Question

The drug ancrod was tested in a double-blind clinical trial in which stroke patients were randomly assigned to receive either ancrod or a placebo. One of the response variables in the study was whether or not the subjects experienced intracranial hemorrhaging. Of the 84 patients who received ancrod, 13 hemorrhaged. Of the 77 patients who received the placebo, only three hemorrhaged.

a.Is there a significant difference in hemorrhaging rates between treatments? Show your work and include all steps for full credit.Use critical value = 3.481

b.Calculate and interpret the relative risk of hemorrhaging fortaking the drug compared to the placebo.

c.Calculate and interpret the odds ratio of hemorrhaging for taking the drug compared to the placebo.

Answer 1

a.Null hypothesis:There is no significant difference in hemorrhaging rates between treatments

Alternative hypothesis:There is significant difference in hemorrhaging rates between treatments

Observed frequency

	Patients who received Arcrod	Patients who received placebo	Total
hemorrhaging	13	3	16
Non hemorrhaging	71	74	145
Total	84	77	161

Expected frequency

	Patients who received Arcrod	Patients who received placebo	Total
hemorrhaging	16*84/161=8.35	16*77/161=7.65	16
Non hemorrhaging	145*84/161=75.65	145*77/161=69.65	145
Total	84	77	161

13	8.35	2.589521
3	7.65	2.826471
71	75.65	0.285823
74	69.35	0.311788
		6.013602

Critical value=3.841, since calculated value is greater than critical value null hypothesis may be rejected.

b.Relative risk= (Probability of event in exposed group) / (Probability of event in not exposed group)

Probability of event in exposed group=

Probability of event in not exposed group=

RR=

it is 4.184 times more likely to have an hemorrhaging after taking the drug in this study than placebo in the study.

c.An odds ratio (OR) is a measure of association between an exposure and an outcome

since OD >1, hemorrhaging for taking the drug higher odds compared to the placebo.