Statistical considerations for use of composite health-related quality-of-life scores in randomized trials.
BACKGROUND
Quality of life instruments are frequently used as outcomes in randomized trials. Instruments that consist of several subscales present researchers with a choice of whether to combine some or all scales into a single composite score. There may be several clinically and scientifically reasonable alternative combinations of subscales for the primary outcome measure.


RESULTS
The statistical efficiency of different combinations of subscales depends on the relative effect size of the intervention on each subscale and the correlation between the subscales. Simple equations can be derived for determining the relative statistical efficiency of each clinically reasonable combination of subscales. Hypothetical scenarios show that the number of patients needed in a clinical trial can be twice as great for some combinations of subscales as for others.


CONCLUSIONS
There are often compelling clinical or scientific reasons to use a particular subscale or composite in a randomized trial. In the case where a number of different alternatives would be reasonable, statistical efficiency can help guide the choice of endpoint.
Vickers
Andrew J
AJ
Integrative Medicine Service, Biostatistics Service, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA. vickersa@mskcc.org
D003627:Data Interpretation, Statistical; D006801:Humans; D008960:Models, Psychological; D010865:Pilot Projects; D011788:Quality of Life / Q000523:psychology*; D016032:Randomized Controlled Trials as Topic / Q000379:methods* / Q000706:statistics & numerical data; D012107:Research Design; D018401:Sample Size; D018434:Sickness Impact Profile; D016896:Treatment Outcome