U.S. flagAn official website of the United States government
                    Here's how you know **Official websites use .gov**
                            
                            A **.gov** website belongs to an official government organization in the United States.
 **Secure .gov websites use HTTPS**
                            
                            A **lock** (                            
                              A locked padlock                                                      
                            
                            ) or **https://** means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.


Menu Agricultural Research Service
						
                    U.S. DEPARTMENT OF AGRICULTURE
 close



Research
                Research Home
                            National Programs
                            Research Projects
                            Scientific Manuscripts
                            International Engagement
                            Scientific Software/Models
                            Databases and Datasets
                            Office of Scientific Quality Review
            Media
                News & Features
                            Multimedia
                            Briefing Room
                            Down on the Farm
                            Press Room
                            Science in Your Shopping Cart
                            Scientific Discoveries
                            Tellus
                            Under the Microscope
            About ARS
                About ARS Home
                            Staff Directory
                            Labs and Research Centers (Map)
                            Headquarter Offices
                            Organizational Chart
                            Employee Services (REE Employees Only)
                            Office of Outreach, Diversity, and Equal Opportunity
            Work With Us
                Work With Us Home
                            ARS Vacancies at USAJOBS
                            Careers at ARS Information
                            Scientific Collaborations
                            Extramural Agreements
                            1890 Faculty Research Sabbatical ProgramSearch small
                    Search
            
            ARS Home
                                About ARS
                                Contact Us




ARS Home Research Publications at this Location





- ARS National Programs
- Search for a scientific manuscript
- Search for a research project
- ARS Software
- Programs and Projects
- Collaborations
- Subjects of Investigation
- Publications

Title:   Author  item
                                            Vanraden, Paul
  Submitted to: Interbull Annual Meeting Proceedings
                                    Publication Type: Proceedings
                                    Publication Acceptance Date: 6/4/2006
                                    Publication  Date: 7/1/2006
                                    Citation: Van Raden, P.M. 2006. Normality and skewness of genetic evaluations. Interbull Annual Meeting Proceedings. Interbull Bull. 35:164-167.Interpretive Summary: Linear models are used for evaluation of linear, normally distributed traits such as milk yield. Traits (e.g. somatic cell count) with skewed distributions are normalized through log or other simple transformation. Traits with nonlinear, non-normal distributions have gained importance, and alternative (e.g. Baysian and threshold) models for evaluation have been developed. These can increase accuracy of EBVs within an evaluation system, but differences in distributions of EBVs among countries may affect the accuracy of international (MACE) EBVs. May 2006 Interbull evaluations were examined for skewness and kurtosis of EBVs on each national scale. Kurtosis was small and positive for most traits and scales, with larger values for calving traits. Some positive kurtosis is expected due to differing reliabilities. For yield traits, skewness was generally small and the larger values may be the result of selection, whereby a country has a small number of very inferior bulls, or has imported a few superior bulls. In almost all countries somatic cell score (SCS) had a moderate, consistent skewness. Clinical mastitis data is binomial, but EBVs are relatively normally distributed, and skewness was similar to that of SCS. Longevity and fertility EBVs are fairly normally distributed although raw data is not. Longevity EBVs had little skewness except in the countries using survival analysis. Fertility EBVs had only slight skewness. Skewness of calving ease (CE) EBVs ranged from –.81 to –.01. Snell score EBVs (Canada) and EBVs on the underlying scale of threshold models (3 other countries), which are reported to Interbull, are more normally distributed than EBVs from the linear models used elsewhere. Rankings for EBVs from linear models are also skewed, and this skewness is doubled when these EBVs are finally transformed to the observed (published) scales in countries exchanging Snell or underlying scores with Interbull. The effect of mixing EBV distributions with different skewness was evaluated by examining sources of extreme bulls for CE. The U.S. supplied 34% of all bulls for direct CE and although the U.S. mean was lower, 45% of the top 100 bulls but only 20% of the poorest were from the U.S. Skewness of stillbirth evaluations is uniform across countries, because only linear models are currently used. Implementation of a threshold model by the U.S. would result in reduced skewness of EBV relative to EBV from other countries. Although elimination of skewness may not be appropriate, uniformity of skewness among contributing populations would improve the accuracy of international evaluations.
                                Technical Abstract: Linear models are used for evaluation of linear, normally distributed traits such as milk yield. Traits (e.g. somatic cell count) with skewed distributions are normalized through log or other simple transformation. Traits with nonlinear, non-normal distributions have gained importance, and alternative (e.g. Baysian and threshold) models for evaluation have been developed. These can increase accuracy of EBVs within an evaluation system, but differences in distributions of EBVs among countries may affect the accuracy of international (MACE) EBVs. May 2006 Interbull evaluations were examined for skewness and kurtosis of EBVs on each national scale. Kurtosis was small and positive for most traits and scales, with larger values for calving traits. Some positive kurtosis is expected due to differing reliabilities. For yield traits, skewness was generally small and the larger values may be the result of selection, whereby a country has a small number of very inferior bulls, or has imported a few superior bulls. In almost all countries somatic cell score (SCS) had a moderate, consistent skewness. Clinical mastitis data is binomial, but EBVs are relatively normally distributed, and skewness was similar to that of SCS. Longevity and fertility EBVs are fairly normally distributed although raw data is not. Longevity EBVs had little skewness except in the countries using survival analysis. Fertility EBVs had only slight skewness. Skewness of calving ease (CE) EBVs ranged from –.81 to –.01. Snell score EBVs (Canada) and EBVs on the underlying scale of threshold models (3 other countries), which are reported to Interbull, are more normally distributed than EBVs from the linear models used elsewhere. Rankings for EBVs from linear models are also skewed, and this skewness is doubled when these EBVs are finally transformed to the observed (published) scales in countries exchanging Snell or underlying scores with Interbull. The effect of mixing EBV distributions with different skewness was evaluated by examining sources of extreme bulls for CE. The U.S. supplied 34% of all bulls for direct CE and although the U.S. mean was lower, 45% of the top 100 bulls but only 20% of the poorest were from the U.S. Skewness of stillbirth evaluations is uniform across countries, because only linear models are currently used. Implementation of a threshold model by the U.S. would result in reduced skewness of EBV relative to EBV from other countries. Although elimination of skewness may not be appropriate, uniformity of skewness among contributing populations would improve the accuracy of international evaluations.


Return to top


Connect with ARS Sign up


ARS Home
                            USDA.gov
                            Plain Writing
                            Policies & Links
                            Civil Rights Statements
                            FOIA
                            Accessibility Statement
                            Privacy Policy
                            Non-Discrimination Statement
                            Quality of Information
                            USA.gov
                            WhiteHouse.gov