Every study that follows for example some ingredient like thimerisol, compares a high thimerisol group to a low thimerisol group that got less vaccines. But parents almost always vaccinate until they see or believe they see vaccine damage in their kid, when they are likely to quit. Whether they are correct or not, this preferentially puts sick kids in the low group. This completely ithese studies.
As a result of this confounding, numerous studies have reported a statistically significant protective effect of thimerisol against autism.
 Thimerosal Exposure in Infants and Developmental Disorders: A Retrospective Cohort Study in the United Kingdom Does Not Support a Causal Association, Nick Andrews; Elizabeth Miller; Andrew Grant et al, Pediatrics September 2004, VOLUME 114 / ISSUE 3 http://pediatrics.aappublications.org/content/114/3/584.full-text.pdf
 Prenatal and Infant Exposure to Thimerosal From Vaccines and Immunoglobulins and Risk of Autism, Cristofer S. Price, William W. Thompson, Barbara Goodson,et al, Pediatrics October 2010, VOLUME 126 / ISSUE 4 http://pediatrics.aappublications.org/content/126/4/656
 Autism Occurrence by MMR Vaccine Status Among US Children With Older Siblings With and Without Autism, Anjali Jain; Jaclyn Marshall; Ami Buikema; et al. JAMA. 2015;313(15):1534–1540. doi:10.1001/jama.2015.3077. http://jama.jamanetwork.com/article.aspx?articleid=2275444