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Biomedical subjects

O Siddiqui

Publications and source records attributed to O Siddiqui.

At least 19 recordsLinked to original sources
PubMed2000

Multiple imputation compared with some informative dropout procedures in the estimation and comparison of rates of change in longitudinal clinical trials with dropouts.

Statistical analysis based on multiple imputation (MI) of missing data when analyzing data with missing observations is gaining popularity among statisticians because of availability of computing softwares; it might be tempting to use MI whenever data is missing. An important assumption behind MI is the "ignorability of missingness." In this paper, we demonstrate the use of MI in conjunction with random effects models and several other methods that are devised to handle nonignorable missingness (informative dropouts). We then compare the results to assess sensitivity to underlying assumptions. Our focus is primarily to estimate and compare rates of change (of a primary variable). The application dataset has a high dropout rate and has features to suggest informativeness of the dropout process. The estimates obtained under random effects modeling with multiple imputation were found to differ substantially from those obtained by methods devised to handle informative dropouts.

Algorithms↗
PubMed2000

Random-effects regression analysis of correlated grouped-time survival data.

Random-effects regression modelling is proposed for analysis of correlated grouped-time survival data. Two analysis approaches are considered. The first treats survival time as an ordinal outcome, which is either right-censored or not. The second approach treats survival time as a set of dichotomous indicators of whether the event occurred for time periods up to the period of the event or censor. For either approach both proportional hazards and proportional odds versions of the random-effects model are developed, while partial proportional hazards and odds generalizations are described for the latter approach. For estimation, a full-information maximum marginal likelihood solution is implemented using numerical quadrature to integrate over the distribution of multiple random effects. The quadrature solution allows some flexibility in the choice of distributions for the random effects; both normal and rectangular distributions are considered in this article. An analysis of a dataset where students are clustered within schools is used to illustrate features of random-effects analysis of clustered grouped-time survival data.

Biometry↗
PubMed1999

The application of Poisson random-effects regression models to the analyses of adolescents' current level of smoking.

BACKGROUND: In school-based smoking prevention research, it is still debatable whether parents or peers are most influential to maintained smoking among adolescents. As a result, this study examines the effects of parental and peer approval of smoking on adolescents' current levels of smoking. METHODS: Poisson random-effects regression models were used to assess the effects of parental and peer approval of smoking on adolescents' (n = 913) current level of smoking. RESULTS: Results of these analyses indicate that a stronger relationship between parental approval of smoking and current level of smoking was found for female adolescents than for male adolescents. Conversely, a stronger relationship between peer approval of smoking and current level of smoking was found for male adolescents than for female adolescents. With respect to race, the influence of parental approval of smoking on adolescents' current level of smoking was generally more pronounced for minority adolescents, relative to white adolescents. However, the influence of peer approval of smoking on current level of smoking was strongest for white adolescents and was less strong for black, Hispanic, and Asian adolescents. CONCLUSION: The findings of this study demonstrated that the effects of parental and peer approval of smoking on adolescents' current levels of smoking were varied by gender and race. These differential effects may have some implication for the development of future school-based smoking prevention and cessation programs.

Adolescent↗
PubMed1999

Linear structural equation model in analyzing quality-of-life-data from clinical trials.

Assessment of quality of life (QOL) in clinical trials becomes a challenging task from the viewpoint of clinical biostatistics. The responses of the items for measuring QOL indices usually vary widely from patient to patient and from time to time. Measurement errors might be present in the responses of the items, and they might be correlated. Hence, in analyzing QOL data, the usual assumption that there are no measurement errors in responses is too liberal. Because the QOL indices are likely to be correlated, separate analysis of each index might not be efficient from the point of view of statistical methodology. We apply linear structural equation modeling (LISREL) in assessing the QOL data obtained from a clinical trial. A basic premise of the LISREL approach is that the abstract concepts (latent constructs) that are not directly measurable can be studied. LISREL is a statistical procedure for conceiving and testing structural hypotheses that cannot be tested adequately with other statistical procedures. It allows us to specify relations between unobserved and observed variables while controlling for measurement errors and correlations among both the measurement errors and the latent constructs.

Arterial Occlusive Diseases↗
PubMed1999

Characteristics of inconsistent respondents who have "ever used" drugs in a school-based sample.

This study examines the predictors of inconsistent responses from adolescents to questions about whether they ever used alcohol, cigarettes, and marijuana. Male adolescents had significantly higher rates of inconsistent responses than female adolescents. Black and Hispanic adolescents had significantly higher rates of inconsistent responses regarding ever using alcohol and cigarettes (only for Black) than White adolescents. The subjects' living status and academic achievements were significant predictors of inconsistent responses regarding ever using marijuana. Thus, these results are consistent with the notion that inconsistent responses may bias the estimation of the prevalence of ever using drugs in multivariate analyses.

Adolescent↗
PubMed1998

A comparison of the random-effects pattern mixture model with last-observation-carried-forward (LOCF) analysis in longitudinal clinical trials with dropouts.

The last-observation-carried-forward imputation method is commonly used for imputting data missing due to dropouts in longitudinal clinical trials. The method assumes that outcome remains constant at the last observed value after dropout, which is unlikely in many clinical trials. Recently, random-effects regression models have become popular for analysis of longitudinal clinical trial data with dropouts. However, inference obtained from random-effects regression models is valid when the missing-at-random dropout process is present. The random-effects pattern-mixture model, on the other hand, provides an approach that is valid under more general missingness mechanisms. In this article we describe the use of random-effects pattern-mixture models under different patterns for dropouts. First, subjects are divided into groups depending on their missing-data patterns, and then model parameters are estimated for each pattern. Finally, overall estimates are obtained by averaging over the missing-data patterns and corresponding standard errors are obtained using the delta method. A typical longitudinal clinical trial data set is used to illustrate and compare the above methods of data analyses in the presence of missing data due to dropouts.

Clinical Trials as Topic↗
PubMed1996

Intraclass correlation estimates in a school-based smoking prevention study. Outcome and mediating variables, by sex and ethnicity.

Most school-based smoking prevention studies employ designs in which schools or classrooms are assigned to different treatment conditions while observations are made on individual students. This design requires that the treatment effect be assessed against the between-school variance. However, the between-school variance is usually larger than the variance that would be obtained if students were individually randomized to different conditions. Consequently, the power of the test for a treatment effect is reduced, and it becomes difficult to detect important treatment effects. To assess the potential loss of power or to calculate appropriate sample sizes, investigators need good estimates of the intraclass correlations for the variables of interest. The authors calculated intraclass correlations for some common outcome variables in a school-based smoking prevention study, using a three-level model-i.e., students nested within classrooms and classrooms nested within schools. The authors present the intraclass correlation estimates for the entire data set, as well as separately by sex and ethnicity. They also illustrate the use of these estimates in the planning of future studies.

Adolescent↗
PubMed1995

The television, school, and family smoking prevention and cessation project. VIII. Student outcomes and mediating variables.

BACKGROUND: This paper presents the student outcomes of a large-scale, social-influences-based, school and media-based tobacco use prevention and cessation project in Southern California. METHODS: The study provided an experimental comparison of classroom delivery with television delivery and the combination of the two in a 2 x 2 plus 1 design. Schools were randomly assigned to conditions. Control groups included "treatment as usual" and an "attention control" with the same outcome expectancies as the treatment conditions. Students were surveyed twice in grade 7 and once in each of grades 8 and 9. The interventions occurred during grade 7. RESULTS: We observed significant effects on mediating variables such as knowledge and prevalence estimates, and coping effort. The knowledge and prevalence estimates effects decayed partially but remained significant up to a 2-year follow-up. The coping effort effect did not persist at follow-ups. There were significant main effects of both classroom training and TV programming on knowledge and prevalence estimates and significant interactions of classroom and TV programming on knowledge (negative), disapproval of parental smoking, and coping effort. There were no consistent program effects on refusal/self-efficacy, smoking intentions, or behavior. CONCLUSIONS: Previous reports demonstrated successful development and pilot testing of program components and measures and high acceptance of the program by students and parents. The lack of behavioral effects may have been the result of imperfect program implementation or low base rates of intentions and behavior.

Child↗
PubMed1994

Differential influence of parental smoking and friends' smoking on adolescent initiation and escalation of smoking.

Smoking-related behaviors and attitudes of significant others (especially friends and parents) are among the most consistent predictors of adolescent smoking. However, theorists remain divided on whether the behaviors of significant others influence adolescent smoking directly or indirectly, and the relative influence of parental and peer smoking on adolescents' own smoking is still a matter of debate. In addition, little research has examined the role of significant others' behavior on different stages of smoking onset. In particular, not much information is available regarding gender and ethnic differences in social influences on smoking behavior. We use structural equation modeling to address these issues. Different theoretical perspectives from cognitive-affective theories (Ajzen 1985; Ajzen and Fishbein 1980) and social learning theories (Akers et al. 1979; Bandura 1969, 1982, 1986) have been integrated into a structural model of smoking influence. The results show that friends' smoking affects adolescent initiation into smoking both directly and indirectly, whereas parental smoking influences smoking initiation only indirectly. The data also show that friends' and parents' smoking affect smoking escalation only indirectly. In general, friends' smoking has a stronger effect on adolescents' smoking behavior, particularly on initiation. Multiple group comparisons of the structural models predicting smoking initiation among males and females reveal that parental approval of smoking plays a significant mediating role for females, but not for males. Comparisons of Whites, Blacks, Hispanics, and other ethnic groups reveal that there are some significant differences in the pathways of friends' influences among the four groups.

Adolescent↗
PubMed1989

Percutaneous absorption of steroids: relative contributions of epidermal penetration and dermal clearance.

The permeation of triamcinolone, hydrocortisone, prednisolone, corticosterone, triamcinolone acetonide, testosterone, and betamethasone-17-valerate through excised human stratum corneum was quantified. The time course of permeation could be adequately described by a simple diffusion model suggesting that shunt transport may not be important. The disappearance of these steroids from aqueous solutions applied to human and rat dermis was also monitored. The concentrations of unbound steroid in the viable epidermis appeared to be mainly related to the blood perfusion rate in the dermis and, more importantly, to the lipophilicity of the steroid. The most lipophilic steroids penetrated the human epidermis at the fastest rates but are cleared from the viable epidermis at rates comparable to those found for more polar steroids.

Diffusion↗
PubMed1989

Iontophoretic transport of weak electrolytes through the excised human stratum corneum.

The effect of iontophoresis on the rate of permeation of a number of therapeutically active weak acids and bases through excised human stratum corneum has been examined, over a range of pH values. It has been shown that the amount of ionized drug species present in the drug solution is an important factor in the delivery of acids and bases by this route and that the molecular weight of the drug does not influence the rate of delivery by iontophoresis.

Chromatography, High Pressure Liquid↗
PubMed1989

Physicochemical, physiological, and mathematical considerations in optimizing percutaneous absorption of drugs.

The percutaneous absorption or transdermal permeation of drugs has gained considerable prominence in recent years through the development of transdermal systemic delivery systems and the related interest in the design and use of topical products. The present review discusses the phenomenon of percutaneous absorption in drug delivery and its importance in regard to the anatomy of skin, its physiological function, and biomechanical properties, as well as the physicochemical properties of the skin and the drug. The mathematical relationships and models used to describe the phenomenon of percutaneous absorption are also reported. The importance of these factors in optimizing percutaneous absorption or transdermal permeation of drugs for local or systemic effect is also discussed.

Animals↗
PubMed1987

Nonparenteral administration of peptide and protein drugs.

Peptides/proteins are an important class of drugs which are usually administered by parenteral route. In recent years, pharmaceutical research has been directed towards developing a nonparenteral route of delivery of peptide/protein drugs. These studies report that it may be possible to administer the peptides/proteins especially insulin, by nasal, buccal, rectal, or even transdermal route. Therefore, there is a great potential for future development of a nonparenteral route of delivery of peptide/protein drugs. The primary objective of this review is to report the present status of research involving nonparenteral administration of macromolecular peptides/proteins, with special emphasis on insulin.

Administration, Cutaneous↗
PubMed1985

The effect of iontophoresis and vehicle pH on the in-vitro permeation of lignocaine through human stratum corneum.

The effect of iontophoresis and the pH of aqueous vehicles on the rate and extent of permeation of lignocaine through excised human stratum corneum was investigated. In the absence of iontophoresis, the rate of penetration was greatest at the higher pH values where lignocaine exists mainly in the unionized form; iontophoresis was most effective at the lower pH values where lignocaine is mainly ionized. At pH 3.4, and 5.2, the flux increased during iontophoresis, by approximately 8.5 and 4 times, respectively, relative to that occurring without iontophoresis. The present results suggest that some weak electrolytes which show poor percutaneous penetration may be administered topically using iontophoresis provided the drug is kept in a highly ionized form.

Humans↗
PubMed1983

Physical factors affecting microencapsulation by simple coacervation of gelatin.

Gelatin has been coacervated at 60 degrees C using sodium sulphate. Interfacial tensions between coacervate and supernatant liquid, coacervate and two oils (with and without one of two drugs, clofibrate and chlormethiazole) and supernatant liquid and the oils (+/- drug) have been measured by a drop volume technique, in the presence and absence of one of three surfactants, cetrimide, sodium lauryl sulphate and hexadecyltrimethylammonium lauryl sulphate (double salt). Spreading coefficients calculated from tensions indicate that coacervate should spread readily over oil droplets in presence of double salt, less readily with cetrimide and spreading is unlikely in the presence of sodium lauryl sulphate. The sign of the charge on coacervate droplets and oil droplets was identified under different conditions and showed coacervate droplets and oil droplets have opposite charges except in the presence of sodium lauryl sulphate. Microcapsules were prepared using cetrimide or 'double salt' as emulsifier and release of drug measured. Those prepared with 'double salt' released more slowly than those prepared with cetrimide.

Capsules↗