BMJ 2001;323:1123-1124 ( 10 November )

Education and debate

Statistics Notes

Analysing controlled trials with baseline and follow up measurements

Andrew J Vickers, assistant attending research methodologist ^a, Douglas G Altman, professor of statistics in medicine ^b. 

^a Integrative Medicine Service, Biostatistics Service, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA, ^b ICRF Medical Statistics Group, Centre for Statistics in Medicine, Institute of Health Sciences, Oxford OX3 7LF

Correspondence to: Dr Vickers vickersa@mskcc.org

The first 150 words of the full text of this article appear below.

In many randomised trials researchers measure a continuous variable at baseline and again as an outcome assessed at follow up. Baseline measurements are common in trials of chronic conditions where researchers want to see whether a treatment can reduce pre-existing levels of pain, anxiety, hypertension, and the like.

Statistical comparisons in such trials can be made in several ways. Comparison of follow up (post-treatment) scores will give a result such as "at the end of the trial, mean pain scores were 15 mm (95% confidence interval 10 to 20 mm) lower in the treatment group." Alternatively a change score can be calculated by subtracting the follow up score from the baseline score, leading to a statement such as "pain reductions were 20 mm (16 to 24 mm) greater on treatment than control." If the average baseline scores are the same in each group the estimated treatment effect will be the same using these two simple approaches. . . . [Full text of this article]

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