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Biomedical subjects

J M Nam

Publications and source records attributed to J M Nam.

At least 19 recordsLinked to original sources
PubMed2001

A pilot study to rate determinants of exposure from videotaped work activities of farmers' use of pesticides.

Industrial hygienists often observe workers to determine who should be measured based on a predicted exposure level. Such evaluations are usually based on real-time observation, yet surprisingly little research has been conducted on what determinants of exposure industrial hygienists use to rank workers and whether the ranking is accurate. Accuracy of the ranking may also be affected by the observation process, which is limited when conditions in the workers' immediate environment are rapidly changing. An alternative to real-time observation is videotaping workers and evaluating the tapes at a later date. A pilot study was conducted using previously collected data to determine if workers could be ranked by exposure level utilizing a scoring system to rate videotaped work practices. Six farmers had been videotaped and their dermal exposures measured as they applied insecticide to hogs. In this study, scores were developed to rate the farmers' working conditions by exposure level. Two types of determinants were used to describe exposure: touching and work practices. Touching included the number of times parts of the body had contact with surfaces possibly contaminated with insecticide. Work practices included the types of clothing and protective equipment worn as well as specific practices used by the farmer (e.g., application method). Two raters conducted independent assessments of the videotape using the same criteria. One rater reviewed the tape twice. Agreement between the raters for the "touching" score was weak (intra-class coefficient (ICC) = 0.28), but there was excellent agreement between the two raters (ICC = 0.92) for overall quality of work practices. As expected, a greater number of touches was moderately correlated with an increase in total exposure (rs = 0.60) and there was a weak inverse relationship between protective work practices and the exposure under the clothing (rs = -0.26). All other relationships with exposure level were contrary to what was expected. Since videotapes provide the industrial hygienist with a record of work events and can capture details that might otherwise be missed or not considered they may play a useful role in exposure assessment, especially if carefully developed procedures are followed to overcome the limitations found by this pilot study.

Agriculture↗
PubMed2000

Interval estimation of the kappa coefficient with binary classification and an equal marginal probability model.

We derive a likelihood score method for interval estimation of the intraclass version of the kappa coefficient of agreement with binary classification using a general theory of Bartlett (1953, Biometrika 40, 306-317). By exact evaluation, we investigate statistical properties of the score method, the chi-square goodness-of-fit procedure (Donner and Eliasziw, 1992, Statistics in Medicine 11, 1511-1519; Hale and Fleiss, 1993, Biometrics 49, 523-534), and a crude confidence interval for small and medium sample sizes. Actual coverage percentages of the score and chi-square methods are satisfactorily close to the nominal confidence coefficient, while that of the crude method is quite unsatisfactory. The expected length of the score method is shorter than that of the chi-square procedure when the response rate is very small or very large.

Black or African American↗
PubMed1999

Association of major histocompatibility complex determinants with the development of symptomatic and asymptomatic genital herpes simplex virus type 2 infections.

The clinical spectrum of herpes simplex virus (HSV) infections, ranging from asymptomatic to frequently distressing outbreaks, suggests that there may be immunologic determinants of disease severity that are associated with human leukocyte antigen (HLA) expression. A controlled, prospective study identified several major histocompatibility complex (MHC) class I and II antigens whose frequencies are associated with HSV-2 infection or with frequent symptomatic genital recurrences. Previous studies were hampered by the inability to serologically identify patients with asymptomatic HSV-2 infection. Clinical evaluation and Western blot assay were used to identify 3 subject cohorts: 1 with no prior HSV infections, 1 with HSV-2 antibodies but no recognized symptoms, and 1 with HSV-2 antibodies and frequent genital recurrences. Statistical comparisons of HLA frequencies among these cohorts showed associations of HLA-B27 and -Cw2 with symptomatic disease. Also, HLA-Cw4 was significantly associated with HSV-2 infection. These associations indicate that immunologic factors linked to the MHC influence the risk of HSV-2 infection and disease expression.

Adult↗
PubMed1999

Power and sample size for testing homogeneity of relative risks in prospective studies.

Power and sample-size formulas for testing the homogeneity of relative risks using the score method are presented. The homogeneity score test (Gart, 1985, Biometrika 72, 673-677) is formally equivalent to the Pearson chi-square test, although they look different. Results of this paper may be useful in assessing the validity of the model of a common relative risk before combining several 2 x 2 tables or in designing a prospective study for detecting heterogeneity of relative risks.

Animals↗
PubMed1998

Human leukocyte antigen class II alleles associated with human T-cell lymphotropic virus type I infection and adult T-cell leukemia/lymphoma in a Black population.

BACKGROUND: Human T-cell lymphotropic virus type I (HTLV-I) is linked to adult T-cell leukemia/lymphoma (ATL) and HTLV-I-associated myelopathy (HAM; also known as tropical spastic paraparesis [TSP]), a chronic neurodegenerative disorder. Worldwide, several million HTLV-I carriers are at risk for disease, with an estimated lifetime cumulative risk of 1%-5%. However, the determinants of disease progression are relatively unknown. We studied human leukocyte antigens (HLA class II) that have been implicated in the pathogenesis of HTLV-I-related diseases. METHODS: We analyzed HLA class II alleles among asymptomatic HTLV-I carriers (n = 45), patients with ATL (n = 49) or HAM/TSP (n = 54), and HTLV-I seronegative control subjects (n = 51). All participants were of African descent and were enrolled in epidemiologic studies conducted at the University of the West Indies, Kingston, Jamaica. We used standard microlymphocytotoxicity assays for HLA antigen serotyping and polymerase chain reaction-based methods to examine HLA class II DRB1 and DQB1 alleles. RESULTS: Two antigens determined by serotyping, DR15 and DQ1, occurred at significantly increased frequency among HTLV-I carriers compared with seronegative control subjects (42% versus 22% for DR15 [odds ratio [OR] = 2.7; 95% confidence interval [CI] = 1.0-7.2] and 78% versus 53% for DQ1 [OR = 3.1; 95% CI = 1.2-8.5]). Asymptomatic carriers were shown to have an HLA class II allele distribution similar to that of patients with ATL, and the frequencies of the alleles DRB1*1501, DRB1*1101, and DQB1*0602 were significantly greater in patients with ATL and asymptomatic carriers than in patients with HAM/TSP. In addition, haplotypes DRB1*1101-DQB1*0301 and DRB1*1501-DQB1*0602 were significantly increased among patients with ATL compared with patients with HAM/TSP. CONCLUSIONS: These data suggest that host genetic background is an important factor in determining whether HTLV-I carriers develop either ATL or HAM/TSP.

Alleles↗
PubMed1997

Testing a genetic equilibrium across strata.

A score statistic for testing the Hardy-Weinberg law for data sampled from several populations with different allele frequencies is derived assuming a common coefficient of disequilibrium (odds ratio-type) across strata. The assumption can be examined by a chi-square goodness-of-fit test. It is unlikely such a model would be rejected in typical studies involving human populations. Under this model, a Wald-type test has substantially less power than the score test. An approximate formula for the sample size required for a specific power of the stratified score test for detecting departure from equilibrium is given.

Chi-Square Distribution↗
PubMed1997

Establishing equivalence of two treatments and sample size requirements in matched-pairs design.

Statistical methods for testing the null hypothesis of a nonzero difference between two treatments and the sample size determination for matched-pairs studies are investigated. A Wald-type test proposed by Lu and Bean (1995, Statistics in Medicine 14, 1831-1839) is anticonservative, i.e., its false positive error rate is greater than specified. Score method and normal deviate based on a restricted maximum likelihood estimation are presented. These two test statistics are shown to be algebraically equal. Their actual type I error probabilities are satisfactorily close to a nominal level. Numerical examinations demonstrate that the sample size formulas using these alternative methods are reasonable while that by Lu and Bean is not. The efficiency of matching in equivalence studies is positively related to intercorrelation or the kappa coefficient of agreements. We recommend the score or ML methods to establish equivalence of two treatments for individually matched samples.

Autopsy↗
PubMed1996

Risk factors for adrenal cancer: an exploratory study.

Adrenal cancer is a heterogeneous group of neoplasms with unknown etiology. In search of risk factors, we conducted a case-control study based on data from the 1986 National Mortality Followback Survey, which included a questionnaire sent to the next of kin of almost 20,000 deceased adults (age > or = 25 year) in the United States. Information was obtained on a large number of items, including use of cigarettes, alcohol, oral contraceptives (OCs), height and weight and food consumption patterns. A total of 176 subjects who died of adrenal cancer (88 men and 88 women) and 352 controls (176 men and 176 women) who died of causes unrelated to smoking, drinking or OCs (for female controls) were included in the study. Although information on histologic type was not available, most cases were estimated from incidence surveys to be adrenocortical carcinoma, with a small percentage being malignant pheochromocytoma or neuroblastoma. An increased risk was associated with heavy smoking (> or = 25 cigarettes/day) among men (odds ratio [OR] = 2.0, 95% confidence interval [CI] 1.0-4.4) but not women. No clear association was seen for alcohol use, height and weight or food consumption patterns in either sex. Among women, increased risks were found for ever users of OCs (OR = 1.8, 95% CI 1.0-3.2) and especially those who used them before age 25 (OR = 2.5, 95% CI 1.2-5.5). When the analysis was restricted to subjects with spousal respondents, more pronounced risks were seen for ever users of OCs and for those who used OCs before age 25. Our findings suggest that cigarette smoking and use of OCs may increase the risk of adrenal cancer, but additional studies are needed with more detailed information on risk factors and histologic type of adrenal cancer.

Adrenal Gland Neoplasms↗
PubMed1996

Distribution of human leukocyte antigens in a population of black patients with human T-cell lymphotrophic virus type I-associated adult T-cell leukemia/lymphoma.

Human leukocyte antigens (HLAs) play an important role in regulating the immune response to infectious agents and determinants of malignant transformation. We compared the HLA frequencies of 25 black patients with adult T-cell leukemia/lymphoma (ATL) referred to the National Cancer Institute for therapy with a racially similar, asymptomatic control population of human T-cell lymphotrophic virus, type I (HTLV-I)-seropositive individuals (n = 45). Serological typing was performed for MHC class I and II antigens. Antigen frequencies were calculated, and corresponding gene frequencies were estimated using the maximum likelihood method. Comparisons between the ATL and control group were made with chi 2 or Fisher's exact test. Three antigens (A36, B18, and DR53) were found to have a higher frequency in the ATL patients than in the controls (uncorrected two-tailed P < 0.05). The gene frequencies for these antigens also were statistically significant in the uncorrected analysis. However, only A36 approached statistical significance after correction of the P value for multiple comparisons (P = 0.08). The results of this pilot study indicate that black patients with ATL may have increased frequencies of certain class I HLA when compared with a racially similar HTLV-I-positive reference population. This suggests that either these antigens may represent markers for a population at greater risk of developing ATL once infected with HTLV-I or that they were acquired at some point in the process of malignant transformation or progression from the carrier state to onset of ATL. These antigens should be targeted in larger studies to confirm or refute these findings.

Adult↗
PubMed1995

Sample size determination in stratified trials to establish the equivalence of two treatments.

When designing a trial to establish that a new treatment is as effective as a standard one, the conventional test procedure and sample size based on a null hypothesis of no difference between two treatments is inappropriate. Several authors have investigated test statistics and corresponding sample sizes based on the null hypothesis that the standard treatment is more effective than the new by at least some specific value for a single 2 x 2 table. This paper considers a trial that involves several 2 x 2 tables and presents an approximate formula for the sample size required to obtain a given power of a one-tailed score test for a null hypothesis of a specific common non-zero difference between two treatments across strata. I show that the sample size for a trial based on an unstratified test is always larger than that based on a stratified test when the design is balanced.

Clinical Trials as Topic↗
PubMed1995

Interval estimation and significance testing for cyclic trends in seasonality studies.

The statistical analysis for detecting a seasonal trend in epidemiologic studies has traditionally employed Edwards' method (1961, Annals of Human Genetics 25, 83-85) or a modification (Roger, 1977, Biometrika 64, 152-155) which are formulated under a simple harmonic model with two parameters, amplitude and phase angle. In seasonality studies, researchers usually have known seasons at which peak and trough incidences occur. Utilizing this information, we present the most efficient interval estimation of the ratio of maximum and minimum seasonal frequencies and the uniformly most powerful unbiased test for detecting the seasonal variation using the general theory of Bartlett (1953, Biometrika 40, 12-19). The asymptotic power function and the approximate formula for sample size are derived. A simulation study shows that actual values of power of this score test are satisfactorily close to nominal values even for small samples. The proposed simple score test is more powerful and requires substantially smaller samples for a specific power than the standard Edwards or Roger tests. An alternative method based on the logarithm of the maximum likelihood estimator of the ratio is comparable to the simple score method.

Biometry↗
PubMed1994

Sample size requirements for stratified prospective studies with null hypothesis of non-unity relative risk using the score test.

The standard test of the null hypothesis of unity of the relative risk seeks to determine if two treatments differ. It does not apply when the requirement is either to establish the equivalence of two treatments or to determine whether the relative risk is less than a specified value other than one. This paper presents the asymptotic power function of the score test for the null hypothesis of a specified value of a common relative risk for stratified prospective studies and proposes an approximate formula for the sample size required for a specific power of the test. One can obtain a sample size formula for stratified studies with the standard null hypothesis of unity relative risk as a special case of this formula.

Child, Preschool↗
PubMed1993

Tobacco use and nasopharyngeal carcinoma in a cohort of US veterans.

The risk of nasopharyngeal carcinoma (NPC), a relatively rare neoplasm in the United States, was examined in relation to tobacco use in a cohort of nearly 250,000 US veterans whose mortality experience was followed for 26 years. A total of 48 NPC deaths were identified during the follow-up period. Current smokers had a nearly 4-fold increase in risk of NPC compared with non-users of any tobacco, with risk increasing to 6.4 among those smoking more than 2 packs daily. After adjustment for age and number of cigarettes smoked, risks were inversely associated with age at starting to smoke, with the highest risk observed among those who started smoking before age 15, although no clear trend associated with duration of smoking was observed. Former cigarette smokers had a small excess risk of NPC, but no association was detected for cigar/pipe users. This study adds strong evidence to the increasing literature linking cigarette smoking to NPC.

Adolescent↗
PubMed1992

Control sample size when cases are given in constant ratio stratum-matched case-control studies.

Strata-matched case-control studies based on a given number of cases and k times as many controls are common. We obtain a necessary and sufficient condition for there to exist a numerical solution for k with a desired level of a power. We derive the maximum power of the Cochran test, which may be less than one, when the number of cases is fixed. We provide an approximate formula for the minimum number of cases required for a specified power. There exists a numerical solution (that is, convergence of an iterative method) for k if the number of cases given is greater than this minimum number and no such a solution otherwise. We also show that the incremental gain in relative efficiency of the test with respect to k is diminished as k gets large.

Adult↗
PubMed1992

Cigarette smoking, alcohol, and nasopharyngeal carcinoma: a case-control study among U.S. whites.

BACKGROUND: Nasopharyngeal carcinoma (NPC) is a relatively uncommon cancer in the United States. Its etiology among White Americans is not well known, but cigarette smoking has been implicated in some epidemiologic studies. PURPOSE: The purpose of this study was to investigate the roles of cigarette use and alcohol consumption as risk factors for NPC in the United States. METHODS: We conducted a case-control study using data from the National Mortality Followback Survey based on information from death certificates. In this study, we compared use of cigarettes and alcohol by 204 White American men and women who died of NPC with that by 408 who died of causes unrelated to cigarette smoking and alcohol use. RESULTS: Risk of NPC increased in proportion to the amount and duration of smoking (with a more than threefold increase among persons smoking heavily) and declined following cessation of smoking. After controlling for smoking, we found an 80% excess risk of NPC among persons whose intake of alcohol was heavy. CONCLUSION: Use of cigarettes and consumption of alcohol were found to be statistically significant risk factors for NPC. The findings are among the strongest to date indicating that use of cigarettes and perhaps alcohol may contribute to the etiology of these relatively rare cancers among Whites in the United States.

Adult↗
PubMed1992

Optimum sample size determination in stratified case-control studies with cost considerations.

We investigate sample size determination for Cochran's test for stratified case-control studies when samples of cases and controls are allocated to maximize the asymptotic efficiency of Cochran's test subject to fixed total cost with cost per control varying by strata. We consider two situations typical of strata-matched case-control studies: when one samples both cases and controls and when cases are given and one samples controls. In each situation we develop and study an asymptotic method for finding the sample size required for a specific power under the optimum allocation proposed by Nam and Fears. Also, for the second situation, we investigate an asymptotic method for determining the common ratio, k, in one-to-k strata-matched case-control studies without cost consideration for a given power. When cases are given, neither the optimum nor the standard control sample sizes appear in a closed form; we present numerical methods for calculating these sample sizes and illustrate them with examples. We find the reduction in total cost obtained under the optimum allocation compared to standard allocation more pronounced as the differences in stratum-specific costs of sampling controls increase.

Adult↗
PubMed1990

Optimum allocation of samples in strata-matching case-control studies when cost per sample differs from stratum to stratum.

We investigate the allocation of samples over strata in case-control studies with consideration of sampling costs. We consider two situations, typical of strata-matched case-control studies. Subject to a fixed total cost we derived optimum allocations that maximize the asymptotic efficiency of Cochran's test, that is, minimize the variance of the maximum likelihood estimator of the common odds ratio when the common odds ratio is one. Our results show that the standard design with equal numbers of cases and controls in each stratum is inefficient when costs per control differ greatly from stratum to stratum. We show that the optimum design suggested in this paper is robust for values of the odds ratio common in epidemiologic studies.

Case-Control Studies↗