Verifiable identification condition for nonignorable nonresponse data with categorical instrumental variables

We consider a model identification problem in which an outcome variable contains nonignorable missing values. Statistical inference requires a guarantee of the model identifiability to obtain estimators enjoying theoretically reasonable properties such as consistency and asymptotic normality. Recently, instrumental or shadow variables, combined with the completeness condition in the outcome model, have been highlighted to make a model identifiable. However, the completeness condition may not hold even for simple models when the instrument is categorical. We propose a sufficient condition for model identifiability, which is applicable to cases where establishing the completeness condition is difficult. Using observed data, we demonstrate that the proposed conditions are easy to check for many practical models and outline their usefulness in numerical experiments and real data analysis

An empirical likelihood approach to reduce selection bias in voluntary samples

We address the weighting problem in voluntary samples under a nonignorable sample selection model. Under the assumption that the sample selection model is correctly specified, we can compute a consistent estimator of the model parameter and construct the propensity score estimator of the population mean. We use the empirical likelihood method to construct the final weights for voluntary samples by incorporating the bias calibration constraints and the benchmarking constraints. Linearization variance estimation of the proposed method is developed. A limited simulation study is also performed to check the performance of the proposed methods

Efficient Multiple-Robust Estimation for Nonresponse Data Under Informative Sampling

Nonresponse after probability sampling is a universal challenge in survey sampling, often necessitating adjustments to mitigate sampling and selection bias simultaneously. This study explored the removal of bias and effective utilization of available information, not just in nonresponse but also in the scenario of data integration, where summary statistics from other data sources are accessible. We reformulate these settings within a two-step monotone missing data framework, where the first step of missingness arises from sampling and the second originates from nonresponse. Subsequently, we derive the semiparametric efficiency bound for the target parameter. We also propose adaptive estimators utilizing methods of moments and empirical likelihood approaches to attain the lower bound. The proposed estimator exhibits both efficiency and double robustness. However, attaining efficiency with an adaptive estimator requires the correct specification of certain working models. To reinforce robustness against the misspecification of working models, we extend the property of double robustness to multiple robustness by proposing a two-step empirical likelihood method that effectively leverages empirical weights. A numerical study is undertaken to investigate the finite-sample performance of the proposed methods. We further applied our methods to a dataset from the National Health and Nutrition Examination Survey data by efficiently incorporating summary statistics from the National Health Interview Survey data.Comment: 2 figure

Alteration of the nucleosomal DNA path in the crystal structure of a human nucleosome core particle

Gene expression in eukaryotes depends upon positioning, mobility and packaging of nucleosomes; thus, we need the detailed information of the human nucleosome core particle (NCP) structure, which could clarify chromatin properties. Here, we report the 2.5 Å crystal structure of a human NCP. The overall structure is similar to those of other NCPs reported previously. However, the DNA path of human NCP is remarkably different from that taken within other NCPs with an identical DNA sequence. A comparison of the structural parameters between human and Xenopus laevis DNA reveals that the DNA path of human NCP consecutively shifts by 1 bp in the regions of superhelix axis location −5.0 to −2.0 and 5.0 to 7.0. This alteration of the human DNA path is caused predominantly by tight DNA–DNA contacts within the crystal. It is also likely that the conformational change in the human H2B tail induces the local alteration of the DNA path. In human NCP, the region with the altered DNA path lacks Mn(2+) ions and the B-factors of the DNA phosphate groups are substantially high. Therefore, in contrast to the histone octamer, the nucleosomal DNA is sufficiently flexible and mobile and can undergo drastic conformational changes, depending upon the environment