@article{feng_xiao_li_chen_ohuma_2020, title={Correlation models for monitoring fetal growth}, volume={29}, ISSN={["1477-0334"]}, DOI={10.1177/0962280220905623}, abstractNote={Ultrasound growth measurements are monitored to evaluate if a fetus is growing normally compared with a defined standard chart at a specified gestational age. Using data from the Fetal Growth Longitudinal Study of the INTERGROWTH-21st project, we have modelled the longitudinal dependence of fetal head circumference, biparietal diameter, occipito-frontal diameter, abdominal circumference, and femur length using a two-stage approach. The first stage involved finding a suitable transformation of the raw fetal measurements (as the marginal distributions of ultrasound measurements were non-normal) to standardized deviations (Z-scores). In the second stage, a correlation model for a Gaussian process is fitted, yielding a correlation for any pair of observations made between 14 and 40 weeks. The correlation structure of the fetal Z-score can be used to assess whether the growth, for example, between successive measurements is satisfactory. The paper is accompanied by a Shiny application, see https://lxiao5.shinyapps.io/shinycalculator/.}, number={10}, journal={STATISTICAL METHODS IN MEDICAL RESEARCH}, author={Feng, Yuan and Xiao, Luo and Li, Cai and Chen, Stephanie T. and Ohuma, Eric O.}, year={2020}, month={Oct}, pages={2795–2813} }
 @article{blanchard_nakamura_cao_chen_moreau_2020, title={Spine and dine: A key defensive trait promotes ecological success in spiny ants}, volume={10}, ISSN={["2045-7758"]}, DOI={10.1002/ece3.6322}, abstractNote={Abstract A key focus of ecologists is explaining the origin and maintenance of morphological diversity and its association with ecological success. We investigate potential benefits and costs of a common and varied morphological trait, cuticular spines, for foraging behavior, interspecific competition, and predator–prey interactions in naturally co‐occurring spiny ants (Hymenoptera: Formicidae: Polyrhachis) in an experimental setting. We expect that a defensive trait like spines might be associated with more conspicuous foraging, a greater number of workers sent out to forage, and potentially increased competitive ability. Alternatively, consistent with the ecological trade‐off hypothesis, we expect that investment in spines for antipredator defense might be negatively correlated with these other ecological traits. We find little evidence for any costs to ecological traits, instead finding that species with longer spines either outperform or do not differ from species with shorter spines for all tested metrics, including resource discovery rate and foraging effort as well as competitive ability and antipredator defense. Spines appear to confer broad antipredator benefits and serve as a form of defense with undetectable costs to key ecological abilities like resource foraging and competitive ability, providing an explanation for both the ecological success of the study genus and the large number of evolutionary origins of this trait across all ants. This study also provides a rare quantitative empirical test of ecological effects related to a morphological trait in ants.}, number={12}, journal={ECOLOGY AND EVOLUTION}, author={Blanchard, Benjamin D. and Nakamura, Akihiro and Cao, Min and Chen, Stephanie T. and Moreau, Corrie S.}, year={2020}, month={Jun}, pages={5852–5863} }
 @article{chen_xiao_staicu_2019, title={A smoothing-based goodness-of-fit test of covariance for functional data}, volume={75}, ISSN={["1541-0420"]}, DOI={10.1111/biom.13005}, abstractNote={Functional data methods are often applied to longitudinal data as they provide a more flexible way to capture dependence across repeated observations. However, there is no formal testing procedure to determine if functional methods are actually necessary. We propose a goodness‐of‐fit test for comparing parametric covariance functions against general nonparametric alternatives for both irregularly observed longitudinal data and densely observed functional data. We consider a smoothing‐based test statistic and approximate its null distribution using a bootstrap procedure. We focus on testing a quadratic polynomial covariance induced by a linear mixed effects model and the method can be used to test any smooth parametric covariance function. Performance and versatility of the proposed test is illustrated through a simulation study and three data applications.}, number={2}, journal={BIOMETRICS}, publisher={Wiley}, author={Chen, Stephanie T. and Xiao, Luo and Staicu, Ana-Maria}, year={2019}, month={Jun}, pages={562–571} }
 @article{chen_morgan_beresford_getz_christopher_langstrom_strickland_wiggenhorn_lyapustina_2019, title={Performance of the Population Bioequivalence (PBE) Statistical Test with Impactor Sized Mass Data}, volume={20}, ISSN={["1530-9932"]}, DOI={10.1208/s12249-019-1507-8}, abstractNote={This article extends previous work studying performance characteristics of the population bioequivalence (PBE) statistical test recommended by the US Food and Drug Administration (FDA) for orally inhaled and nasal drug products. Based on analysis of a metered dose inhaler database for impactor sized mass, a simulation study was designed to compare performance of the recommended PBE approach with several modified or alternative approaches. These included an extended PBE that separately modeled within-batch (can) and between-batch (batch) variability and average bioequivalence (ABE) tests that modeled with or without between-batch variability and with or without log-transformation. This work showed that separately modeling within- and between-batch variability while increasing the number of sampled batches addressed previously identified issues of the PBE approach when between-batch variability was present, namely, (a) increased risk for falsely concluding equivalence and (b) low probability of correctly concluding equivalence. The same modifications were also required of the ABE to achieve expected performance. However, these modifications did not successfully address the issue of equivalence conclusions that depended on the direction of product mean differences (asymmetric performance). This work highlights the importance of understanding decision-making error rates in developing regulatory recommendations to standardize bioequivalence outcomes across products.}, number={7}, journal={AAPS PHARMSCITECH}, author={Chen, Stephanie and Morgan, Beth and Beresford, Hayden and Getz, Elise Burmeister and Christopher, David and Langstrom, Goran and Strickland, Helen and Wiggenhorn, Christopher and Lyapustina, Svetlana}, year={2019}, month={Oct} }
 @article{chen_luo_hou_raubenheimer_ji_jin_jiang_yu_wang_li_et al._2018, title={Nutrient Balancing by Captive Golden Snub-Nosed Monkeys (Rhinopithecus roxellana)}, volume={39}, ISSN={["1573-8604"]}, DOI={10.1007/s10764-018-0070-6}, number={6}, journal={INTERNATIONAL JOURNAL OF PRIMATOLOGY}, author={Chen, Stephanie T. and Luo, Xi and Hou, Rong and Raubenheimer, David and Ji, Weihong and Jin, Xuelin and Jiang, Zhi and Yu, Xuewei and Wang, Jiajia and Li, Min and et al.}, year={2018}, month={Dec}, pages={1124–1138} }
 @article{morgan_chen_christopher_langstrom_wiggenhorn_getz_beresford_hoffelder_acerbi_andrews_et al._2018, title={Performance of the Population Bioequivalence (PBE) Statistical Test Using an IPAC-RS Database of Delivered Dose from Metered Dose Inhalers}, volume={19}, ISSN={["1530-9932"]}, DOI={10.1208/s12249-017-0941-8}, abstractNote={This article reports performance characteristics of the population bioequivalence (PBE) statistical test recommended by the US Food and Drug Administration (FDA) for orally inhaled products. A PBE Working Group of the International Pharmaceutical Aerosol Consortium on Regulation and Science (IPAC-RS) assembled and considered a database comprising delivered dose measurements from 856 individual batches across 20 metered dose inhaler products submitted by industry. A review of the industry dataset identified variability between batches and a systematic lifestage effect that was not included in the FDA-prescribed model for PBE. A simulation study was designed to understand PBE performance when factors identified in the industry database were present. Neglecting between-batch variability in the PBE model inflated errors in the equivalence conclusion: (i) The probability of incorrectly concluding equivalence (type I error) often exceeded 15% for non-zero between-batch variability, and (ii) the probability of incorrectly rejecting equivalence (type II error) for identical products approached 20% when product and between-batch variabilities were high. Neglecting a systematic through-life increase in the PBE model did not substantially impact PBE performance for the magnitude of lifestage effect considered. Extreme values were present in 80% of the industry products considered, with low-dose extremes having a larger impact on equivalence conclusions. The dataset did not support the need for log-transformation prior to analysis, as requested by FDA. Log-transformation resulted in equivalence conclusions that depended on the direction of product mean differences. These results highlight a need for further refinement of in vitro equivalence methodology.}, number={3}, journal={AAPS PHARMSCITECH}, author={Morgan, Beth and Chen, Stephanie and Christopher, David and Langstrom, Goran and Wiggenhorn, Christopher and Getz, Elise Burmeister and Beresford, Hayden and Hoffelder, Thomas and Acerbi, Daniela and Andrews, Stephen and et al.}, year={2018}, month={Apr}, pages={1410–1425} }