T8. The following table, from across-sectionalstudy conducted over 1-year time period, shows the prevalence of autism stratified by MMR Vaccination (exposure). Calculate and interpret the appropriate measure of association and 95% CIs from this study. Is the prevalence of the outcome rare or common? What concerns might you have about using this kind of study design to investigate the relationship between vaccines and autism? Similarly, what concerns might you have had if this study had been an ecologic cross-sectional study of 50 countries? Describe such a study.

Table 1. Prevalence of Autism Among Children Stratified by receipt of MMR Vaccination

	With Autism	Without Autism	Total
MMR vaccination
Yes	3	229	232
No	147	7978	8125
Total	150	8207	8357

You decide, based on the results of this cross-sectional study, that you want to investigate whether MMR vaccination is a cause of autism. Describeat least one limitation of using a cross-sectional study in and how it relates to your exposure and outcome.