If you find this research useful, I would be interested in hearing about how you are using group testing! Please send me an e-mail at chris@chrisbilder.com. Most of this research is performed jointly with Dr. Joshua M. Tebbs and Dr. Christopher S. McMahan.

Grants funded

News stories about the research for NIH award R01 AI121351 are available from the Nebraska Today at

Awards

Shiny web applications

Introductions to group testing

Are you a student interested in learning more about the group testing research problem? If so, here are introductory resources on it:

Journal articles with supplementary material

  1. St. Ville, M., McMahan, C., Bible, J., Tebbs, J., and Bilder, C. (2024). Bayesian additive regression trees for group testing data. Submitted for publication.
  2. McMahan, C., Joyner, C., Tebbs, J., and Bilder, C. (2024). A mixed effects Bayesian regression model for multivariate group testing data. Submitted for publication.
  3. Bilder, C., Hitt, B., Biggerstaff, B., Tebbs, J., and McMahan, C., (2023). binGroup2: Statistical tools for infection identification via group testing. The R Journal 15(4), 21-36.   
  4. Liu, Y., McMahan, C., Tebbs, J., Gallagher, C., and Bilder, C. (2021). Generalized additive regression for group testing data. Biostatistics (22)4, 873-889.
  5. Bilder, C., Tebbs, J., and McMahan, C. (2021). Informative Array Testing with Multiplex Assays. Statistics in Medicine 40(13), 3021-3034.
  6. Mokalled, S., McMahan, C., Brown, D., Tebbs, J., and Bilder, C. (2021). Incorporating the dilution effect in group testing regression: A new approach. Statistics in Medicine 40(11), 2540-2555.  
  7. Joyner, C., McMahan, C., Tebbs, J., and Bilder, C. (2020). From mixed effects modeling to spike and slab variable selection: A Bayesian regression model for group testing data. Biometrics 76(3), 913-923.
  8. Hou, P., Tebbs, J., Wang, D., McMahan, C., and Bilder, C. (2020). Array testing with multiplex assays. Biostatistics 21(3), 417-431. 
  9. Hitt, B., Bilder, C., Tebbs, J., and McMahan, C. (2019). The objective function controversy for group testing: Much ado about nothing? Statistics in Medicine 38(24), 4912-4923.
  10. Bilder, C., Tebbs, J., and McMahan, C. (2019). Informative Group Testing for Multiplex Assays. Biometrics 75(1), 278-288.
  11. Wang, D., McMahan, C., Tebbs, J., and Bilder, C. (2018). Group testing case identification with biomarker information. Computational Statistics and Data Analysis 122, 156-166.
  12. Warasi, M., McMahan, C., Tebbs, J., and Bilder, C. (2017). Group testing regression models with dilution submodels. Statistics in Medicine 36(30), 4860-4872.
  13. Hou, P., Tebbs, J., Bilder, C., and McMahan, C. (2017). Hierarchical group testing for multiple infections. Biometrics 73(2), 656-665.
  14. McMahan, C., Tebbs, J., Hanson, T., and Bilder, C. (2017). Bayesian regression for group testing data. Biometrics 73(4), 1443-1452.
  15. Warasi, M., Tebbs, J., McMahan, C., and Bilder, C. (2016). Estimating the prevalence of multiple diseases from two-stage hierarchical pooling. Statistics in Medicine 35(21), 3581-3864.
  16. Black, M., Bilder, C., and Tebbs, J. (2015). Optimal retesting configurations for hierarchical group testing. Journal of the Royal Statistical Society: Series C (Applied Statistics) 64(4), 693-710.
  17. Tebbs, J., McMahan, C., and Bilder, C. (2013). Two-stage hierarchical group testing for multiple infections with application to the Infertility Prevention Project. Biometrics 69(4), 1064-1073.
  18. McMahan, C., Tebbs, J., and Bilder, C. (2013). Regression models for group testing data with pool dilution effects. Biostatistics 14(2), 284-298.
  19. Zhang, B., Bilder, C., and Tebbs, J. (2013). Regression analysis for multiple-disease group testing data. Statistics in Medicine 32(28), 4954-4966.
  20. Bilder, C. and Tebbs, J. (2012). Pooled testing procedures for screening high volume clinical specimens in heterogeneous populations. Statistics in Medicine 31(27), 3261-3268.
  21. McMahan, C., Tebbs, J., and Bilder, C. (2012). Two-dimensional informative array testing. Biometrics 68(3), 793-804.
  22. McMahan, C., Tebbs, J., and Bilder, C. (2012). Informative Dorfman Screening. Biometrics 68(1), 287-296.
  23. Bilder, C. and Tebbs, J. (2011). Informative Retesting Procedures to Assay High Volume Clinical Specimens. Paper for the 58th World Statistics Congress of the International Statistical Institute.
  24. Bilder, C., Tebbs, J., and Chen, P. (2010). Informative Retesting. Journal of the American Statistical Association 105(491), 942-955.
  25. Bilder, C. (2009). Human or Cylon? Group Testing on Battlestar Galactica. Chance 22(3), 46-50. (Corresponding presentation is at www.chrisbilder.com/bsg)
  26. Chen, P., Tebbs, J., and Bilder, C. (2009). Group Testing Regression Models with Fixed and Random Effects. Biometrics 65(4), 1270-1278.
  27. Bilder, C. and Tebbs, J. (2009). Bias, efficiency, and agreement for group-testing regression models.Journal of Statistical Computation and Simulation 79(1), 67-80.
  28. Bilder, C. and Tebbs, J. (2005). Empirical Bayesian estimation of the disease transmission probability in multiple-vector-transfer designs.Biometrical Journal 47(4), 502-516.
  29. Tebbs, J. and Bilder, C. (2004). Confidence interval procedures for the probability of disease transmission in multiple-vector-transfer designs. Journal of Agricultural, Biological, and Environmental Statistics 9(1), 75-90.

Journal articles without supplementary material

  1. Fang, L., Hu, T., Li, S., Wang, L., McMahan, C., and Tebbs, J. (2024). Probit time-to-event regression for misclassified group testing data. Submitted for publication.
  2. Li, S., Hu, T., Wang, L., McMahan, C., and Tebbs, J. (2024). Regression analysis of group-tested current status data. Biometrika 111(3), 1047-1061.
  3. Warasi, M., Tebbs, J., McMahan, C., and Bilder, C. (2023). Estimating the prevalence of two or more diseases using outcomes from multiplex group testing. Biometrical Journal 65(7), 1-14.
  4. Bilder, C., Tebbs, J., and McMahan, C. (2021). Discussion on "Is Group Testing Ready for Prime-Time in Disease Identification". Statistics in Medicine 40(17), 3881-3886.
  5. Bilder, C., Iwen, P., and Abdalhamid, B. (2021). Pool size selection when testing for severe acute respiratory syndrome coronavirus 2. Clinical Infectious Diseases 72(6), 1104-1105.
  6. Abdalhamid, B., Bilder, C., Garrett, J., and Iwen, P. (2020). Cost effectiveness of sample pooling for SARS-CoV-2 testing. The Journal of Infection in Developing Countries 14(10), 1136-1137.
  7. Bilder, C., Iwen, P., Abdalhamid, B., Tebbs, J., and McMahan, C. (2020). Tests in short supply? Try group testing. Significance 17(3), 15-16.
  8. Abdalhamid, B., Bilder, C., McCutchen, E., Hinrichs, S., Koepsell, S., and Iwen, P. (2020). Assessment of specimen pooling to conserve SARS CoV-2 testing resources. American Journal of Clinical Pathology 153(6), 715-718.
  9. Bilder, C. (2019). Group testing for estimation. Wiley StatsRef: Statistics Reference Online. https://doi.org/10.1002/9781118445112.stat08231. PDF
  10. Bilder, C. (2019). Group testing for identification. Wiley StatsRef: Statistics Reference Online. https://doi.org/10.1002/9781118445112.stat08227. PDF
  11. McMahan, C., Tebbs, J., and Bilder, C. (2015). Rejoinder to "A note on the evaluation of group testing algorithms in the presence of misclassification." Biometrics 72(1), 303-304.
  12. Zhang, B., Bilder, C., and Tebbs, J. (2013). Group testing regression model estimation when case identification is a goal. Biometrical Journal 55(2), 173-189.
  13. Black, M., Bilder, C., and Tebbs, J. (2012). Group testing in heterogeneous populations by using halving algorithms. Journal of the Royal Statistical Society: Series C (Applied Statistics) 61(2), 277-290.
  14. Bilder, C., Zhang, B., Schaarschmidt, F., and Tebbs, J. (2010). binGroup: A Package for Group Testing, The R Journal 2(2), 56-60.
  15. Chen, P., Tebbs, J., and Bilder, C. (2009). Global Goodness-of-fit for Group Testing Regression Models. Statistics in Medicine 28(23), 2912-2928.
  16. Tebbs, J. and Bilder, C. (2006).  Hypothesis tests for and against a simple order among proportions estimated by pooled testing.  Biometrical Journal 48(5), 792-804.
  17. Bilder, C. (2005).  Group testing model estimation and inference.  2005 Proceedings of the American Statistical Association, Biometrics Section [CD-ROM], Alexandria, VA: American Statistical Association, 177-182.
  18. Bilder, C. and Tebbs, J. (2004).  Empirical Bayesian estimation of the disease transmission probability in multiple-vector-transfer designs.  2004Proceedings of the American Statistical Association, Biometrics Section [CD-ROM], Alexandria, VA: American Statistical Association, 277-284.
  19. Tebbs, J., Bilder, C., and Moser, B. (2003).  An Empirical Bayes Group-Testing Approach to Estimating Small Proportions. Communications in Statistics: Theory and Methods 32(5), 983-995.

Software

Presentations

  1. Bilder, C., Nguyen, M., Tebbs, J., and McMahan, C. "How complex is pooled testing for STI detection? A new metric to evaluate its implementation," STI Prevention Conference, September 17, 2024; Atlanta, GA.
  2. Bilder, C., Nguyen, M., Tebbs, J., and McMahan, C. "The complexity of pooled testing for infectious disease detection," Royal Statistical Society International Conference, September 3, 2024; Brighton, England.
  3. Nguyen, M., Bilder, C., Tebbs, J., and McMahan, C. "Time considerations for group testing implementation," Joint Statistical Meetings, August 6, 2024; Portland, OR.
  4. Porter, E., McMahan, C., Tebbs, J., and Bilder, C. "Gradient boosting for group testing data," Joint Statistical Meetings, August 5, 2024; Portland, OR.
  5. Nguyen, M., Bilder, C., Tebbs, J., and McMahan, C. "Specimen pooling: How many stages are too much?" WNAR/IMS Meeting, June 11, 2024; Fort Collins, CO.
  6. Hitt, B., Bilder, C., Tebbs, J., and McMahan, C. "Enhancing testing capacity: Statistical approaches to disease detection using group testing," WNAR/IMS Meeting, June 11, 2024; Fort Collins, CO.
  7. McMahan, C., St. Ville, M., Bible, J., Tebbs, J., and Bilder, C., "Bayesian additive regression trees for group testing data," WNAR/IMS Meeting, June 11, 2024; Fort Collins, CO.
  8. Das, P., Bilder, C., Tebbs, J., and McMahan, C. "Group testing with incomplete block designs and imperfect tests," WNAR/IMS Meeting, June 10, 2024; Fort Collins, CO.
  9. Hitt, B., Bilder, C., Tebbs, J., and McMahan, C. "Statistical tools for infection identification via group testing," Conference on Statistical Practice, February 29, 2024; New Orleans, LA.
  10. Bilder, C., Nguyen, M., Tebbs, J., and McMahan, C. "A new metric to evaluate algorithm complexity for group testing," WNAR/IMS Meeting, June 19, 2023; Anchorage, AK.
  11. McMahan, C., Tebbs, J., and Bilder, C. "A mixed effects Bayesian regression model for multivariate group testing data," IBS ENAR Meeting, March 21, 2023; Nashville, TN.
  12. Nguyen, M., Bilder, C., Tebbs, J., and McMahan, C. "Expected number of stages for hierarchical group testing algorithms," IBS ENAR Meeting, March 19, 2023; Nashville, TN.
  13. Bilder, C. "Group testing for identifying cases of COVID-19: Opportunities and challenges," Panelist, IBS ENAR Meeting, March 16, 2021; Baltimore, MD.
  14. Bilder, C., Hitt, B., Benfer, J., Blaney, K., Carter, B., Eveland, K., McMahan, C., and Tebbs, J. "To pool or not to pool? A web-based Shiny app to help laboratories make the specimen pooling decision," STD Prevention Conference, September 2020; Atlanta, GA.
  15. Bilder, C. and McMahan, C. "JUST GROUP IT. Group testing for identification," Invited introductory overview lecture, Joint Statistical Meetings, August 6, 2020; Philadelphia, PA.
  16. McMahan, C. and Bilder, C. "JUST GROUP IT. Group testing for estimation," Joint Statistical Meetings, August 6, 2020; Philadelphia, PA.  
  17. Bilder, C. "Group testing for efficient SARS-CoV-2 detection during the COVID-19 pandemic," Invited seminar, Department of Statistics, Stanford University, July 7, 2020.
  18. Bilder, C., Hitt, B., Tebbs, J., and McMahan, C. “A Shiny app for making the specimen pooling decision,” UseR! July 2020; St. Louis, MO. (Conference cancelled due to COVID-19)
  19. Bilder, C., Tebbs, J., and McMahan, C. “Sexually transmitted disease detection using array testing algorithms with multiplex assays,” Innovations in Design, Analysis, and Dissemination: Frontiers in Biostatistics and Data Science, April 2020; Overland Park, KS. (Conference cancelled due to COVID-19)
  20. Bilder, C. "Infectious disease detection with multiplex assays and specimen pooling in array configurations," Invited seminar, Department of Biostatistics, University of Nebraska Medical Center, February 28, 2020; Omaha, NE.
  21. Mokalled, S., McMahan, C., Brown, D., Tebbs, J., and Bilder, C. “Acknowledging the dilution effect in group testing regression: A new approach,” Contributed paper, IBS ENAR meeting, March 24, 2020; Nashville, TN. (Student travel award winning paper)
  22. Bilder, C., Tebbs, J., and McMahan, C. "Strategies for pooling in array testing configurations with multiplex assays," Joint Statistical Meetings, July 30, 2019; Denver, CO. (American Statistical Association's Statistical Significance Poster Award winner)
  23. Bilder, C. "Informative group testing for multiplex assays," Webinar for the International Biometric Society Journal Club, June 13, 2019.
  24. Bilder, C., Hitt, B., Tebbs, J., and McMahan, C. "A user-friendly Shiny web application for choosing pool sizes when testing pooled specimens," Association of Public Health Laboratories Annual Meeting, June 4, 2019; St. Louis, MO.
  25. Bilder, C. "What if" Department of Statistics and Actuarial Science, University of Iowa, March 14, 2019; Iowa City, IA.
  26. Bilder, C. "What if" Department of Statistics, University of Nebraska-Lincoln, January 23, 2019; Lincoln, NE.
  27. Bilder, C. "What if" Department of Statistics, Kansas State University, November 1, 2018; Manhattan, KS.
  28. Hitt, B., Bilder, C., Tebbs, J., and McMahan, C. "The optimal group size controversy for infectious disease testing: Much ado about nothing," Royal Statistical Society International Conference, September 6, 2018; Cardiff, Wales.
  29. Tebbs, J., McMahan, C., and Bilder, C. "Pooling for simultaneous detection of Chlamydia trachomatis and Neisseria gonorrhoeae is cost efficient and accurate," STD Prevention Conference, August 29, 2018; Washington, DC.
  30. Hitt, B., Bilder, C., Tebbs, J., and McMahan, C. "Optimal testing configurations for group testing," Joint Statistical Meetings, July 31, 2018; Vancouver, BC.
  31. Hitt, B., Bilder, C., Tebbs, J., and McMahan, C. "Optimal group testing algorithms for infectious disease detection with the binGroup package," UseR!, July 13, 2018; Brisbane, Australia.
  32. Hou, P., Tebbs, J., Bilder, C., and McMahan, C. "Hierarchical group testing for multiple infections," International Biometric Conference, July 10, 2018; Barcelona, Spain.
  33. Joyner, C., McMahan, C., Tebbs, J., and Bilder, C. "From mixed-effects modeling to spike and slab variable selection: a Bayesian regression," IBS ENAR meeting, March 28, 2018; Atlanta, GA.
  34. Mokalled, S., McMahan, C., Tebbs, J., and Bilder, C. "Acknowledging the dilution effect in group testing regression: a new approach," IBS ENAR meeting, March 28, 2018; Atlanta, GA.
  35. McMahan, C., Liu, Y., Tebbs, J., Bilder, C., and Gallagher, C." A Bayesian generalized additive model for group testing data," IBS ENAR meeting, March 26, 2018; Atlanta, GA.
  36. Bilder, C., Tebbs, J., and McMahan, C. "Infectious disease detection using specimen pooling with multiplex assays when risk-factor information is present," IBS ENAR meeting, March 25, 2018; Atlanta, GA.
  37. Bilder, C., Tebbs, J., and McMahan, C. "Hierarchical group testing with multiplex assays in heterogeneous populations," Royal Statistical Society International Conference, September 6, 2017; Glasgow, Scotland.
  38. Bilder, C., Tebbs, J., and McMahan, C. "Hierarchical group testing with multiplex assays in heterogeneous populations," Joint Conference on Biometrics & Biopharmaceutical Statistics, August 30, 2017; Vienna, Austria.
  39. Bilder, C., Tebbs, J., and McMahan, C. "Informative group testing for multiplex assays," Joint Statistical Meetings, July 31, 2017; Baltimore, MD.
  40. Tebbs, J., McMahan, C., Hanson, T., and Bilder, C. "Bayesian regression for group testing data," Conference on Applied Statistics in Agriculture, April 24, 2017; Manhattan, KS.
  41. Mou, X., Tebbs, J., McMahan, C., and Bilder, C. "Back-end disease screening using information theory," IBS ENAR meeting, March 15, 2017; Washington, DC.
  42. Bilder, C., Tebbs, J., McMahan, C., and Hitt, B. "Optimal pool sizes for group testing," IBS ENAR meeting, March 12, 2017; Washington, DC.
  43. McMahan, C., Tebbs, J., and Bilder, C. "Bayesian regression of group testing data," IBS ENAR meeting, March 12, 2017; Washington, DC.
  44. Warasi, M., Tebbs, J., McMahan, C., and Bilder, C. "Using hierarchical group testing to estimate the prevalence of multiple diseases," IBS ENAR meeting, March 12, 2017; Washington, DC.
  45. Bilder, C., Tebbs, J., McMahan, C., and Hitt, B. "Objective functions for group testing," International Biometric Conference, July 15, 2016; Victoria, BC.
  46. Hou, P., Tebbs, J., and Bilder, C. "Hierarchical group testing for multiple infections," IBS ENAR meeting, March 17, 2015; Miami, FL. (Student travel award winning paper)
  47. Bilder, C., Tebbs, J., and Black, M. "Multi-stage group testing with heterogeneous probabilities of disease positivity," WNAR/IMS Conference, June 18, 2014; Honolulu, HI.
  48. Tebbs, J., McMahan, C., and Bilder, C., "Group testing for multiple infections with application to the Infertility Prevention Project," Joint Statistical Meetings, August 4, 2013; Montreal, QC.
  49. McMahan, C., Tebbs, J., Hanson, T., and Bilder, C., "Bayesian regression models for group testing data," Joint Statistical Meetings, August 4, 2013; Montreal, QC.
  50. Black, M., Bilder, C., and Tebbs, J. "Optimal retesting configurations for hierarchical group testing," Joint Statistical Meetings, August 4, 2013; Montreal, QC.
  51. McMahan, C., Tebbs, J., and Bilder, C., "Regression models for group testing data with pool dilution effects," IBS ENAR meeting, March 11, 2013; Orlando, FL.
  52. Zhang, B., Bilder, C., and Tebbs, J., "Incorporating retests into group testing regression model estimation," Joint Statistical Meetings, August 2012; San Diego, CA.
  53. Bilder, C., Zhang, B., and Tebbs, J., "Marginal regression models for multiple-disease group testing data," IBS ENAR meeting, April 2, 2012; Washington, DC.
  54. McMahan, C., Tebbs, J., and Bilder, C., "Two-dimensional informative array testing," IBS ENAR meeting, April 2, 2012; Washington, DC.
  55. McMahan, C., Tebbs, J., and Bilder, C., "Regression models for group testing data with pool dilution effects," IBS ENAR meeting, April 1, 2012; Washington, DC.
  56. Peng, Y., Tebbs, J., and Bilder, C., "Dorfman group screening with multiple infections," IBS ENAR meeting, April 1, 2012; Washington, DC.
  57. Bilder, C. and Tebbs, J., "Informative retesting procedures to assay high volume clinical specimens," 58th World Statistics Congress of the International Statistical Institute, August 24, 2011; Dublin, Ireland.
  58. Zhang, B., Bilder, C., and Tebbs, J. "Group testing regression models for multiple traits," Joint Statistical Meetings, August 2, 2011; Miami Beach, FL.
  59. Bilder, C., Iwen, P., and Tebbs, J., "Alternative pooled testing procedures to evaluate high volume clinical specimens," Association of Public Health Laboratories Annual Meeting, June 6, 2011; Omaha, NE.
  60. Bilder, C. and Tebbs, J., "Using pooled testing to screen heterogeneous populations," Thirteenth Biennial CDC Symposium on Statistical Methods, May 24, 2011; Decatur, GA.
  61. McMahan, C., Tebbs, J., and Bilder, C. "Informative Dorfman screening," IBS ENAR meeting, March 22, 2011; Miami, FL. (Student travel award winning paper)
  62. Bilder, C. "Decoding positive groups through halving," Department of Statistics, University of South Carolina, January 20, 2011; Columbia, SC.
  63. Black, M., Bilder, C., and Tebbs, J. "Group testing in heterogeneous populations using halving algorithms," 2010 Joint Statistical Meetings, August 3, 2010; Vancouver, BC.
  64. McMahan, C., Tebbs, J., and Bilder, C. "Informative array screening," 2010 Joint Statistical Meetings, August 2, 2010; Vancouver, BC.
  65. Bilder, C., Zhang, B., Schaarschmidt, F., and Tebbs, J. "binGroup: a package for group testing," UseR! 2010, July 21, 2010; Gaithersburg, MD.
  66. Bilder, C. "Turning data into knowledge to solve sci-fi problems," Department of Statistics, University of Nebraska-Lincoln, September 2, 2009; Lincoln, NE.
  67. Bilder, C., Tebbs, J., and Chen, P. "Informative retesting," 2009 Joint Statistical Meetings, August 4, 2009; Washington, DC.
  68. Tebbs, J., Chen, P., and Bilder, C. "Inference for variance components in generalized linear mixed models for pooled binary response," 2009 IBS ENAR meeting, March 18, 2009; San Antonio, TX.
  69. Bilder, C. "Human or Cylon? Group testing on the Battlestar Galactica to save humanity," Department of Statistics, Kansas State University, November 14, 2008; Manhattan, KS.
  70. Bilder, C. "Human or Cylon? Group testing on the Battlestar Galactica," Department of Statistics, University of Missouri, October 5, 2007; Columbia, MO.
  71. Bilder, C. "Cylon detection through group testing," Department of Statistics and Actuarial Science, Simon Fraser University, April 13, 2007; Burnaby, BC.
  72. Bilder, C. "Just group it!" Department of Preventive and Societal Medicine and Department of Internal Medicine Section of Infectious Diseases, University of Nebraska Medical Center, November 17, 2005; Omaha, NE.
  73. Bilder, C. "Just group it!" Department of Statistics, University of Nebraska-Lincoln, September 7, 2005; Lincoln, NE.
  74. Bilder, C. "Group testing model estimation and inference," 2005 Joint Statistical Meetings, August 8, 2005; Minneapolis, MN. (handout)
  75. Bilder, C. and Tebbs, J. "Improved group testing estimation of trait prevalence," 2005 IBS ENAR meeting, March 20, 2005; Austin, TX.
  76. Bilder, C. and Tebbs, J. "Empirical Bayesian estimation of the disease transmission probability in multiple-vector-transfer designs," 2004 Joint Statistical Meetings, August 10, 2004; Toronto, ON.
  77. Tebbs, J. and Bilder, C.  "Confidence interval procedures for the probability of disease transmission in multiple-vector-transfer designs," 2003 Joint Statistical Meetings, August 5, 2003; San Francisco, CA.
  78. Tebbs, J. and Bilder, C.  "New interval estimation procedures for the disease transmission probability in multiple-vector transfer designs," Conference on Applied Statistics in Agriculture, April 28, 2003; Manhattan, KS.  
  79. Bilder, C. and Tebbs, J. "Estimating the disease-transmission probability in multiple-vector transfer designs," Oklahoma Chapter of the American Statistical Association, April 10, 2003; Stillwater, OK.