bing to g uni ject of grad due to low performance) on high school graduation rates. Ba. The following table contains data from the Chicago Public School system (CPS) on 6th graders who were retained (forced to repeat grade) or promoted (successfully advanced to the next grade). Calculate the difference in the fraction of students who drop out, comparing students who were retained versus those who were promoted. 6th graders Retained Promoted Fraction retained 0.417 Educational attainment as of fall 2005 Graduated from CPS 0.296 0.404 Dropped out 0.572 0.527 Transferred to private school 0.036 0.035 Moved to another public school district 0.160 0.154 Still enrolled in the CPS 0.132 0.068 Age when started high school 15.218 14.909 Age when left high school 17.243 17.283 Number of credits completed when 86.430 98.469 left high school Age when dropped out 17.025 17.044 Number of credits completed in high school 42.863 47.855 before dropping out 3b. Explain why the difference you calculated does not represent the causal effect of being retained. 3c. Provide one example of an omitted variable that could be driving/biasing this relationship. Justify your answer clearly, explaining how the variable is related to both graduation rates and grade retention. Determine the sign/direction of the bias (is the true/causal relationship greater or less than the number that you calculated above?)