1,824 research outputs found
Rank Intraclass Correlation for Clustered Data
Clustered data are common in biomedical research. Observations in the same
cluster are often more similar to each other than to observations from other
clusters. The intraclass correlation coefficient (ICC), first introduced by R.
A. Fisher, is frequently used to measure this degree of similarity. However,
the ICC is sensitive to extreme values and skewed distributions, and depends on
the scale of the data. It is also not applicable to ordered categorical data.
We define the rank ICC as a natural extension of Fisher's ICC to the rank
scale, and describe its corresponding population parameter. The rank ICC is
simply interpreted as the rank correlation between a random pair of
observations from the same cluster. We also extend the definition when the
underlying distribution has more than two hierarchies. We describe estimation
and inference procedures, show the asymptotic properties of our estimator,
conduct simulations to evaluate its performance, and illustrate our method in
three real data examples with skewed data, count data, and three-level ordered
categorical data
Understanding Difference-in-differences methods to evaluate policy effects with staggered adoption: an application to Medicaid and HIV
While a randomized control trial is considered the gold standard for
estimating causal treatment effects, there are many research settings in which
randomization is infeasible or unethical. In such cases, researchers rely on
analytical methods for observational data to explore causal relationships.
Difference-in-differences (DID) is one such method that, most commonly,
estimates a difference in some mean outcome in a group before and after the
implementation of an intervention or policy and compares this with a control
group followed over the same time (i.e., a group that did not implement the
intervention or policy). Although DID modeling approaches have been gaining
popularity in public health research, the majority of these approaches and
their extensions are developed and shared within the economics literature.
While extensions of DID modeling approaches may be straightforward to apply to
observational data in any field, the complexities and assumptions involved in
newer approaches are often misunderstood. In this paper, we focus on recent
extensions of the DID method and their relationships to linear models in the
setting of staggered treatment adoption over multiple years. We detail the
identification and estimation of the average treatment effect among the treated
using potential outcomes notation, highlighting the assumptions necessary to
produce valid estimates. These concepts are described within the context of
Medicaid expansion and retention in care among people living with HIV (PWH) in
the United States. While each DID approach is potentially valid, understanding
their different assumptions and choosing an appropriate method can have
important implications for policy-makers, funders, and public health as a
whole
Sensitivity Analysis of Per-Protocol Time-to-Event Treatment Efficacy in Randomized Clinical Trials
Assessing per-protocol treatment effcacy on a time-to-event endpoint is a common objective of randomized clinical trials. The typical analysis uses the same method employed for the intention-to-treat analysis (e.g., standard survival analysis) applied to the subgroup meeting protocol adherence criteria. However, due to potential post-randomization selection bias, this analysis may mislead about treatment efficacy. Moreover, while there is extensive literature on methods for assessing causal treatment effects in compliers, these methods do not apply to a common class of trials where a) the primary objective compares survival curves, b) it is inconceivable to assign participants to be adherent and event-free before adherence is measured, and c) the exclusion restriction assumption fails to hold. HIV vaccine efficacy trials including the recent RV144 trial exemplify this class, because many primary endpoints (e.g., HIV infections) occur before adherence is measured, and nonadherent subjects who receive some of the planned immunizations may be partially protected. Therefore, we develop methods for assessing per-protocol treatment efficacy for this problem class, considering three causal estimands of interest. Because these estimands are not identifiable from the observable data, we develop nonparametric bounds and semiparametric sensitivity analysis methods that yield estimated ignorance and uncertainty intervals. The methods are applied to RV144
Testing for Network and Spatial Autocorrelation
Testing for dependence has been a well-established component of spatial
statistical analyses for decades. In particular, several popular test
statistics have desirable properties for testing for the presence of spatial
autocorrelation in continuous variables. In this paper we propose two
contributions to the literature on tests for autocorrelation. First, we propose
a new test for autocorrelation in categorical variables. While some methods
currently exist for assessing spatial autocorrelation in categorical variables,
the most popular method is unwieldy, somewhat ad hoc, and fails to provide
grounds for a single omnibus test. Second, we discuss the importance of testing
for autocorrelation in network, rather than spatial, data, motivated by
applications in social network data. We demonstrate that existing tests for
autocorrelation in spatial data for continuous variables and our new test for
categorical variables can both be used in the network setting
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Virologic, Immunologic and Clinical Responses in Foreign-Born versus US-Born HIV-1 Infected Adults Initiating Antiretroviral Therapy: An Observational Cohort Study
Introduction: Mortality rates within the first year of combination antiretroviral therapy (cART) initiation are several-fold higher in resource-limited countries than in resource-replete settings. However studies in western countries examining virologic, immunologic and clinical responses after cART initiation in indigenous versus non-indigenous populations have shown mixed results. This study aimed to determine whether there is a difference in these outcomes in a United States setting between foreign-born and US-born patients. Methods: This retrospective observational cohort study of HIV-1 infected adults in one urban clinic in the United States compared virologic suppression, immune recovery and rates of AIDS defining events (ADEs) within the first year of cART using linear mixed effect models, log rank tests and Cox proportional hazard models. Data were analyzed for 94 foreign-born and 1242 US-born patients. Results: Foreign-born patients were younger (31.7 years versus 38.5 years), more often female (38.3% versus 27.1%), less often injection drug users (3.2% versus 9.5%) or men who have sex with men (19.0% versus 54.5%), and had higher loss to follow-up rates (14.9% versus 6.2%). No significant differences were detected between the groups in suppression of plasma HIV-1 RNA, CD4+ cell recovery or development of ADEs. Conclusions: During the first year on cART, virologic suppression, immune recovery and development of ADEs were comparable between foreign-born and US-born patients in care in a US clinic. Differential rates of loss to follow-up warrant further investigation in the foreign-born population
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