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Difficulty: HardInferential Statistical Tests

A hospital's infection control committee is evaluating the efficacy of a novel chlorhexidine bathing protocol for reducing methicillin-resistant Staphylococcus aureus (MRSA) colonization. The study enrolls a cohort of 150 patients undergoing long-term hemodialysis. At baseline, each patient undergoes a nasal swab and is categorized as either MRSA-positive or MRSA-negative. The entire cohort then follows the novel bathing protocol for six months. At the conclusion of the trial, the exact same 150 patients are swabbed again and categorized as MRSA-positive or MRSA-negative. The researchers aim to determine if there is a statistically significant change in the proportion of patients colonized with MRSA following the six-month intervention. Which of the following is the most appropriate statistical test to analyze these study results?

  1. A
    Chi-square test
  2. B
    Fisher exact test
  3. McNemar testAnswer
  4. D
    Paired Student t-test
  5. E
    Wilcoxon signed-rank test

Answer

The most appropriate statistical test is the McNemar test.
The correct statistical test is the McNemar test. Test selection depends fundamentally on two factors: the study design (independent vs. paired/dependent samples) and the type of data being analyzed (continuous, ordinal, or categorical). In this scenario, the exact same 150 patients are evaluated at baseline and again at six months, making the data paired. The outcome variable is MRSA colonization status, which is recorded as positive or negative, making the data binary categorical (dichotomous). The McNemar test is specifically designed to analyze paired nominal data by focusing on the discordant pairs (patients whose status changed from positive to negative, or vice versa).

Step-by-Step Solution

1
Identify the independent variable and the study design.
The intervention is evaluated before and after in the exact same 150 patients. Therefore, the samples are dependent (paired).
Recognizing whether samples are independent or paired is the first critical step in selecting a statistical test.
2
Identify the nature of the dependent (outcome) variable.
The outcome is MRSA colonization status, categorized strictly as 'MRSA-positive' or 'MRSA-negative'. This is a binary categorical (nominal) variable.
The data type of the outcome variable determines which family of statistical tests can be applied.
3
Select the test that matches both criteria: paired design and categorical outcome.
The McNemar test is specifically designed to analyze paired dichotomous data by evaluating discordant pairs.
Matching the paired study design with the binary outcome excludes tests meant for independent groups or continuous data.

Key Concept

The McNemar test is used to compare dichotomous (binary categorical) outcomes between two correlated or paired groups.
Estimated Time:1m 15s
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Inferential Statistical Tests

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