264
LETTERS TO THE EDITOR
settle the question definitively. However, indirect evidence makes Comstock's alternatives seem unlikely. As to availability of medical care, especially hospitalization, it is a fact that only a small part of veterans' hospital utilization is within the VA system. The National Center for Health Statistics has estimated that during the period 1970-1973, of the 29 million veterans, 2.81 million were annually admitted to hospitals. Of these, 2.4 million men were admitted to non-Federal hospitals only, and less than 15 per cent went to any Federal hospital (1). Since so small a proportion of all hospitalizations of veterans was provided by the VA, it is difficult to see how the availability of VA services could have had a large effect on the mortality experience of all veterans. It is well known that mortality rates are higher among persons of lower socioeconomic class; however, the degree to which the provision of educational benefits under the GI Bill raised the average socioeconomic status of veterans in general is speculative as is the influence that such change might have had on mortality experience. The potent effect on subsequent mortality rates of screening by physical examination has long been known to life insurance companies. The Impairment Study of the Society of Actuaries (2) showed that the existence of apparently minor physical impairments at the time that an insurance
policy was initiated was associated with excessive mortality over a subsequent 15year period, as compared with experience among policyholders not noted to have had impairments. For example, those with an intermittent trace of albumin, but who were issued standard policies, had a mortality ratio of 133; those with gastric ulcer, with an operation several years previously, had a ratio of 231. These high ratios are not surprising, but it follows that if, through physical examination, subgroups having high mortality are screened out, then the residual group will show low mortality. It may be that pur results to some degree have been affected by the factors that Comstock mentions; but we continue to believe that medical selection through the induction physical examination made the most important contribution. REFERENCES
1. Unpublished data from the Health Interview Survey, National Center for Health Statistics, Department of Health, Education, and Welfare 2. The Society of Actuaries: Impairment Study. Society of Actuaries, New York, 1954
Carl C. Seltzer Harvard School of Public Health Boston, Massachusetts Seymour Jablon National Research Council Washington, D. C.
MAIL SURVEY RESPONSE BY SMOKING Seltzer et al. (Am J Epidemiol 100:453-457, 1974) report confirmation of a Kaiser-Permanente finding that cigarette smokers are slower to respond to a mailed questionnaire than nonsmokers. This interesting result certainly needs consideration in design of future mail surveys. Granted that bias lurks almost everywhere to trap the unwary, I do not think
STATUS
that Seltzer et al. have correctly stated the problem when they warn that "the implications of this bias become significant if, in addition to the underrepresentation of smokers responding to questionnaires, there are also differences in the health status between responding smokers and responding nonsmokers." It seems to me that a necessary ingredient for bias in
LETTERS TO THE EDITOR
265
estimates of the association between smok- biased, since they invariably show differing and health is that responding smokers ences between responding smokers and are different in health status from responding nonsmokers. nonresponding smokers (or responding Harold A. Kahn nonsmokers are different from nonresponding nonsmokers). Statistician, Office of Biometry If Seltzer's criterion is right, and I do not and Epidemiology National Eye Institute think it is, all major studies of smoking and Bethesda, Maryland 20014 health that deviate from equal representation of smoking categories are significantly
THE AUTHOR'S In the paper cited by Dr. Kahn, we referred to an earlier Kaiser-Permanente mail survey (Am J Epidemiol 98:50-55, 1973) in which ill smokers were found to respond more quickly than well smokers. The actual data were suggested but not fully reported in table 5 of that study. The pertinent results showed the following: Percentage of ill people in category Respondents to 1st mailing
Male smokers Male nonsmokers
N 16/124 12/162
% 12.9 7.4
Nonrespondents to 1st mailing (who responded to subsequent solicitations) N % 11/158 7.0 9/141 6.4
REPLY
According to Kahn, a "necessary ingredient for bias in estimates of the association between smoking and health is that responding smokers are different in health status from nonresponding smokers." These data are consistent with his statement, and augment the suspicion that many important studies of smoking and health may be significantly biased because of differential response by smoking and health status. Carl C. Seltzer Harvard School of Public Health Boston, Massachusetts
LETTERS TO THE EDITOR
settle the question definitively. However, indirect evidence makes Comstock's alternatives seem unlikely. As to availability of medical care, especially hospitalization, it is a fact that only a small part of veterans' hospital utilization is within the VA system. The National Center for Health Statistics has estimated that during the period 1970-1973, of the 29 million veterans, 2.81 million were annually admitted to hospitals. Of these, 2.4 million men were admitted to non-Federal hospitals only, and less than 15 per cent went to any Federal hospital (1). Since so small a proportion of all hospitalizations of veterans was provided by the VA, it is difficult to see how the availability of VA services could have had a large effect on the mortality experience of all veterans. It is well known that mortality rates are higher among persons of lower socioeconomic class; however, the degree to which the provision of educational benefits under the GI Bill raised the average socioeconomic status of veterans in general is speculative as is the influence that such change might have had on mortality experience. The potent effect on subsequent mortality rates of screening by physical examination has long been known to life insurance companies. The Impairment Study of the Society of Actuaries (2) showed that the existence of apparently minor physical impairments at the time that an insurance
policy was initiated was associated with excessive mortality over a subsequent 15year period, as compared with experience among policyholders not noted to have had impairments. For example, those with an intermittent trace of albumin, but who were issued standard policies, had a mortality ratio of 133; those with gastric ulcer, with an operation several years previously, had a ratio of 231. These high ratios are not surprising, but it follows that if, through physical examination, subgroups having high mortality are screened out, then the residual group will show low mortality. It may be that pur results to some degree have been affected by the factors that Comstock mentions; but we continue to believe that medical selection through the induction physical examination made the most important contribution. REFERENCES
1. Unpublished data from the Health Interview Survey, National Center for Health Statistics, Department of Health, Education, and Welfare 2. The Society of Actuaries: Impairment Study. Society of Actuaries, New York, 1954
Carl C. Seltzer Harvard School of Public Health Boston, Massachusetts Seymour Jablon National Research Council Washington, D. C.
MAIL SURVEY RESPONSE BY SMOKING Seltzer et al. (Am J Epidemiol 100:453-457, 1974) report confirmation of a Kaiser-Permanente finding that cigarette smokers are slower to respond to a mailed questionnaire than nonsmokers. This interesting result certainly needs consideration in design of future mail surveys. Granted that bias lurks almost everywhere to trap the unwary, I do not think
STATUS
that Seltzer et al. have correctly stated the problem when they warn that "the implications of this bias become significant if, in addition to the underrepresentation of smokers responding to questionnaires, there are also differences in the health status between responding smokers and responding nonsmokers." It seems to me that a necessary ingredient for bias in
LETTERS TO THE EDITOR
265
estimates of the association between smok- biased, since they invariably show differing and health is that responding smokers ences between responding smokers and are different in health status from responding nonsmokers. nonresponding smokers (or responding Harold A. Kahn nonsmokers are different from nonresponding nonsmokers). Statistician, Office of Biometry If Seltzer's criterion is right, and I do not and Epidemiology National Eye Institute think it is, all major studies of smoking and Bethesda, Maryland 20014 health that deviate from equal representation of smoking categories are significantly
THE AUTHOR'S In the paper cited by Dr. Kahn, we referred to an earlier Kaiser-Permanente mail survey (Am J Epidemiol 98:50-55, 1973) in which ill smokers were found to respond more quickly than well smokers. The actual data were suggested but not fully reported in table 5 of that study. The pertinent results showed the following: Percentage of ill people in category Respondents to 1st mailing
Male smokers Male nonsmokers
N 16/124 12/162
% 12.9 7.4
Nonrespondents to 1st mailing (who responded to subsequent solicitations) N % 11/158 7.0 9/141 6.4
REPLY
According to Kahn, a "necessary ingredient for bias in estimates of the association between smoking and health is that responding smokers are different in health status from nonresponding smokers." These data are consistent with his statement, and augment the suspicion that many important studies of smoking and health may be significantly biased because of differential response by smoking and health status. Carl C. Seltzer Harvard School of Public Health Boston, Massachusetts
