Biomedical subjects

J M Nam

Publications and source records attributed to J M Nam.

33 records · Page 2Linked to original sources

PubMed1990

Comparison of simple and modified Bernstein's methods in estimation of recessive allele frequency in the HLA system.

Bias and inefficiency of Bernstein's estimator of the recessive allele frequency in generalized ABO-like systems are important for situations where the frequency is small and the number of codominant alleles is large. A modified Bernstein's estimator of the recessive (Smith, 1967) has much smaller bias and higher efficiency than Bernstein's for large samples. Analogous findings are generally observed for small and moderate samples by Monte Carlo experiments. The modified method is preferred for the recessive frequency in the HLA system. The results are illustrated by numerical examples.

ABO Blood-Group System↗

PubMed1990

Approximate interval estimation of the difference in binomial parameters: correction for skewness and extension to multiple tables.

Recently, Beal (1987, Biometrics 43, 941-950) found Mee's modification of Anbar's approximate interval estimation for the difference in binomial parameters to be a good choice in small sample sizes. As this method can be derived from the score theory of Bartlett, it is easily corrected for skewness. Exact numerical evaluation shows that this correction is not as important for this case as for the ratio of binomial parameters (Gart and Nam, 1988, Biometrics 44, 323-338). The score theory is also used to extend this method to the stratified or multiple-table case. Thus, good approximate interval estimates for differences, ratios, and odds ratios of binomial parameters can all be derived from the same general theory.

Binomial Distribution↗

PubMed1989

HLA associations with obesity.

A subgroup of 351 subjects with human leukocyte antigen (HLA) typing were available from the Framingham Heart Study for analyses to identify associations with obesity. The subjects consisted of 143 males and 208 females aged 58-88 years at the 15th biennial examination in 1978. The obese classification was based on maximum body mass index (BMI) over the 16 available biennial examinations of the Framingham Heart Study. The subjects were classified as obese if they exceeded the 95th percentile of BMI for 20- to 29-year-old subjects as described in the NHANES II study; males were obese if BMI greater than 31.1 and females were obese if BMI greater than 32.3. There were 27 obese males (18.9%) and 44 obese females (21.2%) in the sample. Gene frequencies were compared between the nonobese and obese groups for the pooled sample as well as by sex. Among alleles previously shown to be related to obesity, HLA Bw35 appeared to be more frequent in obese females but these data did not confirm a difference for the B18 or Cw4 alleles. More importantly, HLA Aw30 was found to be significantly higher among the obese subjects in both males and females. Further analyses adjusting for potential confounding variables reduced the estimated relative risk for obesity for subjects with the Bw35 allele to approximately 1.30 and no longer significant for this sample size. In contrast, the relative risk for Aw30, while reduced, remained significant after adjustment for confounding variables. Based on these data, individuals with the Aw30 allele have a relative risk of 2.61 for obesity.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗

PubMed1988

The equivalence of two tests and models for HLA data with no observed double blanks.

Gart and Nam (1984, Biometrics 40, 887-894) consider the analysis of an ABO-like model based on the Hardy-Weinberg law in the commonly occurring case of human leukocyte antigen (HLA) data where there are no double blanks, that is, no recessive homozygotes. They derive a score test, based on the truncated likelihood, of the hypothesis that the true recessive gene (or allele) frequency is zero. Yasuda (1968, Biometrics 24, 915-935) considers a similar codominant system wherein the true recessive gene frequency is assumed zero, but the Hardy-Weinberg law does not hold. In particular, he considers the possibility of a nonzero inbreeding coefficient. We show that the two models are equivalent; each likelihood can be shown to be a reparameterization of the other. Furthermore, the score test of the zero gene frequency in Gart and Nam is identical to the score test for a zero inbreeding coefficient given by Yasuda. The results are applied to an example wherein it appears that the Hardy-Weinberg model is appropriate. Thus, it is not possible in this population to identify homozygous individuals without error from phenotypic data alone.

Consanguinity↗

PubMed1987

On two tests of fit for HLA data with no double blanks.

Stevens suggests a test of fit, based on Bernstein's estimators, of the Hardy-Weinberg law for the ABO system. Nam and Gart extend this test to the generalized ABO-like system and apply it to HLA data. When the recessive gene is rare, Huether and Murphy recall Haldane's point that its Bernstein's estimator is negatively biased and go on to suggest novel corrected versions of it. With the identification of more HLA antigens, it is not uncommon to find, in certain populations, that the sample data contain no double blanks; that is, every individual reacts to at least one antigen for a given locus. Gart and Nam give a simple score test of a zero true recessive-gene frequency for such situations. Here we examine the extended test of Stevens as a test of this hypothesis. We find that it is fully efficient for two codominant alleles but that when the number exceeds two its efficiency may be 50% or lower or as high as 100%, depending on the number of alleles and the pattern of gene frequencies. The tests are applied to a set of HLA data.

ABO Blood-Group System↗

PubMed1987

A simple approximation for calculating sample sizes for detecting linear trend in proportions.

A simple approximate formula for sample sizes for detecting a linear trend in proportions is derived. The formulas for both the uncorrected and corrected Cochran-Armitage test are given. For two binomial proportions these reduce to those given by Casagrande, Pike, and Smith (1978, Biometrics 34, 483-486). Some numerical results of a power study for small sample sizes show that the nominal power corresponding to the approximate sample size is a reasonably good approximation to the actual power.

Analysis of Variance↗

PubMed1984

A score test for the possible presence of recessive alleles in generalized ABO-like genetic systems.

In generalized ABO-like genetic systems, particularly for certain human leukocyte antigen (HLA) loci, it is quite likely, even for large sample sizes, that no double recessives (or 'double blanks') may be found. It has been suggested that all such data should be analyzed as if the true recessive gene frequency were 0. We derive an easily computed score [or C(alpha)] statistic which tests for the possibility that, in such instances, this frequency may exceed 0. An associated point estimator is suggested. The methods are applied to the data analyzed by Smouse and Williams (1982, Biometrics 38, 757-768).

ABO Blood-Group System↗

PubMed1980

Skin melanoma and seasonal patterns.

While laboratory experiments have never proved that solar ultraviolet radiation (UV-B) causes skin melanoma, epidemiological research has strongly suggested that this is the case. Monthly patterns of the solar radiation which reaches the earth's surface show that the greatest intensities of UV-B occur during the summer months. This study uses first-order harmonics (sine curve models) to analyze the monthly incidence of skin melanoma in the United States and to test for seasonal patterns. The association between occurrence of melanoma and the degree of solar insolation may provide new insight into the mechanics of the biologic action spectrum and the dose-response relationships suspected for this disease. Does short term, high intensity UV-B promote tumor development? Using data from the Third National Cancer Survey (1969-1971), seasonal patterns for whites were studied in detail by anatomic site, sex, age and geographic region. Overall, a strong seasonal pattern with summertime peak was observed for females. This was most pronounced for women under age 55 years, and for women of all ages with melanomas of the upper and lower extremities. Among men, a seasonal pattern with summertime peak was observed only for melanomas of the upper extremities. From the data available, it is difficult to determine if the seasonal patterns of melanoma result from the promoting effects of UV-B exposure or from enhanced recognition during the summer months.

Adult↗

PubMed1976

Bernstein's and gene-counting methods in generalized ABO-like systems.

Although the simple and adjusted Bernstein's methods are fully efficient for m=2 and m=3 (ABO system) respectively, their efficiency declines for larger values of m. For m greater than or equal to 4, the adjusted or modified Bernstein's method with a single counting iteration leads to a nearly efficient estimator. A single degree of freedom chi-square test of the Hardy-Weinberg law for all m is derived. Some findings on the statistical efficiency of typing various numbers of antigens are given. All the results are illustrated in numerical examples.

ABO Blood-Group System↗

PubMed1973