Simpson’s Paradox is suppression, but Lord’s Paradox is neither: clarification of and correction to Tu, Gunnell, and Gilthorpe (2008)

Table 2 Associations between type of surgery and surgical outcome.

	Open surgery	Percutaneous nephrolithotomy
Association between type of surgery and surgical outcome, disregarding kidney-stone size
Failure	77	61
Success	273	289
Total	350	350
Percentage success	78%	83%	Positive association

	Large stone		Small stone
	Open surgery	Percutaneous nephrolithotomy	Open surgery	Percutaneous nephrolithotomy
Association between type of surgery and surgical outcome for each kidney-stone size
Failure	71	25	6	36
Success	192	55	81	234
Total	263	80	87	270
Percentage success	73%	69%	93%	87%	Negative associations

Surgical outcome coded 0 = failure, 1 = success; type of surgery coded 0 = open surgery, 1 = percutaneous nephrolithotomy; kidney-stone size coded 0 = large stone, 1 = small stone