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Difficulty: MediumInferential Statistical Tests

A hospital's intensive care unit implements a new early goal-directed therapy protocol for patients presenting with septic shock. To evaluate the protocol's impact on resource utilization, researchers compare the duration of ICU admission (measured in days) between 120120 patients managed under the new protocol and 125125 historical controls managed with previous standard care. Preliminary data analysis reveals that the duration of admission values in both groups are highly positively skewed, with a small subset of patients experiencing exceptionally prolonged stays. Which of the following statistical tests is most appropriate to determine if there is a significant difference in the duration of ICU admission between the two groups?

  1. Mann-Whitney U testAnswer
  2. B
    Independent two-sample t-test
  3. C
    Paired t-test
  4. D
    Wilcoxon signed-rank test
  5. E
    Chi-square test

Answer

The Mann-Whitney U test is the most appropriate statistical method for evaluating these data.
The correct answer is the Mann-Whitney U test. The study design involves comparing a continuous numerical outcome (duration of intensive care unit admission) between two separate, unrelated sets of patients (those managed with the new protocol versus historical controls). Because these represent two independent groups, paired tests are excluded. Furthermore, the stem explicitly states that the outcome data is highly positively skewed. Because the data is not normally distributed, parametric tests like the independent two-sample t-test cannot be reliably used. The Mann-Whitney U test (also known as the Wilcoxon rank-sum test) is the appropriate non-parametric alternative for comparing continuous or ordinal outcomes between two independent groups.

Step-by-Step Solution

1
Identify the independent variable (exposure or grouping variable) and its classification.
The independent variable is the treatment group (new protocol versus historical controls). This creates two completely independent, categorical groups.
Determining the number and relationship of the study groups is the first step in selecting a statistical test.
2
Identify the dependent variable (outcome variable) and its distribution characteristics.
The dependent variable is the duration of ICU admission measured in days. This is a continuous numerical variable, but the data is explicitly described as highly positively skewed rather than normally distributed.
The nature of the dependent variable (continuous vs. categorical) and its distribution (normal vs. skewed) dictate whether parametric or non-parametric testing should be applied.
3
Select the correct statistical test by synthesizing the group structure and data distribution.
Comparing a continuous, non-normally distributed outcome between two independent groups requires a non-parametric approach. Therefore, the Mann-Whitney U test is chosen.
The Mann-Whitney U test evaluates differences between two independent groups when the dependent variable is ordinal or continuous but not normally distributed, serving as the non-parametric alternative to the independent t-test.

Key Concept

Selection of the Mann-Whitney U test for comparing continuous or ordinal outcomes between two independent groups when the data lacks a normal distribution.
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Topics

Inferential Statistical Tests

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