected age groups shows "consistent differences at each age in favor of Newburgh" does not provide an estimate of examiner variability such as could have been obtained readily by normal statistical methods.
In addition to the changes in the examiners and in the examination methods, there were changes in the statisticians. The report after three years of fluoridation was made in collaboration with one statistician; those after four, six and eight years with a different one; and a third statistician was employed in the preparation of the final report.
Different adjustment procedures. In most of the tables in this study a "Crude rate" and an "Adjusted rate" are shown. The incongruity of making these small adjustments to rates that were obtained by combining data from children of considerably different ages does not appear to have been realized. In some cases even data from children aged between six and twelve years were added (Ast et al., 1950, 1951; Ast and Chase, 1953), the great increase in the caries attack rate between those ages being ignored. The adjustments were made (depending on the type of data) to the tooth population, the first permanent molar population, or the distribution of children. In the first three reports of dental findings (Ast et al., 1950, 1951; Ast and Chase, 1953), they were all made to the appropriate situation in Kingston during the 1955-6 examinations, but the adjustment system was then changed, the crude rates after eight years of fluoridation being adjusted to the situation in Kingston in 1953-4 (Ast et al., 1955), and those shown in the final report to that present in the control city in 1954-5 (Ast et al., 1956).
Variations in age groups. In discussing the Evanston study, it has already been pointed out that the method of combining the results of different age groups may result in "weighting" the data, so that comparisons between the test and the control cities may be affected. In the examples given >from other fluoridation trials in which this method was used, the age groups were consistent from examination to examination; but in the Newburgh-Kingston study the groups varied between examinations, between comparisons made from data obtained during the same examinations, and even the age range of the subjects inspected varied from time to time. In regard to the DMF rate per 100 erupted permanent teeth, the groups were as follows: 6-7, 8-9,10-12 (Ast et al. 1950); 6, 7, 8, 9, 10, 11 and 12 (Ast et al., 195 1; Ast and Chase, 1953); 6, 7, 8, 9 and 10 (Ast et al., 1955); and 6-9, 10-12, 13-14 and 16 years (Ast et al., 1956).
Changes in the age groups were also made in reporting the other data presented in this study, but in many cases the groups were different from those which have just been mentioned.
Grouping of data hinders comparisons. In the final report, Ast et al. (1956) said: "The data are combined for six to nine year old children because these children in Newburgh had used fluoridated water throughout their lives"; and the age groups ten to twelve years and thirteen to fourteen years were associated with the tooth calcification pattern. No explanation has been found for the grouping used by Ast et al. in 1950, but this matter will be considered later.
Whatever may have been the reason for adding the data of children of different ages, it has the unfortunate result of making it very difficult to compare the rates which were present in the test (and in the control) city at different stages of the trial, especially as, in the 1955 report of Ast et al., the rates obtained from the clinical examinations were not shown separately from those computed from the combined clinical and X-ray results.
"Weighting". Even if the explanation advanced by the authors of this study is considered to be a reasonable one, there remains the danger of "weighting" the data by combining into one category such divergent material as is provided by children of different yearly ages. One of the tables in which obvious "weighting" is seen is Table I of the first report (Ast et al., 1950), "weighting" being present in several different forms. In the control city, the total DMF rate per 100 teeth (ages six to twelve years) is "weighted"; for the total number of teeth examined is made up (in 1945-6) of only 11 per cent from the six to seven years age group, with its comparatively low DMF rate, and of 67 per cent from the ten to twelve years group with its comparatively high rate (22 per cent was from age eight to nine years). In the latest examination shown in that table (1947-8), the two percentages were 17 and 59 respectively, so that the comparison between the results of the two examinations is also "weighted". Similar instances of "weighting" are also seen in the data >from the test city; but as these are of a different degree, the comparison between Newburgh and Kingston is another instance of "weighting" (Table 1, Ast et al., 1951). It can be seen that some "weighting" occurred within the age groups used in the baseline examinations, principally in the eight to nine years group in both cities.
Fewer erupted teeth than expected. In the final report (Ast et al., 1956), from Table I it can be calculated that the number of erupted permanent teeth in the six to nine years group in Newburgh was less than the number expected, on the assumption that the mean age of eruption of each type of tooth was the same as in the children in Kingston. Also, in the ten to twelve years group (by assuming that in these children at least the eight incisors and the four first molars would have erupted) the number of erupted permanent canines, bicuspids and second molars was fewer in Newburgh than would be expected. Statistically speaking, both these differences are highly significant.
Delayed eruption or "weighting"? These results could have arisen by there being a delay in the eruption of these teeth in the Newburgh children, for it is unlikely that the eruption rate altered in the Kingston subjects. However, Ast et al., (195 1) said that "there does not seem to be any change in the eruption pattern among the children in Newburgh, the study city, as compared with those in Kingston, the control city." No definition of an "erupted tooth" was given, but it is presumed that the authors of this study did not adopt the odd method used in Evanston, where "Only teeth which were 50 per cent or more erupted were considered. A carious or filled tooth was, of course, considered regardless of its stage of eruption" (Hill et al., 1955).
The conclusion of Ast et al. that has just been mentioned was reached only four years after the commencement of fluoridation, and as the teeth considered were partially formed prior to the commencement of that process, they would not show effects which the ingestion of fluoridated water may produce on the early stages of tooth development.
If delay in eruption did not occur in Newburgh, the lower number of permanent teeth present at those ages in that city compared with that present in Kingston was due to a "weighting" effect; there having been, in proportion, more young children in each of these age groups in Newburgh than there were in Kingston. If this is the case, as it is reasonable to assume that the mean DMF rates of the younger children were lower than those of the older ones, it would appear that in these age groups the contrast between the DMF rates in Newburgh and those in the control city was exaggerated in the final report.
"Smoothing" of initial rates. In 1951 Ast et al. reported that the "initial clinical examinations made in Newburgh and Kingston in 1944-1946 were made by one examiner, at which time the DMF rates were the same." However, reference to Table 2 in that paper shows that the DMF rates per 100 erupted permanent teeth were, in Newburgh and Kingston respectively, at age six years, 8.5, 7.2; age seven years 11.7, 12.0; age eight years 17.1, 17.3; age nine years 21.2, 18.9; age ten years 21.9, 21.3; age eleven years 21.8, 21.8, and age twelve years 25.3, 25.4. Also, Table 5. which shows the DF rates per 100 deciduous teeth present, gives the rates in Newburgh and Kingston respectively as 27.2, 21.5 at age five years; 34.2, 32.1 at age six years; 42.3, 43.3 at age seven years, and 48.0, 47.2 for the eight-year-old children. Data for the DF rates of the deciduous teeth of older children were not provided.
In the first report of this study (Ast et al., 1950) no results were given for the deciduous teeth, and the results for the permanent ones were presented in three age groups, six to seven, eight to nine and ten to twelve years. It can be seen that by adding the data from children aged six years, in whom the DMF rate in Newburgh was higher than that in Kingston, to those of the seven-year-old children, in whom the reverse situation was present, the divergence between the rates prevalent in the two cities was reduced. In Table I (Ast et al., 1950) the combined rate was shown as 10.7 in Newburgh and 10.8 in Kingston. In a similar manner, the addition of the data for eight and nine-year old children and those of children who were ten, eleven and twelve years of age produced a levelling effect between the rates of the two cities in these two combined age groups. This process of combining data from children of different ages, when reporting the DMF rate per 100 erupted permanent teeth, although it was employed in only the first and the last dental reports, may have been used in order to simplify the presentation of the data; but it had the unfortunate effect of disguising differences between the DMF rates in the two cities at the time of the basic examinations. In the next report the situation was stated more accurately, Ast and Chase (1953) saying that "the DMF rates in both cities were approximately the same at the start of the study".
Fluctuations in the control city. In this, as in other studies, it is found that the comments made in the text tend to underrate the changes that took place in the dental caries attack rates in the control city. In the summary of the paper by Ast et al. (195 1) it was stated that "the DMF rates in the control city of Kingston show no changes." In that paper, Table 2 shows the DMF rates per 100 erupted permanent teeth; in Kingston the "per cent change" in the rates of the four age categories six, seven, eight and nine years were 30.5, 7.5, 0.6 and 9.5 respectively. Small changes were shown for ages ten, eleven and twelve years. It should be noted that the six, seven and eight-year-old children all showed decreased rates between 1945-6 and 1949. No attempt was made to explain these decreases, and the water of Kingston "remained fluoride deficient throughout the study period" (Ast et al., 1956).
Fluctuations disguised. The method used by Ast et al. in 1951 was to compute the mean DMF rate per 100 teeth in all the children aged six to twelve years; the Kingston rate for this combined age group declining slightly from 20.2 to 19.9 between 1945-6 and 1949. However, by adjusting to the "permanent tooth population in Kingston 1945-6 examinations", the authors showed that the rate of 19.9 became 20.2. On this basis it could be claimed that the "rate" in Kingston had not changed, but the incorrect statement was made that the DMF "rates" in the control city of Kingston showed no "changes". These rates of 19.9 and 20.2 were produced by combining the data of young children - that had few erupted permanent teeth and relatively low DMF rates per 100 teeth with data of older children that had most of their teeth erupted, and considerably higher DMF rates per 100 teeth. The rate obtained in 1949 was then adjusted. This procedure, no doubt unintentionally, disguised the fluctuations in the rates in the control city.
In Table 2 (Ast et al., 195 1) the "per cent change" in the Kingston children aged nine years was shown as 9.5, but if the figures 18.9 and 19.1 are the correct ones for the years 1945-46 and 1949, the "per cent change" should have been stated as 1.1, not 9.5.
Variability of caries rates. Unfortunately, the variability of even the mean caries rates cannot be studied, for the rates of yearly age groups were not published in the first and the last reports (Ast et al., 1950, 1956), and the only results shown in the 1955 report of Ast et al. were based on a combined clinical and X-ray examination.
The meagre data supplied for deciduous teeth. Data regarding the caries rates of the permanent teeth were shown in each report of this study; however, only very meagre data were published for the deciduous ones. None were made available in the first report (Ast et al., 1950). In the following year (Ast et al., 195 1) the DF rates per 100 deciduous teeth were given, but only for children aged five, six, seven and eight years, and in each age group the rates had decreased both in the test and in the control cities.
Unexplained marked decreases in the control. No explanation was given by Ast et al. (1951) for the decreases in the DF rates in Kingston, where the greatest relative decrease, from 32.1 per cent DF to 24.8 per cent DF, was seen in the teeth of the six year-old children. It would have been of great interest to see whether this trend was maintained in later years, but DF rates were not stated in the tables contained in any of the later reports. However, in the following one (Ast and Chase, 1953) the situation in regard to the deciduous teeth of children five, six, seven and eight years old (now termed "def teeth per 100 deciduous teeth present") was depicted diagrammatically by means of a histogram, these unexplained decreases in the def rates in the control city being clearly seen, a small one at age five years, and considerable ones at the ages of six, seven and eight years.
The increase in caries-free teeth in the control. The only other information published regarding the deciduous teeth was expressed in terms of "Children with caries free deciduous cuspids, first and second molars." This type of table appeared first in the 1951 report of Ast et al., and the results were given for only those children who were five or six years of age. In both age groups in Kingston the figures suggest an increase in these caries free teeth, the six-year-old children changing, between 1945-6 and 1949, from 17.2 per cent to 25.5 per cent free from caries. These changes were mentioned, but no attempt was made to explain them. In the next report (Ast and Chase, 1953) data for children aged seven years were also included. This report showed that, between 1945-6 and 1951-2, the percentage of children in Kingston who had these deciduous teeth free from caries showed a slight decrease at age five years (28.2 per cent to 26.4 per cent); but in the six-year old children the percentage increased from 17.2 to 26.3; and in those who were seven years of age, it practically doubled (8.3 to 16.5). On this occasion, these changes in the control city were not even mentioned.
"Analysis" of findings. In the 1955 report of Ast et al. it was stated that "As an indication of the benefits of water fluoridation to deciduous teeth, a previous report [in 1953] analyzed the findings among the 6 to 7 year old children in each city after six to seven years following the initiation of water fluoridation." Actually the report showed findings for the children aged five, six and seven years who had caries-free deciduous cuspids, first and second deciduous molars. At the ages of six and seven years, in both the test and the control cities, there were increases in the percentages of these teeth that were free from caries; therefore, although these increases were greater in Newburgh, they should not have been attributed solely to water fluoridation. In any case, the publication of one table showing, in this selected group of deciduous teeth, the percentage changes that have just been mentioned, a histogram depicting the def rates, and twenty lines of comment in the text on the results displayed, can hardly be said to indicate the benefits of fluoridation to deciduous teeth, or even to constitute an adequate analysis of the findings in regard to the deciduous teeth present in children aged six and seven years.
Changes in caries-free teeth in the control. In the 1955 report of Ast et al. the age range was changed by not publishing the results for the five-year old children, but showing, for the first time, the results for caries-free deciduous cuspids first and second deciduous molars, for eight and nine-year old children. However, these results cannot be compared with those of the previous years, as they were based on a combined clinical and X-ray examination. Nevertheless, a comparison can be made with the rates shown in the final report (Ast et al., 1956). In the year between the 1953-4 and 1954-5 examinations, the rates in Kingston for the ages six, seven, eight and nine years changed from 10.6, 7.0, 7.9 and 0.0 to 11.1, 4.7, 1.8 and 1.6 for the respective ages. Such changes are not unexpected, for marked variations were seen in Evanston, where, also, the examinations were made by a clinical plus X-ray procedure. For instance, the percentages of children aged seven years who were drinking fluoridated water and who had caries free deciduous teeth were, in successive examinations, 11.33 (pre-fluoridation), 8.71, 3.87, 10,66, 13.01 and 17.86 (Hill et al., 1956). It would seem that assessments made on the basis of caries-free groups of deciduous teeth are not very reliable.
Changes in the sampling method. Consideration of these five dental reports shows that the sampling method changed from time to time, and that the method used in the control city was sometimes the same and sometimes different from that used in the test one. In the first report (Ast et al., 1950) it was stated: "we are considering only those children age 6-12 who were in the original base study and who have had each successive examination until they reach age 12. Also included are new school children who entered the study at age 6 subsequent to the first examination and were present at each of the successive examinations. Thus, this study group will have only those children who we are assuming have had continuous residence in their respective cities."
Continuous residence only assumed. It can be seen that the "continuous residence" of each subject was based on assumption only, and not on statements made in a questionnaire, such as was used in Evanston (Blayney and Tucker, 1948). Therefore, it is possible that children could have been absent from the city for considerable periods between the times of successive examinations. Also, there is no assurance that the six-year-old children entering the study in any of the post-fluoridation examinations had not come to live in the area since the commencement of the study. Therefore, it is doubtful whether the objective of having "reasonable assurance that the children studied had had continuous residence in their respective cities" (Ast et al., 195 1) can be said to have been attained.
Population changes in Newburgh. "Early in 1950 questionnaires were given to more than 3,200 children in the Newburgh schools for completion by their parents" (Ast et al., 195 1). The questions asked were not stated, nor was the number of replies received, but it was said that: "An analysis of the answers to those questionnaires shows that the Newburgh population is a relatively stable one and that the inclusion of the small migrant groups does not alter the caries picture to any significant degree. Consequently, in this report there are included all 5 to 12 year old children present in the schools in Newburgh and Kingston on the days the examinations were made" (Ast et al., 195 1).
Since information in regard to the caries attack rates in these migrant groups could not have been obtained directly from the questionnaires, it is presumed that the dental record cards of those children were grouped and that the cards of the children who were judged from the answers not to be migrants were also grouped, and the data contained in the two groups in regard to the caries attack rates were compared. If that process was carried out, it was not mentioned, nor were data published which would enable the reader to assess the situation. If no differences were found between the two groups, it must be considered to be strange because by that time it was said that "The DMF rates among permanent teeth of 6 to 12 year old children in Newburgh show a consistent downward trend" (Ast et al., 195 1). The United Kingdom Mission (1953) reported that the authors of this study had "found that the proportion of immigrants in Newburgh and Kingston was too small to affect the comparison." However, although the Newburgh population was said to be "relatively stable", in the 1954-5 examinations in that city 24 per cent of the children were excluded because they failed to fulfil the residence qualifications (Ast et al., 1956).
The workers who conducted the paediatric study in these cities, Schlesinger et al., in 1950 said that in each city "An effort was made to select... children from families which might reasonably be expected to remain for the duration of the study." In spite of that precaution, they found that 29.9 per cent of their subjects in Newburgh moved from the city during the period of the study (Schlesinger, Overton, Chase and Cantwell, 1956).
Population movement in Kingston. No mention was made of the issue of a questionnaire to children in the control city; apparently it was assumed that migrants to that city would have come from areas with "fluoride-free" water supplies. Schlesinger et al. (1956) found that 22.2 per cent of the children included in the paediatric examinations moved from Kingston during the period of the study; presumably a similar number of new residents settled in the city.
It may be considered that in moving from one locality to another, interruptions could occur to regular conservative and prophylactic treatment of the children, so that their dental health may not have been as good as that of children who lived for many years in the same city. It is possible also that regular dental examinations, by stimulating interest in the teeth, may improve eating habits and oral hygiene measures.
Considerable alterations in populations. In Table I of Ast et al. (1950) the number of permanent teeth erupted is shown. The numbers given for Newburgh in the examination of 1944-5 for the three age groups six to seven, eight to nine and ten to twelve years are respectively 3,579, 7,937 and 24,586. However, by adding in Table I of Ast and Chase (1953), the number of erupted teeth - for the same age groups, and in the same examination - are 5,379, 10,033 and 27,186. It was stated in the former report that "we are considering only those children age 6-12 who were in the original base study and who have had each successive examination until they reach age 12." It therefore appears that to meet those requirements, it was necessary to exclude, for the three age groups, 33 per cent, 21 per cent and 10 per cent of the number of erupted teeth, and, presumably, similar percentages of children. A like situation was seen in regard to the Kingston data, the percentages of teeth excluded being 24, 26 and 12. After only four years, it was apparently necessary to omit these large proportions of the data in order to consider only those children who were "continuous residents", no other explanation being evident for the different numbers of erupted teeth that were stated in the two papers. Although the population of Newburgh may have been "relatively stable" when compared with some unnamed population, it is obvious that the number of migrants was so great that they should have been excluded from the study.
Data of migrants excluded only in Newburgh. The necessity for excluding the data of migrants was later realized, and the method of including in the study all the children present in the schools on the day of the examination - although it was continued in Kingston - was abandoned in Newburgh. Ast et al. (1955) stated: "Based on residence histories, the Newburgh study group was limited to those who had used Newburgh water since the introduction of sodium fluoride on May 2, 1945." In the final report, also, only those children who had lived continuously in Newburgh were included, but "All the Kingston children examined are included in this report" (Ast et al., 1956).
Alterations in sample size. The sample size and the age distribution of the children were altered during the course of this study. The data included in the first three dental reports were obtained from the "entire elementary school populations" (Ast and Chase, 1953), except that in some years some of the children were excluded in Newburgh on residential grounds, and that in 1951-2, owing to the loss of an examiner, only half of the children in each city were included. However, in the 1953-4 series the age range was restricted to six to ten years, and the number of children examined was only a small fraction of those inspected in the same age groups during other examinations. Ast et al. (1956) said that the preceding report "dealt with rather small groups of children (about 375 children ages six to ten in each city), and there was considerable difference in age distribution."
Sampling by selection. The method of sampling used in the 1953-4 examination must be considered to be unorthodox, and was described by Ast et al. (1955) in these words: "The current series includes a limited number of schools which were chosen because of the availability of X-ray facilities. From previous data on DMF rates by school, it was determined that the selected Kingston school had a caries rate which was among the lowest in the city, while the rates for the three Newburgh schools were distributed through the range of rates for that city. This has the effect of minimizing the difference in the DMF rates between the two cities."
A decrease in the "per cent difference". In the final report (Ast et al., 1956, Table 1) the "per cent difference" between the DMF rate per 100 erupted teeth of children aged six to nine years in Newburgh and Kingston was given as 56.7. This is a smaller difference than any of those shown for the ages six, seven, eight and nine years (74.7, 68.3, 58.1 and 66.0 respectively), in the previous (1955) report, despite the fact that it was stated in that report, that the sampling method used had minimized the difference between the DMF rates in the two cities. A trial period of ten to twelve years was suggested by Ast (1943), and was mentioned in the authors' first report (Ast et al., 1950). In view of the decrease in the "per cent difference" between the test and the control cities, which was revealed in the final report, it is unfortunate that the trial was stopped as soon as the minimum period proposed by the authors had elapsed.