An article in the British Medical Journal \"Comparision of Treatmentof renal cal

Question

An article in the British Medical Journal "Comparision of Treatmentof renal calculi by operative surgery, percutaneousnephrolithotomy, and extracorporeal shock wave lithotripsy,"provided the following discussion of success rates in kidney stoneremovals. Opening discussion of success rates in kidney stoneremovals. Open surgery had a success rate of 78% (273/350) while anewer method, percuataneous nephrolithotomy (PN), had a successrate of 83% (289/350). This newer method looked better, but resultschanges when stone diameter was considered. For stones withdiameters less than two centimeters, 93% (81/87) of cases of opensurgery were successful compared with only 83% (234/270) of casesof PN. For stones greater than or equal to two centimeters, thesuccess rates were 73% (192/263) and 69% (55/80) for open surgeryand PN, respectively. Opern surgery is better for both stone sizes,but less successful in total. In 1951, E.H.Simpson pointed out thisapparent contradiction (known as Simpson'sParadox) but the hazard still persists today. Explain howopen surgery can be better for both stone sizes but worse intotal.

Explanation / Answer

The key to this problem is the sample size of the individual experiments.  You can see that there is a huge difference in the numbers, for instance, of the stones less than two centimeters.  One size is 87 and the other is 270.  Since each value of a smaller sample size has a greater influence on the total statistic, this can often create a contradiction.

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.