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#1
08-05-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,867 Threads: 149 Thanked 2,107 Times in 1,049 Posts Reputation: 2127
Which of the following tests would be most appropriate

A pharmaceutical researcher is examining the ulcerogenic potential of a new nonsteroidal anti-inflammatory drug.
He gives 20 rats a single subcutaneous injection of the drug every day for one week, and gives a similarly
matched group of 20 animals daily saline injections for one week. 24 hours after the final injection, the
investigator sacrifices the rats, removes their stomachs, and examines them to determine if any ulcers were
produced. He obtains the following data:
.................Ulcers present.....................Ulcers absent

Drug................12............................ .............8

No Drug............8................................. ........12

Which of the following tests would be most appropriate for determining if administration of the drug increased the
incidence of stomach ulcers?

A. Analysis of variance (ANOVA)

B. Chi-squared test

C. Linear regression

D. Paired t-test

E. Pearson correlation coefficient

#2
08-05-2012
 USMLE Forums Scout Steps History: 1+CK+CS+3 Posts: 93 Threads: 21 Thanked 37 Times in 16 Posts Reputation: 47

B Chi squared test?
#3
08-05-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,867 Threads: 149 Thanked 2,107 Times in 1,049 Posts Reputation: 2127

Explanation:

The correct answer is B. The most commonly used method for calculating p values from a two-by-two
contingency table is the chi-squared test. This test is used for frequency data (such as those above) rather
than for comparison of means. Since the investigator scored the stomachs as either containing ulcers or not
containing ulcers, without attempting to quantify the number of ulcers, the data represent frequencies rather
than means. Chi-squared is calculated as the sum for all cells of (Observed - Expected)2/Expected. The p value
for this value of chi-squared is obtained from a table using degrees of freedom equal to (number of rows - 1) x
(number of columns - 1).

ANOVA (choice A) is used to determine if the difference between two or more groups is significantly different.
This method is not applicable to raw frequency data.

Linear regression (choice C) is the process of fitting a straight line to a set of correlational data points by
minimizing the sum of squares of the vertical distances from the points to the line. This method would not be
suitable for the above data.

A paired t-test (choice D) is used to compare the means of two groups. It is not used to compare raw frequency
data.

Correlation is used when two variables are simultaneously measured in a sample. The Pearson correlation
coefficient (choice E) is used when both the independent and dependent variable are continuous. In this case,
the independent variable is not continuous (since the drug is either given or not given), and the dependent
variable is also not continuous (the ulcers are either present or absent).

Last edited by Novobiocin; 08-05-2012 at 04:12 PM.
 The above post was thanked by: Mondoshawan (08-05-2012)

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