A method of utilizing the subjects' initial caries experience to increase efficiency in caries clinial trials A L B E R T
KiNGMAN
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Biometry Section, National Caries Program, National Institute of Dental Research, Bethesda, Maryland, U.S.A. ,-
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Kingman, A.: A method of utilizing the subjects' initial earies experience to inerease efficiency in caries clinical trials. Community Dent. Oral Epidemiol. 1979: 7: 87-90. Abstract - A problem which often occurs in caries clinical trials is the existence of an imbalance by group in the initial DMFS scores among subjects completing the study. Four methods of incorporating the study subjects' initial caries experience were identified and compared in six recently conducted clinical trials to see which of the methods was the most efficient in making treatment group comparisons of study subjects' caries increment scores. The consistent method of choice was one which utilized an adaptation of GRAINGER'S earies severity index as a stratification factor in a blocking analysis of caries inerements. The results suggest that more information regarding future caries exists in knowing where decay is present in the dentition than in knowing the subjects' total DMFS counts. Key words: clinical trials; dental earies.
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A. Kingman, Biometry Section, National Caries Program, National Institute of Dental Research, Bethesda, Maryland 20016, U.S.A. •
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Accepted for publication 22 October 1978.
Several investigators have considered various characteristics of study subjects as potential predictors of caries increment in longitudinal clinical trials (1). Although these attempts have met with different degrees of success as measured by the associated coefficient of determination (R^), the subject characteristic most consistently correlated with caries increment has been the initial DMFS score. In caries clinical trials the groups examined at the conclusion of the trial are often imbalanced as to their initial DMFS scores, even though strict randomization procedures were followed initially when making assignments to study groups, whether or not stratification was used. This is usually the result of participants dropping out of the study prematurely. In caries clinical trials conducted in the U. S. A. the dropout rate is typically between 10 % and 15 % per year. Two general methods have been used to statisti-
cally adjust the caries incidences for such group imbalances. The first utilizes the total initial DMFS score as a covariate with the DMFS increment scores being adjusted by the analysis of covaHance. The second consists of grouping the total initial DMFS scores into distinct strata and then utilizes a blocking analysis to adjust caries incidence for differences among strata. Gompared with the analysis of covariance, the blocking method generally produces smaller mean square errors and requires fewer assumptions concerning the form of the relationship between an individual's initial DMFS and subsequent caries incidence. Its principal disadvantage is that the method of grouping by initial DMFS scores is done arbitrarily, which can make it difficult to compare results from similar studies. Gonsequently another method of assessing the severity of initial DMFS scores was considered. Because attack rates for different teeth and surfaces
O3Ol-5661/79/020087-O4$O2.50/0 © 1979 Munksgaard, Copenhagen
88
KINGMAN
vary, the type of surface in conjunction with its loeation in the dentition was considered rather than the subject's total DMFS experience. GRAINGER (4) developed a Garies Severity Index based upon such an approach. His index was developed as a simplified method of measuring both the prevalence and severity of dental caries in a populatioti. In the present study GRAINGER'S Severity Index (GSI) and a modification of his index (MGSI) were considered as potential blocking variables for analyzing caries trials data. The purposes of this study were to identify a blocking factor which could: 1) increase the efficiency of treatment/group comparisotis, 2) correct for initial DMFS imbalances among study groups, and 3) facilitate comparing results amotig similar studies.
METHODS
V
Classification of dental caries by location and type of affected surfaces represents a somewhat different approach to assessing severity of caries than the total DMFS score. When using the GSI method five distinct regions of the permanent dentition are examined for evidence of earies. Specifically, these five regions or zones are: Zone 1 - Occlusal pit and fissure surfaces of molars and premolars and the buccal pits and lingual grooves of molars. Zone 2 - Interproximal surfaces of posterior teeth including the distal surface of canines. Zone 3 — Interproximal surfaces of the maxillary anterior teeth excluding the distal surface of canines. Zone 4 ~ Labial surfaces of the anterior teeth. Zone 5 - Interproximal surfaces of the mandibular teeth excluding the distal surfaee of canines. As originally proposed the investigator would examine an individual by first.examining those surfaces in Zone 5 for evidence of caries, then proceed to those surfaces in Zone 4, etc., with the examination terminating as soon as some evidence of caries was detected. The seore assigned the subject was the number of the first zone examined in which caries was detected. The external validity of this index, as tested by POULSEN & HOROWITZ (3), and later by
KATZ & MESKIN (2), was found to be more than adequate in measuring the severity of decay as compared with the DMFS score. These authors, however, questioned the internal validity of the index. They pointed out that all individuals classified in Zone 4 would not necessarily qualify for membership in Zones 3 or lower had a full-mouth examination been conducted, and suggested using combinations of these zones to make this index more hierarchical in nature. The extent of the zone misclassification problem due to the GSI scoring method was investigated. For purposes of this study zone misclassification was said to occur when-
ever an individual assigned to Zone " i " would not have also qualified for all less severe Zones " j " if a full mouth examination had been conducted. Zone and study misclassification rates were then calculated for each study population. The zone misclassification rates were computed as the number of individuals scored in Zone i who did not also qualify for membership in all Zones j of less severity (0
KiNGMAN
• ; - i ' ; . ' • . • -
Biometry Section, National Caries Program, National Institute of Dental Research, Bethesda, Maryland, U.S.A. ,-
:
" ;'• ,
; ; ; i •;
••.•/'. •:•
• : ; > ' • " , .'/^ .-•
,__',--
;; ,.,
;...: .. :'
Kingman, A.: A method of utilizing the subjects' initial earies experience to inerease efficiency in caries clinical trials. Community Dent. Oral Epidemiol. 1979: 7: 87-90. Abstract - A problem which often occurs in caries clinical trials is the existence of an imbalance by group in the initial DMFS scores among subjects completing the study. Four methods of incorporating the study subjects' initial caries experience were identified and compared in six recently conducted clinical trials to see which of the methods was the most efficient in making treatment group comparisons of study subjects' caries increment scores. The consistent method of choice was one which utilized an adaptation of GRAINGER'S earies severity index as a stratification factor in a blocking analysis of caries inerements. The results suggest that more information regarding future caries exists in knowing where decay is present in the dentition than in knowing the subjects' total DMFS counts. Key words: clinical trials; dental earies.
•
>
A. Kingman, Biometry Section, National Caries Program, National Institute of Dental Research, Bethesda, Maryland 20016, U.S.A. •
.
•-
^ i (
~ i k
:••/
.-
Accepted for publication 22 October 1978.
Several investigators have considered various characteristics of study subjects as potential predictors of caries increment in longitudinal clinical trials (1). Although these attempts have met with different degrees of success as measured by the associated coefficient of determination (R^), the subject characteristic most consistently correlated with caries increment has been the initial DMFS score. In caries clinical trials the groups examined at the conclusion of the trial are often imbalanced as to their initial DMFS scores, even though strict randomization procedures were followed initially when making assignments to study groups, whether or not stratification was used. This is usually the result of participants dropping out of the study prematurely. In caries clinical trials conducted in the U. S. A. the dropout rate is typically between 10 % and 15 % per year. Two general methods have been used to statisti-
cally adjust the caries incidences for such group imbalances. The first utilizes the total initial DMFS score as a covariate with the DMFS increment scores being adjusted by the analysis of covaHance. The second consists of grouping the total initial DMFS scores into distinct strata and then utilizes a blocking analysis to adjust caries incidence for differences among strata. Gompared with the analysis of covariance, the blocking method generally produces smaller mean square errors and requires fewer assumptions concerning the form of the relationship between an individual's initial DMFS and subsequent caries incidence. Its principal disadvantage is that the method of grouping by initial DMFS scores is done arbitrarily, which can make it difficult to compare results from similar studies. Gonsequently another method of assessing the severity of initial DMFS scores was considered. Because attack rates for different teeth and surfaces
O3Ol-5661/79/020087-O4$O2.50/0 © 1979 Munksgaard, Copenhagen
88
KINGMAN
vary, the type of surface in conjunction with its loeation in the dentition was considered rather than the subject's total DMFS experience. GRAINGER (4) developed a Garies Severity Index based upon such an approach. His index was developed as a simplified method of measuring both the prevalence and severity of dental caries in a populatioti. In the present study GRAINGER'S Severity Index (GSI) and a modification of his index (MGSI) were considered as potential blocking variables for analyzing caries trials data. The purposes of this study were to identify a blocking factor which could: 1) increase the efficiency of treatment/group comparisotis, 2) correct for initial DMFS imbalances among study groups, and 3) facilitate comparing results amotig similar studies.
METHODS
V
Classification of dental caries by location and type of affected surfaces represents a somewhat different approach to assessing severity of caries than the total DMFS score. When using the GSI method five distinct regions of the permanent dentition are examined for evidence of earies. Specifically, these five regions or zones are: Zone 1 - Occlusal pit and fissure surfaces of molars and premolars and the buccal pits and lingual grooves of molars. Zone 2 - Interproximal surfaces of posterior teeth including the distal surface of canines. Zone 3 — Interproximal surfaces of the maxillary anterior teeth excluding the distal surface of canines. Zone 4 ~ Labial surfaces of the anterior teeth. Zone 5 - Interproximal surfaces of the mandibular teeth excluding the distal surfaee of canines. As originally proposed the investigator would examine an individual by first.examining those surfaces in Zone 5 for evidence of caries, then proceed to those surfaces in Zone 4, etc., with the examination terminating as soon as some evidence of caries was detected. The seore assigned the subject was the number of the first zone examined in which caries was detected. The external validity of this index, as tested by POULSEN & HOROWITZ (3), and later by
KATZ & MESKIN (2), was found to be more than adequate in measuring the severity of decay as compared with the DMFS score. These authors, however, questioned the internal validity of the index. They pointed out that all individuals classified in Zone 4 would not necessarily qualify for membership in Zones 3 or lower had a full-mouth examination been conducted, and suggested using combinations of these zones to make this index more hierarchical in nature. The extent of the zone misclassification problem due to the GSI scoring method was investigated. For purposes of this study zone misclassification was said to occur when-
ever an individual assigned to Zone " i " would not have also qualified for all less severe Zones " j " if a full mouth examination had been conducted. Zone and study misclassification rates were then calculated for each study population. The zone misclassification rates were computed as the number of individuals scored in Zone i who did not also qualify for membership in all Zones j of less severity (0
