With the advent of large-scale and high-throughput data collection coupled with the creation and implementation of complex statistical algorithms for data analysis, the reproducibility of modern data analyses, meaning the ability of independent analysts to recreate the results claimed by the original authors using the original data and analysis techniques, has become an important topic of discussion. I have contributed to developing reproducibility standards for machine learning in the life sciences (1), and also in public health (2). Furthermore, I am also an Associate Editor for Reproducibility at the Journal of the American Statistical Association (JASA) where I helped launch the JASA Reproducibility Award (3).
Heil BJ, Hoffman MM, Markowetz F, Lee S-I, Greene CS, Hicks SC. (2021). Reproducibility standards for machine learning in the life sciences. Nature Methods.
Peng RD, Hicks SC. (2021). Reproducible Research: A Retrospective. Annual Review of Public Health.
Analysis of high-throughput biological data
Statistical methods for normalizing and removing biases in high-throughput data
Normalization is an essential step for the analysis of genomics high-throughput data. Quantile normalization is one of the most widely used multi-sample normalization tools for applications including genotyping arrays, RNA-Sequencing (RNA-Seq), DNA methylation (DNAm), ChIP-Sequencing (ChIP-Seq) and brain imaging. However, quantile normalization relies on assumptions about the data-generation process that are not appropriate in some contexts. To address this, I developed two statistical methods and open-source software for normalizing high-throughput data (1, 2). More recently, I have developed methods and software for correcting and removing compositional biases found in metagenomic data (3), and co-expression analysis of correlated genes (4).
- Hicks SC, Irizarry RA (2015). quantro: a data-driven approach to guide the choice of an appropriate normalization method. Genome Biology.
- Hicks SC, Okrah K, Paulson JN, Quackenbush J, Irizarry RA, Bravo HC (2018). Smooth quantile normalization. Biostatistics.
- Kumar MS, Slud EV, Okrah K, Hicks SC, Hannenhalli S, Corrada Bravo H (2018). Analysis and correction of compositional bias in sparse sequencing count data. BMC Genomics.
- Wang Y, Hicks SC, Hansen KD (2022). Addressing the mean-correlation relationship in co-expression analysis. PLOS Computational Biology.
Statistical methods for deconvolution bulk tissue samples
Bulk tissue samples are often profiled due to their ease of access and low cost to sequence. A major challenge in the analysis of bulk tissue samples is they are typically made up of heterogenous cell types, for example whole blood or brain tissue. This intra-sample cellular heterogeneity is a source of variation than can sometimes be confounded with an outcome or phenotype of interest. If unaccounted for, false positives ensue. Previously, I developed statistical methods to estimate the cell type proportions in whole blood DNAm samples (1). Currently, I am developing cellular deconvolution methods for human brain RNA-seq data (R01MH13183). One major source of variation in brain issue is how the cell sizes and cell abundance varies across cell types. I recently developed a data-driven method to estimate the RNA abundance from cell types in heterogenous tissues using sc/snRNA-seq and smFISH (2).
Hicks SC, Irizarry RA (2019). methylCC: technology-independent estimation of cell type composition using differentially methylated regions. Genome Biology.
Huuki-Myers LA, Montgomery KD, Kwon SH, Page SC, Hicks SC, Maynard KR, Collado-Torres L. (2022). Data-driven Identification of Total RNA Expression Genes (TREGs) for Estimation of RNA Abundance in Heterogeneous Cell Types. bioRxiv.
Developing and benchmarking open-source software for genomics data
In addition to developing statistical methods, I am strongly invested in implementing methods in open-source software for the analysis of genomics data leading to improved reproducibility and understanding of biological data. Bioconductor is an open-source, open-development software project in the R programming language for the analysis and comprehension of high-throughput genomics and molecular biology data. As one of the leaders of the Bioconductor project, I have not only contributed software for the analysis of single-cell RNA-sequencing (scRNA-seq) data, but also I have led a team of investigators to address a growing need of how to approach single-cell genomic data in a comprehensible way published in Nature Methods (1), which included creating a freely available, 42-chapter online book of single-cell methods and tools for prospective users and was highlighted in a Nature Biotechnology editorial in 2020.
- Amezquita RA, Lun ATL, Carey VJ, Carpp LN, Geistlinger L, Marini F, Rue-Albrecht K, Risso D, Soneson C, Waldron L, Pages H, Smith M, Huber W, Morgan M, Gottardo R, Hicks SC. (2020). Orchestrating Single-Cell Analysis with Bioconductor. Nature Methods.
In addition, I evaluate open-source software packages for their accuracy, usability and reproducibility. For example, in high-throughput studies, hundreds to millions of hypotheses are typically tested. Statistical methods that control the false discovery rate (FDR) have emerged as popular and powerful tools for error rate control. While classic FDR methods use only p-values as input, more modern FDR methods have been shown to increase power by incorporating complementary information as “informative covariates” to prioritize, weight, and group hypotheses. To address this, we investigated the accuracy, applicability, and ease of use of two classic and six modern FDR-controlling methods by performing a systematic benchmark comparison using simulation studies as well as six case studies in computational biology (2).
- Korthauer K, Kimes PK, Duvallet C, Reyes A, Subramanian A, Teng M, Shukla C, Alm EJ, Hicks SC (2019). A practical guide to methods controlling false discoveries in computational biology. Genome Biology.
Most recently, we performed a benchmark comparison of 18 scRNA-seq imputation methods across multiple experimental protocols and datasets (3).
- Hou W, Ji J, Ji H, Hicks SC (2020). A systematic evaluation of single-cell RNA-sequencing imputation methods. Genome Biology.
Analysis of genomic data from single cells
Single-cell RNA-Sequencing (scRNA-seq) data has become the most widely used high-throughput method for transcription profiling of individual cells. This technology has created an unprecedented opportunity to investigate important biological questions that can only be answered at the single-cell level. However, this technology also brings new statistical and computational challenges (1, 2).
Amezquita RA, Lun ATL, Carey VJ, Carpp LN, Geistlinger L, Marini F, Rue-Albrecht K, Risso D, Soneson C, Waldron L, Pages H, Smith M, Huber W, Morgan M, Gottardo R, Hicks SC. (2020). Orchestrating Single-Cell Analysis with Bioconductor. Nature Methods.
Lähnemann D, Koester J, Szczurek E, McCarthy D, Hicks S, Robinson MD, Vallejos C, Beerenwinkel N, et al. (2020). Eleven grand challenges in single-cell data science. Genome Biology.
Methods to identify and remove systematic errors and biases in single-cell and single-nucleus data
I was one of the first to demonstrate that systematic errors can explain a substantial percentage of observed cell-to-cell expression variability scRNA-seq, which in turn can lead to false discoveries, for example, when using unsupervised learning methods (1). To address this, we developed a fast, scalable statistical framework for feature selection and dimensionality reduction using generalized principal component analysis (GLM-PCA) for scRNA-seq data, which permits the identification of low-dimensional representations of cells measured with unique molecular identifiers (UMI) count data using a multinomial model (2). More recently, we developed a probabilistic model to identify low-quality cells in scRNA-seq data (3). Finally, we have evaluated probability distributions to accurately model single-nucleus RNA-sequencing (snRNA-seq) data and showed how snRNA-seq has a gene length bias present in both exonic and intronic reads (4).
- Hicks SC, Townes FW, Teng M, Irizarry RA (2018). Missing data and technical variability in single-cell RNA-sequencing experiments. Biostatistics.
- Townes FW, Hicks SC, Aryee MJ, Irizarry RA (2019). Feature Selection and Dimension Reduction for Single Cell RNA-Seq based on a Multinomial Model. Genome Biology.
- Hippen AA, Falco MM, Weber LM, Erkan EP, Zhang K, Dohert JA, Vähärautio A, Greene CS, Hicks SC. (2021). miQC: An adaptive probabilistic framework for quality control of single-cell RNA-sequencing data. PLOS Computational Biology.
- Kuo A, Hansen KD, Hicks SC. (2022). Quantification and statistical modeling of Chromium-based single-nucleus RNA-sequencing data. bioRxiv.
Methods for fast, memory-efficient distance metrics and unsupervised clustering for single-cell data
Computational methods and open-source software to store, access, and analyze single-cell data are essential. Most importantly, they need to be fast, scalable, and memory efficient. For example, the k-means algorithm is a classic algorithm used in the analysis of scRNA-seq data. However, with increasing sizes of single-cell data, new methods are needed that are fast, scalable and memory-efficient. To address this, we implemented the mini-batch optimization for k-means clustering proposed in Sculley (2010) for large single cell sequencing data (1), including fast and memory-efficient k-means++ center finding with distance measures designed for count data and probability distributions (2). The mini-batch k-means algorithm can be run with data stored in memory or on disk (e.g. HDF5 file format). In addition, we have developed fast and scalable discordance metrics to use as a performance metric to evaluate the predicted labels for a general dissimilarity measure (e.g. Euclidean distance) (3). Finally, we have also implemented scalable data infrastructure to store and access single-cell data that can be explored with hierarchical structures (e.g. cell types) (4).
- Hicks SC, Liu R, Ni Y, Purdom E, Risso D. (2020). mbkmeans: fast clustering for single cell data using mini-batch k-means. PLOS Computational Biology.
- Baker DN, Dyjack N, Braverman V, Hicks SC, Langmead B. (2021). minicore: Fast scRNA-seq clustering with various distance measures. ACM Conference on Bioinformatics, Computational Biology and Biomedicine.
- Dyjack N, Baker DN, Braverman V, Langmead B, Hicks SC. (2022). A scalable and unbiased discordance metric with H+. Biostatistics.
- Huang R, Soneson C, Ernst FGM, Rue-Albrecht KC, Guangchuang Y, Hicks SC, Robinson MD. (2020). TreeSummarizedExperiment: a S4 class for data with hierarchical structure. F1000Research.
Methods to identify differences between groups in single-cell data
Another important question is how to identify differences between biological groups or conditions in single-cell data. For example, these count-based models, such as Poisson or negative binomial, can be generalized using the Tweedie family of distributions, which we used to develop a statistical method to identify differentially expressed genes (1). Similarly, we have developed methods for differential pseudotime analysis (gene expression or cell abundance) with multiple samples per group using B-splines (2).
- Mallick H, Chatterjee Su., Chowdhury S, Chatterjee Sa., Rahnavard A, Hicks SC. (2021). Differential expression of single-cell RNA-seq data using Tweedie models. Statistics in Medicine.
- Hou W, Ji Z, Chen Z, Wherry EJ, Hicks SC, Ji H. (2021). A statistical framework for differential pseudotime analysis with multiple single-cell RNA-seq samples. bioRxiv.
High-grade serous ovarian cancer subtypes with single-cell profiling
The goal of this project is to identify the biological basis of subtypes of high-grade serous ovarian cancers (HGSOC) using bulk and single-cell gene expression data. This is highly relevant to public health because HGSOC is a particularly deadly cancer that is often only identified at late stage and treatment options are limited. The long-term impact of this project will be a key step towards developing targeted treatments for HGSOCs. Towards this goal, we demonstrated that genetic demultiplexing from single-cell cancer samples can be used for better experimental design and increase cost savings (1). Most recently, we performed a systematic benchmark of algorithms for bulk deconvolution for HGSOC tumors (2).
- Weber LM, Hippen AA, Hickey PF, Berrett KC, Gertz J, Doherty JA, Greene CS, Hicks SC. (2020). Genetic demultiplexing of pooled single-cell RNA-sequencing samples in cancer facilitates effective experimental design. GigaScience.
- Hippen AA, Omran DK, Weber LM, Jung E, Drapkin R, Doherty JA, Hicks SC, Greene CS. (2022). Performance of computational algorithms to deconvolve heterogeneous bulk tumor tissue depends on experimental factors. bioRxiv.
Single-cell atlases in neuroscience
Single-nucleus RNA-sequencing (snRNA-seq) has become the preferred experimental technology, compared to scRNA-seq, to profile gene expression in frozen cells or cells that are hard to dissociate, such as brain tissue. Previous studies have shown that snRNA-seq offers substantial advantages over scRNA-seq, including reduced dissociation bias and the ability to capture rare cell types in these tissues including pan regions of the brain (1). For example, we recently used snRNA-seq to explore activity-regulated gene expression across cell types of the mouse hippocampus (2).
- Tran MN, Maynard KR, Spangler A, Collado-Torres L, Sadashivaiah V, Tippani M, Barry BK, Hancock DB, Hicks SC, Kleinman JE, Hyde TM, Martinowich K, Jaffe A. (2020). Single-nucleus transcriptome analysis reveals cell type-specific molecular signatures across reward circuitry in the human brain. Neuron.
- Nelson ED, Maynard KR, Nicholas KR, Tran MN, Divecha HR, Collado-Torres L, Hicks SC, Martinowich K. (2022). Activity-regulated gene expression across cell types of the mouse hippocampus. bioRxiv.
Analysis of genomic data with spatial resolution
Spatially-resolved transcriptomics (SRT), the Nature Methods 2020 Method of the Year, is poised to transform our understanding of how the spatial organization and communication of cells in complex tissues. These technologies have created an unprecedented opportunity to investigate important biological questions that can only be answered in a spatial context. However, these technologies also brings new statistical, computational and methodological challenges.
Data storage and data visualization
To address some of these challenges, we have developed standardized data infrastructure to storage SRT data within the Bioconductor framework (1) along with tools to interactively visualize SRT data (2-3). Most recently, we developed a performant web-based interactive visualization tool for spatially-resolved transcriptomics experiments called Loopy Browser, which leverages GeoTIFF file formats and OpenLayers for geomaps (4).
- Righelli D, Weber LM, Crowell HL, Pardo B, Collado-Torres L, Ghazanfar S, Lun ATL, Hicks SC, Risso D (2021). SpatialExperiment: infrastructure for spatially resolved transcriptomics data in R using Bioconductor. Bioinformatics.
- Pardo B, Spangler A, Weber LM, Hicks SC, Jaffe AE, Martinowich K, Maynard KR, Collado-Torres L. spatialLIBD: an R/Bioconductor package to visualize spatially-resolved transcriptomics data. BMC Genomics.
- Tippani M, Divecha HR, Catallini JL, Weber LM, Spangler A, Jaffe AE, Hicks SC, Martinowich K, Collado-Torres L, Page SC, Maynard KR. (2021). VistoSeg: a MATLAB pipeline to process, analyze and visualize high-resolution histology images for Visium spatial transcriptomics data. bioRxiv.
- Sriworarat C, Nguyen AB, Eagles NJ, Collado-Torres L, Martinowich K, Maynard KR, Hicks SC. (2023). Performant web-based interactive visualization tool for spatially-resolved transcriptomics experiments. bioRxiv.
Methods to identify spatially variable genes
Feature selection to identify spatially variable genes (SVGs) is a key step during analyses of SRT data. We developed a scalable approach to identify SVGs based on nearest-neighbor Gaussian processes (GPs), which uses gene-specific estimates of length scale parameters within GP models, and scales linearly with the number of spatial locations (1).
- Weber LM, Saha A, Datta A, Hansen KD, Hicks SC. (2022). nnSVG: scalable identification of spatially variable genes using nearest-neighbor Gaussian processes. bioRxiv.
Spatial atlases in neuroscience
We have created transcriptome-wide spatial atlases using the 10x Genomics Visium platform using postmorteum human brain regions, including dorsolateral prefrontal cortex (1) and locus coeruleus (2). This is highly relevant to public health because these technologies will provide insights into topographical and pathological changes in gene expression for example in the aging human brain or in patients affected by psychiatric diseases.
- Maynard KR, Collado-Torres L, Weber LM, Uytingco C, Barry BK, Williams SR, II JLC, Tran MN, Besich Z, Tippani M, Chew J, Yin Y, Kleinman JE, Hyde TM, Rao N, Hicks SC, Martinowich K, Jaffe AE (2021). Transcriptome-scale spatial gene expression in the human dorsolateral prefrontal cortex. Nature Neuroscience.
- Weber LM, Divecha HR, Tran MN, Kwon SH, Spangler A, Montgomery KD, Tippani M, Bharadwaj R, Kleinman JE, Page SC, Hyde TM, Collado-Torres L, Maynard KR, Martinowich K, Hicks SC. 2023. The gene expression landscape of the human locus coeruleus revealed by single-nucleus and spatially-resolved transcriptomics. eLife.
Data Science Education
An increase in demand for statistics and data science education has led to changes in curriculum, specifically an increase in computing. While this has led to more applied courses, students still struggle with effectively deriving knowledge from data and solving real-world problems. In 1999, Deborah Nolan and Terry Speed argued the solution was to teach courses through in-depth case studies derived from interesting scientific questions with nontrivial solutions that leave room for different analyses of the data. This innovative framework teaches the student to make important connections between the scientific question, data and statistical concepts that only come from hands-on experience analyzing data (1-3).
- Hicks SC, Irizarry RA (2018). A Guide to Teaching Data Science. The American Statistician.
- Hicks SC (2017). Greater Data Science Ahead. Journal of Computational Graphical Statistics.
- Peng RD, Chen A, Bridgeford E, Leek JT, and Hicks SC. (2021). Diagnosing Data Analytic Problems in the Classroom. Journal of Statistics and Data Science Education.
Open Case Studies
To address this, I built the Open Case Studies (OCS) (1) data science educational resource of case studies that educators can use in the classroom to teach students how to effectively derive knowledge from data. This project was selected as a High-Impact Project in 2019-2020 by the Bloomberg American Health Initiative and Bloomberg Philanthropies (2). A list of available cases studies are listed in the teaching section.
- Wright C, Meng Q, Breshock MR, Atta L, Taub MA, Jager LR, Muschelli J, Hicks SC (2023). Open Case Studies: Statistics and Data Science Education through Real-World Applications. arXiv.
Theory of Data Analysis
The data science revolution has led to an increased interest in the practice of data analysis. While much has been written about statistical thinking, a complementary form of thinking that appears in the practice of data analysis is design thinking – the problem-solving process to understand the people for whom a product is being designed. There can be significant or subtle differences in how a data analyst (or producer of a data analysis) constructs, creates, or designs a data analysis, including differences in the choice of methods, tooling, and workflow. These choices can affect the data analysis products themselves and the experience of the consumer of the data analysis. Therefore, the role of a producer can be thought of as designing the data analysis with a set of design principles (1,2). Our work leads to several insights: it suggests a formal mechanism to describe and evaluate (3) data analyses based on the design principles for data analysis, and a framework for diagnosing data analytic problems in the classroom (4).
- D’Agostino McGowan L, Peng RD, Hicks SC. (2022). Design Principles for Data Analysis. Journal of Computational and Graphical Statistics.
- Hicks SC, Peng RD. (2019). Elements and Principles for Characterizing Variation between Data Analyses. arXiv.
- Hicks SC, Peng RD. (2019). Evaluating the Success of a Data Analysis. arXiv.
- Peng RD, Chen A, Bridgeford E, Leek JT, and Hicks SC. (2021). Diagnosing Data Analytic Problems in the Classroom. Journal of Statistics and Data Science Education_.