5. Exercise 9.16. Blood clots in immobilized patients, continued. Only need to d

Question

5. Exercise 9.16. Blood clots in immobilized patients, continued. Only need to do (a). (Page 227 - 228 in 3rd edition) 9.16 Blood clots in immobilized patients, continued. The Fragmin study from Exam- ple 9.13 compared patients treated with Fragmin with patients given a placebo in a randomized, double-blind design. Of the 1473 immobilized patients given a placebo, 73 experienced a complication from DVI (a) Compute the proportion of patients given a placebo who experienced a complication from DVT. What are the risk and the odds of experiencing a complication from DVT when an immobilized patient is given a placebo! How do these values compare? (b) Compare your results with those of Example 9.13. What do you conclude? We will see in Chapter 20 how to formally compare risks or odds for two groups. Example: 9.13 EXAMPLE 9.13 Blood clots in immobilized patients Patients immobilized for a substantial amount of time can develop deep vein throm- bosis (DVT), a blood clot in a leg or pelvis vein. DVT can have serious adverse health effects and can be difficult to diagnose. On its website, drug manufacturer Pfizer reports the outcome of a study looking at the effectiveness of the drug Fragmin (dalteparin) in preventing DVT in immobilized patients. Of the 1518 randomly chosen immobilized patients given Fragmin, 42 experienced a complication from DVT (the remaining 1476 patients did not).12 The proportion of patients experiencing DVT complications is 42/ 1518 = 0.0277 or 2.77%. We can use this information to compute the risk and odds of experiencing DVT complications for immobilized patients treated with Fragmin: risk .= 0.0277, or 2.77% odds = 0.0277/(1-0.0277) = 42/ 1476 = 0.0285 The odds of experiencing DVT complications among immobilized patients given Frag min are 42:1476, or about 1:35. That is, for every such patient experiencing a DVT complication, about 35 do not experience a DVT complication. The numerical values for the risk and odds in this example are very close,0.277 and 0.0285. In general, when the sample size is very large and the undesirable event not very frequent, risk and odds give similar numerical values. In other situations risk and odds can be very different

Explanation / Answer

a) The total number of patients in this case who are given placebo are 1473. Out of this 73 developed DVT complications. Hence the proportion is calculated by the formula in the example shown (i.e) 73/ 1473 = 0.049 or 4.9 % (converting to percentage means multiplying into 100). Hence the proportion of patients who experienced DVT, but were on placebo is 4.9%.

As per the example, the risk is also 4.9%.

To calculate odds we have to divide 73 by number of patients who did not get DVT (i.e) 1473-73 = 1400.

73/ 1400 = 0.05 = 5.0% (i.e) 73:1400 is the odds of experiencing DVT in patients on placebo that is 1:19 (divide 1400 by 73). Hence for every patient experiencing DVT, 19 patients do not.

These values are very close as is the case with the example. The same inference is applicable to both.

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