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Exercise: Statins, pneumonia and confounding, 4

Question 4: After using splitting into fifths to check the propensity scores, the authors say that
they entered the score into their regression models as a continuous variable.
What are the advantages and disadvantages of using score as a continuous variable rather than
as a categorical variable with five categories?

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Suggested answer

The advantages of using propensity score as a continuous variable are:

- we use all the information available in the continuous propensity score,
- we need estimate only one coefficient if we assume the relationship is linear,
although we would need more if it were not.

The disadvantages of using propensity score as a continuous variable are:

- we need to assume that the relationship between outcome and score has a particular form,
such as linear on the logistic scale,
- the coefficents are difficult to interpret.

The advantages of using score as a categorical variable with five categories are:

- we do not need to assume the relationship is linear,
- we estimate coefficients with a fairly simple interpretation,
as log odds ratios comparing each fifth to a reference category.

The disadvantages of using score as a categorical variable with five categories are:

- we do not use all the information in the propensity score,
though with five categories we do not lose a great deal,
- we must estimate four coefficients rather than one.

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Last updated: 12 December, 2006.

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