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
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
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
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.
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).
* 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.
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.
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
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MUSKEGON
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THE GRAND RAPIDS STUDY
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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.
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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.
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