Dejian Lai - Academia.edu (original) (raw)

Papers by Dejian Lai

Frontiers in applied mathematics and statistics, Mar 27, 2024

Propensity score is one of the most commonly used score functions in adjusting for covariates eff... more Propensity score is one of the most commonly used score functions in adjusting for covariates effect in statistical inference. It is important to understand the impact with propensity score in case some of the prespecified covariates are severely imbalanced. In this article, we performed simulation evaluation the empirical type 1 error and empirical power under scenario of imbalanced covariates in several nonparametric two sample tests with propensity score or with other covariate adjustments. Our results suggest common propensity score approaches might have type 1 error inflation at scenarios with severe imbalanced covariates or model is mis-specified.

The Kolmogorov-Smirnov (KS) test has been popular in many applied fields. Published research has ... more The Kolmogorov-Smirnov (KS) test has been popular in many applied fields. Published research has suggested the utility of the KS test in image processing, histogram analysis, and PET/CT scans. However, the fundamental assumption of independence in a statistical model is easily overlooked. When the KS test is applied to spatial analysis, autocorrelation may cause the KS test to have an inflated type I error (small p-values) if no adjustments for spatial correlation are applied. To adjust for autocorrelation, the KS test must incorporate spatial adjustment. The spatially-adjusted KS has a controlled type I error and non-inferior power compared to the original KS test. Utilizing the KS test with spatial adjustment, we reanalyzed a trial comparing two types of stress medications: regadenoson (administered using different timings) versus dipyridamole. To analyze the PET scans with spatial autocorrelation, we introduced a novel way of reconstructing the shape of the human heart using spherical coordinates, and compared the KS test with spatial adjustment to a KS test with adjustment for correlation. The results showed that the reconstructed PET scans analyzed by the KS test with spatial adjustment have controlled p-values.

$Research paper thumbnail of Sequential monitoring clinical trial by conditional power under drift fractional Brownian motion$

Communications in Statistics - Simulation and Computation, Dec 12, 2022

American Journal of Hypertension, Dec 26, 2018

Elevations of fasting glucose (FG) levels are commonly encountered in people treated for hyperten... more Elevations of fasting glucose (FG) levels are commonly encountered in people treated for hypertension (HTN). A high prevalence of insulin resistance 1,2 and of obesity 3 underlie these associations. The choice of medications used for the treatment of HTN also influences the risk for incident glucose elevations: thiazide diuretics increase risk, 4 whereas ACE inhibitors (ACEi) lower risk. 5 In the Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial (ALLHAT) the risk of incident diabetes mellitus (DM) (≥126 mg/dl) at 2-year follow-up among nondiabetic individuals at baseline was highest among users of the thiazide-like diuretic chlorthalidone (9.3%) and lowest among users of the ACEi lisinopril (5.6%). 6 Users of the calcium blocker amlodipine had an intermediate risk (7.2%). Over the past 15 years, many gene variants have been identified that increase the risk for elevated FG levels and for DM. 7 To our knowledge, no study has examined whether these genes interact with medications used to treat HTN to increase FG levels. In this study we determine (i) if a gene risk score (GRS), based on gene polymorphisms that predispose to glucose elevation, interacts with the 3 primary antihypertensive medications used in ALLHAT to increase the risk of FG level elevations in nondiabetic participants (drug-gene interaction) and (ii) whether a GRS improves the discriminatory ability of traditional risk factors for predicting new-onset glucose disorders in persons treated for HTN with different antihypertensive medications.

During spaceflight, skeletal unloading results in loss of bone mineral density (BMD). This occurs... more During spaceflight, skeletal unloading results in loss of bone mineral density (BMD). This occurs primarily in the spine and lower body regions. This loss of skeletal mass could prove hazardous to astronauts on flights of long duration. In this study, intense resistance exercise was used to test whether a training regimen would prevent the loss of BMD that accompanies disuse. Nine subjects (5 men, 4 women) participated in a supine maximal resistance exercise training program during 17 wk of horizontal bed rest. These subjects were compared with 18 control subjects (13 men, 5 women) who followed the same bed rest protocol without exercise. Determination of treatment effect was based on measures of BMD, bone metabolism markers, and calcium balance obtained before, during, and after bed rest. Exercisers and controls had significantly (P Ͻ 0.05) different means, represented by the respective following percent changes: lumbar spine BMD, ϩ3% vs. Ϫ1%; total hip BMD, ϩ1% vs. Ϫ3%; calcaneus BMD, ϩ1% vs. Ϫ9%; pelvis BMD, Ϫ0.5% vs. Ϫ3%; total body BMD, 0% vs. Ϫ1%; bone-specific alkaline phosphatase, ϩ64% vs. 0%; alkaline phosphatase, ϩ31% vs. ϩ5%; osteocalcin, ϩ43% vs. ϩ10%; 1,25 dihydroxyvitamin D, ϩ12% vs. Ϫ15%; parathyroid hormone intact molecule, ϩ18% vs. Ϫ25%; and serum and ionized calcium, Ϫ1% vs. ϩ1%. The difference in net calcium balance was also significant (ϩ21 mg/day vs. Ϫ199 mg/day, exercise vs. control). The gastrocnemius and soleus muscle volumes decreased significantly in the exercise group, but the loss was significantly less than observed in the control group. The results indicate that resistance exercise had a positive treatment effect and thus might be useful as a countermeasure to prevent the deleterious skeletal changes associated with long-duration spaceflight. microgravity; bed rest; bone resorption; bone formation; strength training UNDER CONDITIONS OF REDUCED weight bearing, bone resorption increases and bone mineral density (BMD) decreases. The response of bone to microgravity is an extreme case of the reduced weight-bearing response and has been well documented in US and Russian crew members on long-duration missions aboard the Russian space station Mir and the International Space Station (ISS). To date, we have collected preand postflight bone densitometry measurements on 47 individuals from such flights. Although losses show significant heterogeneity among individuals and between bones of a given subject, bone loss is a consistent finding after spaceflight. Among astronauts and cosmonauts who participated in longduration (average of 170 days) flights aboard Mir and the ISS,

PubMed, Mar 1, 2000

The prevalence of ascariasis is very high in parts of rural areas in southern China. The high pre... more The prevalence of ascariasis is very high in parts of rural areas in southern China. The high prevalence of ascariasis is one of the most neglected public health problems. A longitudinal investigation on the prevalence of ascariasis in a rural community in China was reported previously. In this article, we re-analyze the reported age specific prevalence of ascariasis using a life table method. The results from our analysis may provide a better summary of the impact of the disease on the studied population.

International Journal on Disability and Human Development, Sep 14, 2013

In this study, we performed a weighted principal component analysis on human development indicato... more In this study, we performed a weighted principal component analysis on human development indicators to measure and analyze the progress of well-being throughout the world from 1999 to 2010. The main principal component (MPC) was employed to quantify the temporal changes of the human development of several selected countries by the proposedZ-test. The trends of the human development in some countries with high human development, Sub-Saharan African countries, Brazil, Russia, India, China (BRIC), the USA, and Qatar were discussed in terms of the impact of public health and economic development. We also studied the association of the MPC obtained from our study and the human development index reported by the United Nations Development Program using the Spearman’s rank correlation coefficient.

Medicine and Science in Sports and Exercise, Jun 1, 2006

To examine the associations between physical activity and body mass index (BMI) among females age... more To examine the associations between physical activity and body mass index (BMI) among females aged 14 to 17 yr. A convenience sample of 65 Mexican American, 58 African American, and 49 non-Hispanic white girls in an urban high school in Texas participated in this study. Physical activity was assessed by ankle actigraphy. Average activity per period (before, during and after school) of the day and total activity were derived by a software program as movements per minute. The Center for Disease Control and Prevention 2000 BMI charts were used to calculate overweight status. Pearson correlation coefficient and analysis of variance were used to determine the strength of association and to compare activity levels by BMI status. There was a statistically significant inverse association between total diurnal physical activity level and BMI (r=-0.37; P&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;0.05); and a statistically significant association between after-school activity and BMI (r= -0.28; P&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;0.05). After adjusting for the effects of age, race, and sexual maturity, total diurnal activity level was 10% less (P=0.03) for overweight females; for females at risk of overweight, diurnal activity level was 6% less (P=0.04) than for normal-weight females. Ethnic differences showed an 8% lower activity level among African American than for non-Hispanic white and 6% lower than for Mexican American females. The study indicated that adolescent females in schools are at risk for inactivity at certain times of the day. Schools in partnership with their communities should assume a major role to promote participation in physical activity among adolescents through innovative activity programs in schools.

American Journal of Obstetrics and Gynecology, 2022

Communications in Statistics - Simulation and Computation, Feb 1, 2021

Statistics, 2000

We introduce local asymptotic normality (LAN) for the log-likelihood ratio and the contiguity pro... more We introduce local asymptotic normality (LAN) for the log-likelihood ratio and the contiguity property of a class of location-scale processes in this paper. Using the contiguity property, we study the asymptotic distribution of the estimated Brock-Dechert-Scheinkman (BDS) statistic from the standardized residuals of fitting location-scale type processes.

Communications in Statistics - Simulation and Computation, Mar 31, 2010

Brownian motion has been used to derive stopping boundaries for group sequential trials, however,... more Brownian motion has been used to derive stopping boundaries for group sequential trials, however, when we observe dependent increment in the data, fractional Brownian motion is an alternative to be considered to model such data. In this article we compared expected sample sizes and stopping times for different stopping boundaries based on the power family alpha spending function under various values of Hurst coefficient. Results showed that the expected sample sizes and stopping times will decrease and power increases when the Hurst coefficient increases. With same Hurst coefficient, the closer the boundaries are to that of O'Brien-Fleming, the higher the expected sample sizes and stopping times are; however, power has a decreasing trend for values start from H = 0.6 (early analysis), 0.7 (equal space), 0.8 (late analysis). We also illustrate study design changes using results from the BHAT study.

Public Health, Apr 1, 2013

Background To estimate the potential gain in life expectancy from addressing modifiable risk fact... more Background To estimate the potential gain in life expectancy from addressing modifiable risk factors for all-cause mortality (excluding suicide and deaths from accidents or violence) across specific serious mental illness (SMI) subgroups, namely schizophrenia, schizoaffective disorders, and bipolar disorders in a Western population.

Computational Statistics & Data Analysis, Apr 1, 2004

In this paper, we use a direct maximum likelihood method in estimating the Hurst coe cient for fr... more In this paper, we use a direct maximum likelihood method in estimating the Hurst coe cient for fractional Brownian motion with a short length of observations. We show that the estimate is asymptotically unbiased and derive an easy to use formula for computing the variance of the estimate. We also investigate the ÿnite sample properties via Monte Carlo simulations. The simulation studies indicate that the theoretical formulas based on large sample arguments can be used when the number of observations is relatively small. A real example for monitoring a clinical trial is used to illustrate and compare various methods.

JP journal of biostatistics, Jul 10, 2019

Scandinavian journal of social medicine, Dec 1, 1997

In this paper, we study a nonparametric spatial pattern test statistic, the D statistic. The D st... more In this paper, we study a nonparametric spatial pattern test statistic, the D statistic. The D statistic is an effective test statistic for testing spatial patterns in regional health data. Comparison studies of the D statistic with fixed weights and random weights are illustrated on the atlas of the Chinese cancer mortality rates and on other cancer atlas. Some social, economic and environmental reasons for statistically significant spatial autocorrelations of the Chinese cancer mortality rates were given in the discussion section. The method for calculating the mean and the variance of the randomly weighted D statistic is given in the appendix.

International Journal of Epidemiology, Oct 1, 2000

Journal of Applied Statistics, 2000

The purpose of this paper is to extend the widely used classical Brownian motion technique for mo... more The purpose of this paper is to extend the widely used classical Brownian motion technique for monitoring clinical trial data to a larger class of stochastic processes, i.e. fractional Brownian motion, and compare these results. The beta-blocker heart attack trial is presented as an example to illustrate both methods.

International Journal of Epidemiology, Oct 1, 1999

The human life span can be characterized in several stages according to various point of views. E... more The human life span can be characterized in several stages according to various point of views. Economically, for example, the early years (0-14) are devoted to an investment in future years, the years between 15 and 64 are dominated by the productive (working) period and the years beyond 65 are characterized as a period of consumption. 1 People of working age usually assume a greater responsibility for the daily activities of a society than those in other ages. Deaths occurring in the working age groups lead to heavier social and economic burdens on society. Accurately measuring the impact of premature deaths from various diseases in people of working age is important for allocating limited resources which aim to reduce the burden of such diseases on society and is also essential for suggesting future research directions as well as intervention programmes that are likely to be of significant public health importance.

Estimating HCV incidence and viral clearance in other than young, White, injection drug users (ID... more Estimating HCV incidence and viral clearance in other than young, White, injection drug users (IDUs), negative for HBV and HIV has not been undertaken. We enrolled 1,260 mostly African American injecting and non-injecting drug users negative for HIV and HBV prospectively from urban areas in Houston, TX, from 2005-2009, to determine HCV incidence and spontaneous viral clearance. Socio demographic and high risk behavioral data and blood samples were collected every 6 months for two years. Serum was tested for antibodies to HCV by enzyme immunoassay (EIA), and confirmed for HCV RNA by polymerase chain reaction (PCR). Kaplan-Meier failure curves and Cox proportional hazard regression analyses were performed to identify factors for HCV incidence. A follow-up HCV RNA PCR test was used to determine viral clearance in HCV incident cases. We followed 840 anti-HCV negative participants resulting in 19 HCV seroconversions per 1436.67 person years for an HCV incidence of 13.2/1000 PY (95% Confidence Interval [CI], 8.420.7). IDU’s risk of contracting HCV compared to non-injecting drug users (62.5 per 1000PY, 95% CI, 33.6-116.2, vs. 7.1 per 1000PY, 95% CI 3.7-13.6) was significantly higher. White/Hispanic race, injecting drugs at least daily, and being 40 years of age or older were significant risk factors for HCV seroconversion. Spontaneous viral clearance was observed in 42% of HCV incident cases. HCV incidence remains high among HB vaccinated drug users. Daily injecting drug use is a predictor for new HCV infection. An HCV vaccine is urgently needed, especially in high risk populations.