Suppose researchers do an epidemiologic investigation of people entering a sexua

Question

Suppose researchers do an epidemiologic investigation of people entering a sexually transmitted disease clinic. Given the observed table below and = 0.05, conduct a chi-square test for association to determine if there is an association between present diagnosis and prior episodes of urethritis.

PRESENT DIAGNOSIS: YES / NO

GONNORHEA 160 / 40   

NONGONOCOCCAL URETHRITIS 50 /105

A) What is the alternative hypothesis?

B) What is the critical value?

C) What is the expected value of those with a present diagnosis of gonorrhea not having a previous episode of urethritis?

D) What is the expected value of those with a present diagnosis of nongonococcal urethritis having a previous episode of urethritis?

E) What is the expected value of those with a present diagnosis of nonogonococcal urethritis not having a previous episode of urethritis?

F) What is the test statistic?

Explanation / Answer

a) Ha: There is an association between present diagnosis and prior episodes of urethritis.

b)

The Expected Frequencies, fe are:

(200)(210)/355=

118.3098592

(200)(145)/355=

81.69014085

(155)(210)/355=

91.69014085

(155)(145)/355=

63.30985915

c) 81.69014085

d) 91.69014085

e) 63.30985915

f)

X2=14.69081+21.27635+18.95589+27.45335=82.3764

Observed Frequencies, fo, are: Prior episodes of urethritis PRESENT DIAGNOSIS Yes No Total GONNORHEA 160 40 200 NONGONOCOCCAL URETHRITIS 50 105 155 Total 210 145 355

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