Modelling health scores with the multivariate skew normal
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If you have a question about this talk, please contact Michael Sweeting.
Health care interventions which use quality of life or health scores often
provide data which are skewed and bounded. The scores are typically formed
by adding up responses to a number of questions. Different questions might
have different weights, but the score will be bounded, and might be scaled
to the range 0 to 100. If improvement in health over time is measured,
scores will tend to cluster near the ‘healthy’ or ‘good’ boundary as time
progresses, leading to a skew distribution. Further, some patients will
drop out as time progresses, so the scores reflect a selected population.
We fit multivariate skew normal distributions to data from a randomised
controlled trial of four treatments for sprained ankles, in which scores
were recorded at baseline and 1, 3 and 9 months. In these data, the scores
at 3 and 9 months have skew marginal distributions, but the variance is
similar across the four times points. We consider the extent to which
variance and skewness can be explained by covariates including treatment
and age. In order to address the effect of clustering at the boundary, we
consider censored multivariate normal and skew model. The extended skew
normal is used to model the selection due to drop-out.
This talk is part of the MRC Biostatistics Unit Seminars series.
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