This supplement concerns Appendix E of the "What Works Clearinghouse (WWC) Procedures Handbook, Version 4.1." The supplement extends the range of designs and analyses that can generate effect size and standard error estimates for the WWC. This supplement presents several new standard error formulas for cluster-level assignment studies, in addition to introducing a needed bias correction term to the cluster design effect size formula. However, some extensions also expand options for computing standard errors for individual-level assignment studies. This supplement introduces a general approach, for both individual- and cluster-level assignment studies, that leverages the study-reported standard errors for regression coefficients. These enhancements were identified after the initial publication of the "WWC Procedures Handbook, Version 4.1," in January 2020. No reviews were conducted under Version 4.1 prior to the publication of this supplement. This supplement is specific to Version 4.1, and its content will be directly integrated into future versions of the "WWC Procedures Handbook." [For "What Works Clearinghouse™ Procedures Handbook, Version 4.1," see ED602035.]