Simpson's paradox An association or comparison that holds for all of several groups can reverse direction when the data are combined (aggregated) to form a single group. This reversal is called Simpson's paradox. Hospital A Hospital B Example: On the surface, Died 63 16 Survived 2037 784 Hospital B would Hospital death Total 2100 800 seem to have a % surv. 97. 0% 98. 0% better record. rates But once patient Patients in good condition Patients in poor condition condition is taken Hospital A Hospital B Hospital A Hospital B Died 6 8 Died 57 8 into account, we Survived 594 592 Survived 1443 192 see that hospital A Total 600 600 Total 1500 200 has in fact a better % surv. 99.0% 98.7% % surv. 96.2% 96.0% record for both patient conditions (good and poor). Patient condition is the lurking variable. Now see Ex. 2.40-2.41, p. 144...Simpson's paradox An association or comparison that holds for all of several groups can reverse direction when the data are combined (aggregated) to form a single group. This reversal is called Simpson's paradox. Hospital A Hospital B Example: On the surface, Died 63 16 Survived 2037 784 Hospital B would Hospital death Total 2100 800 seem to have a % surv. 97. 0% 98. 0% better record. rates But once patient Patients in good condition Patients in poor condition condition is taken Hospital A Hospital B Hospital A Hospital B Died 6 8 Died 57 8 into account, we Survived 594 592 Survived 1443 192 see that hospital A Total 600 600 Total 1500 200 has in fact a better % surv. 99.0% 98.7% % surv. 96.2% 96.0% record for both patient conditions (good and poor). Patient condition is the lurking variable. Now see Ex. 2.40-2.41, p. 144