Clinical Biostatistics: breast pump trial exercise, answer 1

Question

There are at least three things wrong with this test. What are they?

Suggested answer

1. They have treated 42 observations on 14 subjects as independent. They are not, because repeated observations on the same woman will be more like one another than they are like observations on a different woman.

2. They have ignored that the same women use both pump and manual methods and treated them as though they are independent. They have carried out a two sample t test on paired data.

3. The standard deviations are different. We cannot assume they are the same. Also, the condition with the bigger standard deviation also has the bigger mean. When the standard deviation increases with the mean, we usually have skewness.

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Last updated: 2 July, 2008.

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