Suggested answer to exercise: Analyses for qualitative data, 5

Question 5: What is a trend test and how would you interpret the one presented here?

Suggested answer

The trend test is the Armitage chi-squared test for trend or the Mantel-Haenszel test for trend. It works by assigning numerical values to each category and then estimating the best prediction of one variable by the other as a simple

y = constant1 + constant2 × x.

The chi-squared test for trend tests the null hypothesis that there is no such prediction and constant2 = 0. The calculated chi-squared value is 20.6 with 1 degree of freedom with an associated P-value smaller than 0.001. This provides strong evidence for a trend in the proportions and so we would conclude that the proportion of males who are seropositive increases with age.

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Last updated: 27 July, 2009.

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