The Greatest Fraud Fluoridation: Errors and Ommissions in Experimental Trials -part II by Dr. Philip R.N. Sutton

Errors and Omissions in Experimental Trials - 1a

PHILIP R. N. SUTTON
D.D.Sc (Melb), L.D.S. (Vic.)
Senior Research Fellow, Department of Oral Medicine and Surgery
Dental School, University of Melbourne

MELBOURNE UNIVERSITY PRESS

First published in 1959
Second edition, enlarged, 1960
Printed and bound in Australia by
Melbourne University Press, Parkville N.2, Victoria
Registered in Australia for transmission by post as a book
London and New York: Cambridge University Press

PREFACE TO THE SECOND EDITION

Soon after the publication of the first edition of this monograph, in September 1959; copies were sent, by the Australian Dental Association, to the workers in charge of all the studies considered. As a result, critical reviews were published in the February 1960 issue of the Australian Dental Journal. The New Zealand Dental Journal of January 1960 also contained a critical review. These have not indicated the necessity for any modifications in Parts One and Two which are, therefore, reprinted unchanged. However, in this edition a Part Three has been added in which these criticisms are reprinted, at length, and some comments made. It is again stressed that in this book consideration is limited to some aspects of five crucial experimental trials of artificial fluoridation. Results reported from "naturally fluoridated" areas are not considered.

P.R.N.S.
Dental School, University of Melbourne
June 1960

PREFACE TO THE FIRST EDITION

Endorsements of the process of the mechanical addition of fluorides to public water supplies, with the aim of reducing the incidence of dental caries, rely mainly on the results published from five trials which were set up to test, primarily, the efficacy of this process.

Important deficiencies in the methods used were revealed during a preliminary investigation of reports of these trials. Therefore this study was undertaken in an attempt to answer the question: Can the claims of considerable dental benefits as a result of artificial fluoridation be regarded as established, or are they based on an unsound foundation.

P.R.N.S.

Dental School, University of Melbourne

February 1959

ACKNOWLEDGMENTS

Members of the Department of Statistics, University of Melbourne, have given most careful consideration to all the statistical matters mentioned in Part One of this monograph they have checked the computations in Part Two and have also given advice regarding statistical matters mentioned in Part Three. Their assistance is gratefully acknowledged. Part One was published in the Medical Journal of Australia, 1 February 1958, pages 139-40. 1 should like to express my thanks to my co-author, and to the Editor of the journal for permission to reprint the paper and also to the Editors of the Australian Dental Journal and the New Zealand Dental Journal for permission to reprint the book reviews shown in Part Three. Extracts from Part Two were presented at the fifteenth Australian Dental Association Congress, Adelaide, 23-7 February 1959. Professor Sir Arthur Amies and Dr Paul Pincus have suggested improvements to the draft of Part Two, and Miss H. N. Rankine, the Librarian of the Dental School of the University of Melbourne, has given valuable assistance. This investigation has been supported by grants from the University of Melbourne Research Fund.

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PART ONE

SOME STATISTICAL OBSERVATIONS ON FLUORIDATION TRIALS*

The suggestion that domestic water supplies should be-fluoridated, with the aim of partially preventing the development of dental caries, has gained wide support, and moves are being made in Australia for the widespread introduction of this measure. Much confusion of thought clouds the issue of the desirability, the method of action and the safety of this process. This uncertainty is reflected in two recent events. In November 1956, a Reference Committee of the American Medical Association (1957) stated that "there is a definite need for a re-evaluation of the problem of fluoridation",† and in March 1957, after a public hearing, the proposal to fluoridate the water supply of New York was not put into practice (Nesin, B.C., personal communication, 1957).

Apart from these considerations, an examination reveals that there are aspects that call for a very careful appraisal of the figures presented in the reports of the experimental trials which have been conducted in Brantford, Canada, and in Grand Rapids, Newburgh and Evanston, U.S.A., and upon the results of which proposals to fluoridate domestic water are almost entirely based.

A preliminary survey of the methods used, of the published figures and of the method of their presentation discloses some disturbing facts. Some of these are as follows. (i) In the clinical examinations no attempt was made to devise a randomization procedure, which would have eliminated bias on the part of the examiners. However, the necessity for such a precaution was recognized by Ast, Bushel, Wachs and Chase (1955) in the Newburgh-Kingston trial, when they instituted a combined clinical and X-ray study eight years after the commencement of the ten-year investigation. (ii) No estimate was made of variability between examiners, although in some studies several operators were employed, some being changed from year to year (Blayney and Tucker, 1948; Arnold, Dean and Knutson, 1953); some of the examinations in Kingston were made by two dental hygienists (Ast, Finn and McCaffrey, 1950). Furthermore, there appears to be no estimate of variability within the examiner-that is, the variability of individual examiners from inspection to inspection. (iii) The importance of random variation in the DMF rate (decayed-missing-filled permanent teeth rate) does not appear to have been recognized, or else it has been ignored. (iv) Bias is suggested by the presentation of some results, so that the casual reader may be misled (Ontario Department of Health, 1956).

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* Reprinted from a paper by Philip R. N. Sutton. D.D.Sc. (Melb.), L.D.S. (Vic.) and Arthur B. P Amies, C.M.G., D.D.Sc. (Melb.), F.R.C.S. (Edin.), F.R.A.C.S., originally published in the Medical Journal of Australia, 1 February 1958. † In December 1957, the American Medical Association endorsed the principle of fluoridation, but that decision cannot affect the facts which have been stated in this paper.

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The following observations will serve as illustrations.

1. In each of these studies it has been emphasized that the maximum benefits of fluoridated water are seen only in those subjects who have consumed it during the total period of enamel formation. Therefore, it would be expected that only a slight decrease (due to the possible topical effect of the fluorine) would be seen in the DMF rate between successive years during approximately the first six years of the project, until the first permanent teeth which had been completely formed under its influence had erupted, and that the advent of these "resistant" teeth would thereafter produce a greater drop in DMF rate between succeeding years. However, in the first three years of each project there is a marked relative fall in the reported DMF rate, particularly in the younger age groups; while in the six years-old group in Brantford the rate reached after ten years is no lower than it was after only four years of fluoridation, (Ontario Department of Health, 1956; Hutton, Linscott and Williams, 1956). It would appear that the results reported are not those which would be expected if the theory mentioned above is correct.

2. In four of these studies (Hutton et al., 1956; Hill, Blayney and Wolf, 1956; Arnold, Dean and Knutson, 1953; Ontario Department of Health, 1956) the method of expressing changes in caries experience was the same. The final rate was subtracted from the baseline rate, and the difference was expressed as a percentage of the latter rate. It is obvious, therefore, that with this method, relatively small variations in the baseline values will produce substantial alterations in the percentage reduction obtained. For instance, in the seven-year-old children in Evanston, during the last five years reported, the increase in caries immune deciduous dentitions was 361 per cent, but for the whole of the nine-year period 1946-55 the increase was only 58 per cent "Hill et al., 1956). The authors claim that "difference between 1946 and 1955 rates is statistically significant" However, such a claim is not warranted, owing to the marked variation in the values observed in the intervening years. The effect of variations between years is seen in the six-yearold group in Brantford. By the use of this method of calculation the reduction in the DMF rate for the period 1944-50 was 82 per cent, but the apparent benefit had dropped to 52 per cent, a decrease of 30 per cent, after an additional two years fluoridation (Ontario Department of Health, 1956). An improved method of indicating relative changes in the DMF rate would seem to be desirable - in particular, one which would permit statistical tests to be applied.

3. As an instance of the divergent results which can be reported by different examiners, those from the two independent trials in Brantford may be compared (Ontario Department of Health, 1956). The National Health and Welfare authors reported a reduction in the DMF rate in the six to eight years age group of 69 per cent from the inception of their examinations in 1948 to the 1954 results. However, in the same city, in the same age range and between the same years, the reduction in the DMF rate obtained by the City Health Department examiner was only 25 per cent, less than half of that claimed by the authors of the other study. The Health Department DMF figures for 1954 were given for individual age groups without statement of the number of children involved in each group. The 25 per cent reduction is based on a DMF rate obtained by simple averaging of the six, seven and eight year DMF rates. For 1948 the actual numbers of children are available (Hutton, Linscott and Williams, 1951). The uncertainty in the computed reduction of 25 per cent is most unlikely to account for the gross difference between it and the figure of 69 per cent quoted by the National Health and Welfare authors.

4. In Table II of the Report of the Ontario Department of Health (1956) to the Ontario Minister of Health, the mean numbers of decayed or filled deciduous teeth are shown. In the column headed "% Reduction Since 1948", there are dashes Opposite the control cities of Sarnia and Stratford. These, surely, would lead the reader to suppose that no reductions had taken place in these cities, particularly as the footnote states that "the rates for Stratford, which has had natural fluoridation for 30 years, and Sarnia, which has no fluoride in its water, have remained about the same". However, in the nine to eleven years age group in Stratford there was a slight decrease of 5 per cent (by the use of the DMF rate reduction method common in these studies), and in Sarnia the same age group showed a decrease of no less than 16 per cent. One would like to know the reason for the omission of these figures, particularly as the latter reduction is almost as high as the 18 per cent claimed for children of the same age in the test city.

Whilst we do not question the integrity of workers in this field, it must be pointed out that the evidence tendered in favour of fluoridation reveals two disturbing features. The first is that what must be essentially a statistical study does not appear to have been planned as such. The second is that even when sufficient information is presented, no comprehensive attempt at statistical evaluation has been considered.

It is possible that a case for fluoridation can be solidly based, but until adequate statistical treatment of all the pertinent factors has been carried out and this would be quite a major undertaking the question should not be regarded as settled. In the meantime, claims concerning the amount of caries reduction are open to doubt.

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PART TWO

FLUORIDATION TRIAL CONTROLS: ERRORS, OMISSIONS AND MIS-STATEMENTS

INTRODUCTION

The fluoridation trials that were conducted in the cities of Grand Rapids, Newburgh and Evanston, in the U.S.A., and the two independent ones in Brantford, Canada, are of more than ordinary importance, because they constitute the main experimental evidence which has led to the introduction of this process as a public health measure. The fluoridation hypothesis is "that a concentration of about I part per million of fluoride in the drinking water, mechanically added, inhibits the development of dental caries in the teeth of the users of the water" (Brown, McLaren and Stewart, 1954b). In 1956 Nesin pointed out: "It must be emphasized that the fluoridation hypothesis in its entirety rests on a very narrow base of selected experimental information. It is this very base which is vulnerable to scientific criticism. And, it is upon this very narrow base that the very impressive array of endorsement rests like an inverted pyramid."

The safety of artificial fluoridation has been questioned by a number of eminent authorities such as Hicks (1956) and Sinclair and Wilson (1955). In 1955 Box stated: "It is my considered opinion that the artificial fluoridation of water supplies, on a wholesale basis, should not be advocated or adopted until fully sufficient findings show that there are no harmful sequelae from a gingival or periodontal standpoint."

However, these questions need be considered only if the overall dental benefits of fluoridation are demonstrated beyond reasonable doubt, and are also found to be worthwhile from a socio-economic point of view. No suggestion has been made that fluoridation has other than dental benefits.

It has been widely accepted that the existence of marked dental benefits has been established, and the literature abounds with references to reductions of about 60 per cent in dental caries as a result of fluoridation. However, the published works contain little consideration of the numerical data reported from these trials, as distinct from mere statements of percentage reductions in the caries attack rates.

A preliminary examination revealed that reports of these studies contain errors and show omissions, and statements made in regard to results are not justified by published data; therefore further study has been made of these crucial trials. This study attempts to evaluate their controls, and the discussion is limited to examination of published reports of (i) method of selection of control cities; (ii) their suitability; (iii) the experimental and statistical processes used in gathering and analysing the data (iv) the results stating the dental caries attack rates; (v) some comments made by the authors of these trials (and by others) on these results.

The aim will be to investigate the reliability of the results reported, to assess the adequacy of the controls that were set up and to evaluate the accuracy of the statements made concerning the data obtained.

BASIC CONSIDERATIONS

Before discussing the procedure adopted in each of these studies, several basic matters that are of importance in a fluoridation trial will be considered.

The necessity for controls. Blayney and Tucker (1948) were correct in stating that "A study of this nature must have an adequate control." The necessity for such a procedure was recognized by the authors of four out of five of these studies. Cities with "fluoride-free" water supplies were selected as controls, and comparisons were made with towns which possessed water supplies with a fluoride content obtained from natural sources, which approximated the concentration which has been called the "optimum" one (Dean, Arnold, Jay and Knutson, 1950; Brown, 1951; Ast and Chase, 1953; Hill, et al., 195 1). It is to be noted that in the trial conducted in Brantford by the City Health Department (Hutton et al., 1951) no provision for controls was made.

Requirements of a control. In an experiment such as the fluoridation of the water supply of a city, whereby the whole of its population is subjected to treatment (fluoridation), it is necessary to obtain the control data from subjects who live in a city or cities with "fluoride-free" water supplies. In determining the cities which are to participate in the trial, in order to increase the sensitiveness of the experiment, it is advantageous to employ ones which are alike in as many respects as it is practically convenient to consider. Of course, as Fisher (1951) pointed out, "the uncontrolled causes which may influence the result are always strictly innumerable."

Because of the nature of these experiments, three main points of similarity must be considered and described. These are (a) the water supply; (b) the climate; and (c) the dental caries attack rates. Other factors, such as socioeconomic status, are of less importance; their influence may be reflected in the caries attack rates.

(a) In its statement of its official policy on this matter, the American Water Works Association (1949) said 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." The Association referred to "the possible influence, on the fluoride potency, of other chemical constituents of natural waters, insofar as these and other variables may affect the action of fluoride on the control of caries in a human population." In 1942 Deatherage reported that "It is these soft waters which cause the most severe mottled enamel." Therefore, the fact that both the test and the control city in a fluoridation trial obtain their water from the same source does not remove the necessity for a study of the composition of the water. Dean, Jay, Arnold, McClure and Elvove (1939) recognized this, stating, "the possibility that the composition of the water in other respects may also be a factor should not be overlooked. For this reason it seems highly desirable that dental caries studies should be accompanied by complete chemical analyses of the dam waters, including a search for the comparatively rare elements." However, in none of these trials was the composition of the water stated.

(b) The climate of a city is an important factor in determining the average amount of salts ingested from the water supply, because of its influence on the volume of water consumed by humans. Therefore, cities that are to be compared should not only have water supplies that have a closely comparable composition, but the climates of the cities should also be very similar.

(c) As the main aim of fluoridation is to reduce the dental caries attack rates, it is obviously of importance that the cities to be compared should have closely comparable dental caries rates within yearly age groups, of children. This information can be obtained only by conducting at least one survey in the cities that are suitable for comparison on other grounds, so that the fact that the caries attacks rates are similar is established prior to the fluoridation of the water supply of one of them.

Random sampling. The fundamental importance of random sampling has been acknowledged for many years. In designing an experiment, as Quenouille (1952) said, "it is necessary to allot the treatments to the available material at random if unbiased estimates of both the effect of the treatments and also the reproducibility of the effects are to be obtained." Therefore, a random device should be employed to determine which of the participating cities is to be the test one.

Variation. Fisher (1950) emphasized this important matter when he said that "from the modem point of view, the study of the causes of variation of any variable phenomenon, from the yield of wheat to the intellect of man, should be begun by the examination and measurement of the variation which presents itself." As was pointed out by Hill et al. in 1950: "It is to be expected that the rate of caries in all teeth varies from year to year due to chance." Therefore, a basic requirement of a fluoridation study is the assessment of the variability of the caries attack rates.

Examiner variability. In experiments in which, of necessity, the subjective judgment of examiners is employed, an important consideration is the assessment of "between-examiner" and "within-examiner" variability. The former type of variability is disclosed when different examiners observe the same subjects, and the latter type is seen in the different results reported by the same examiner inspecting the same subjects on different occasions, but which are sufficiently close together to ensure that the dental condition has not undergone appreciable change.

The important effect which examiner variability can have on the results of a study of dental caries attack rates was pointed out by Radusch (1941) and by Dunning (1950). A recent example is seen in the paper of McCauley and Frazier (1957). Their Table I shows that in the examinations made by one examiner in 1955 of Negro boys and girls who were six years old, in both sexes the DMF rate per 100 teeth erupted, and also the DMF rate per child, were found to be about four times as great as those reported for the same age groups in 1952 when they were examined by several examiners. The authors considered that "it is entirely possible that the 1952 findings were influenced by a bias stemming from subjective differences in the appraisal of tooth decay by different dentists". Between-examiner variability of such a magnitude can, of course, vitiate the results of a study. Unless the examiner variability is determined, and is taken into account, the conclusions drawn from a study of caries attack rates must be treated with reserve.

Examiner bias. In designing an experiment of this nature, one aim should be to eliminate examiner bias. This may arise if the examiners know whether the children they are examining belong to the test or to the control city. One method of doing this is to transport to a common examination centre the small number of children, some from the test and some from the control city, that can be examined each day; the examinations being conducted in a random order which is unknown to the examiners. It is not suggested that in the absence of such precautions the examiners exhibited intentional bias; indeed, as Armitage (1954) pointed out, "through fear of being biased" the judgment of an examiner may be influenced.

THE GRAND RAPIDS STUDY

The city selected as the "fluoride-free" city for comparison with Grand Rapids was Muskegon, Michigan, "whose source of drinking water supply and geographical and climatological characteristics were similar to those of Grand Rapids" (Dean et al. 1950). This city was the only control one in which the caries attack rates in each year were published for each yearly age group. Unfortunately, its usefulness was marred by a number of features.

Large differences in sample size. The reliability of a mean rate is greatly influenced by the number of observations on which it is based. Because of the small number of subjects included in some age groups in some years in Muskegon, little reliance can be placed on the values stated. In twelve categories fewer than twenty children were examined. One "group" consisted of only one child, whereas one contained 462 children (Arnold et al., 1953). In the test city the variation in sample size was even greater, from 18,606 to 3 subjects.

Sampling by school class. "selected age groups of children are examined within each of the schools. Selection is made on the basis of school grade or class, using all children present in a class or grade of a school." (Arnold et al. 1953). These grades or classes were examined in 1945, an additional grade being examined in 1946, 1947 and 1949, and two more in 1950, making a total of eight grades in 1950 and 1951. In the last mentioned year Muskegon ceased to act as a control.

Different methods of sampling. In Grand Rapids the "annual study sample was selected after careful review of census data and consultation with city planning department officials. On the basis of available information, the 31 school districts of Grand Rapids were classified on a socio-economic basis. From the 79 schools in these districts, 25 representative schools were selected, and the examiners were assigned schools on a basis of equal sized samples of comparable population groups" (Arnold, et al., 1953). However, that strange procedure was not followed in Muskegon, the same authors stating that "In Muskegon, the annual examinations have been conducted in almost all schools, excluding only a few small schools on the periphery of the city where many students are from rural areas."

Changes in examiners. In the report of this study up to and including 1951 (in July 1951 Muskegon ceased to be a control), it was stated: "There have been changes in dental examiners with the exception of one officer who has participated in each series of examinations. Each new examiner has been calibrated against this one officer to standardize diagnostic criteria" (Arnold et al., 1953). The degree of success of this odd calibration procedure was not stated.

Examiner variability not assessed. In 1953 Arnold et al said that "Bite-wing X-ray examinations were made of a representative sample of children examined by the different examiners to evaluate, in part, the "examiner error"." However, such a procedure cannot replace the data that could have been obtained by a correctly designed examination process, which would have enabled the determination of between-examiner and within-examiner variability.

Late examination of control city. The authors of this study did not determine the caries attack rates in children in Muskegon prior to accepting this city as a suitable control, for comparison of two of their statements makes it clear that the results of the basic examination in the control city were not known until after the water of the test city was fluoridated. They stated that "Fluoridation of the Grand Rapids water supply was started January 25, 1945" (Dean et al., 1950), and that the "basic examinations in Muskegon were not done until late spring of 1945" (Arnold et al., 1953).

Water of control fluoridated. Another fact which limits the usefulness of Muskegon as a control city is that its water supply was fluoridated in July 1951 (Arnold et al., 1953), so that the results obtained after that date had no value as controls for those of Grand Rapids. This event occurred six and a half years after the institution of fluoridation in Grand Rapids, and therefore at a time when, in the latter city, few of the permanent teeth had erupted in the children that had been ingesting fluoridated water since birth.

Ignorance of commencement of fluoridation in control city. The fact that Muskegon had ceased to be a control by having its water fluoridated in July 1951 was not always realized. For instance, Black (1955) in a paper "Presented before Section on Public Health Dentistry, ninety-fifth annual session, American Dental Association, Miami, Fla., November 8, 1954" - over three years after the institution of fluoridation in Muskegon (Arnold et al. 1953) - said that "At Muskegon, Mich, the control city where fluoride-free water is used, the incidence of dental caries is unchanged and approximates the norm." Black was commenting on the findings made "After eight years of fluoridation at Grand Rapids" (that is eighteen months after the fluoridation of the Muskegon water). No information has been found in the literature with regard to the "national norm", in fact a feature of these trials has been the divergent pre-fluoridation rates. The differing caries attack rates seen in different localities, even in the same state, were illustrated by Hagan (1947) and by Hadjimarkos and Storvick (1949, 1950).

A similar statement to that of Black (1955) was made by Martin (1956) in a lecture delivered at the fourteenth Congress, Australian Dental Association, Melbourne, March 1956; that is, over four and a half years after the Muskegon water was fluoridated. He said: "The decay rates in the fluoride free control area (Muskegon) have remained unchanged." The paragraph containing the above-mentioned quotation cites as reference Arnold et al. (1953), who in that paper said: "The water supply at Muskegon remained unchanged until July 1951, when the city started adding fluorides to its water supply."

Variations in Muskegon rates. Tables 3 and 4 of the report of the seventh year of the Grand Rapids study (Arnold et al., 1953) show that both the def (decayed, extraction indicated, or filled deciduous teeth) and the DMF rates reported from Muskegon from year to year differed considerably from those of the first examination. Despite this fact, the statement that the incidence of dental caries in Muskegon was unchanged was made by Black (1955) and Martin (1956), amongst others. These statements are at variance with that of the authors of the study (Arnold et al., 1953), for they mentioned the changes in these words: "A similar comparison of results at Muskegon shows the percentage reduction to range from 1.5 percent in 6-year olds to a high of 15.5 percent in the 11 year olds" in the permanent teeth.

The magnitude of the changes. The statement which has been quoted above does not reveal the magnitude of the changes which were observed in the DMF rates in the control city. The percentage reductions given were obtained by the method commonly used in all these studies, that is, the difference between the most recent and the original DMF rate was expressed as a percentage of the original rate, the variations obtained in the intervening years being ignored. The changes which occurred would have been more obvious if, for instance, the results for Muskegon had been computed in 1946 instead of in 1951. In that case the "reduction" would have been 40.7 per cent instead of 1.5 per cent in the six-year old, and 32.7 per cent instead of 15.5 per cent in the eleven-year-old children.

Sampling error. Arnold et al. (1953) stated that the percentage reductions obtained in Muskegon "may, in part, represent sampling error." It was not conceded that such an error could also apply to the results from the test city, nor were suggestions made as to the cause of the remainder of the reductions in the control city. These cannot be attributed to changes in the water supply, for they stated: "The water supply at Muskegon remained unchanged until July 1951, when this city started adding fluorides to its water supply" (Arnold et al., 1953).

Variation in Muskegon. The variability from year to year in the mean rates reported from Muskegon, which, it will be recalled, were stated to have remained unchanged (Black, 1955; Martin, 1956) are illustrated in Figs. 1 and 2. The data from which these figures were drawn are from Tables 2 and 3 (with errata corrected) of Arnold, Dean, Jay and Knutson (1956). The point shown in Fig. 2 for the sixteen year-old children in 1946 should be disregarded, as this age "group" consisted of only one child.

Comparison of the series of baseline rates for DMF in Grand Rapids and in Muskegon does not reveal that one series was consistently higher than the other. However, with the exception of the eleven and twelve-year old children, the def rates in Muskegon were higher than they were in the test city.

Comparison with Aurora. Arnold et al. (1953) said: "To establish what might be termed an "expectancy curve" for this study, a natural fluoride area, the city of Aurora, Ill., was selected. The Aurora water supply contains 1.2 ppm F and has a reliable "history of constancy back to 1895." It was not stated whether factors other than the fluoride content of the water supply were considered in selecting this city. The fact that other influences can be of importance was shown in the recent study by Russell (1956) in Montgomery-Prince Georges counties. Prior to the institution of fluoridation, in the total sample of subjects the def rates for children whose mean ages were 5.44, 6.47, 7.45 and 8.49 years were lower than those of children of similar ages in Aurora.

Limitations of Aurora data. The caries attack rates reported from Aurora consist of a single series obtained by several examiners in 1945-6. Therefore, there is no information with regard to variations from year to year in the mean value of the rates, and examiner variability was not considered.

MUSKEGON
MUSKEGON		THE GRAND RAPIDS STUDY
Figure 1. The mean number of def deciduous teeth per child in Muskegon, Michigan, the "fluoride-free" control city for Grand Rapids, Michigan, at each year of examination. The 1945 examination was made in the "late spring", those of the other years, in October and November. Data from Table 2, Arnold et al., 1956. Three months prior to the 1951 examination the water of this city was fluoridated. It has been stated (see p. 145 - 146) that these rates "remained unchanged" during the period shown.		Figure 2. The mean number of DMF permanent teeth per child in Muskegon, Michigan, the "fluoride-free" control city, at each year of examination. Data from Table 3, Arnold, et al., 1956. It has been stated (see p. 145 - 146) that these rates "remained unchanged" during the period shown.