Simpson’s Paradox

Here are comparative data for surgery for a particular condition at two hospitals:

If you had the condition in question, which hospital would you prefer for surgery? Charity seems the obvious choice. But now look at the figures when they are disaggregated to take into account whether patients are in good condition or bad condition before surgery:

For patients initially in good condition the results are:

For patients initially in bad condition the results are:

Whether you are in good condition or bad, your prospects now seem better at Mercy Hospital.