no capability not death

Just a quick note following a twitter exchange I had regarding whether capabilities as valued by the ICEPOP team (the ICECAP-O was referenced in the original paper) are “QALY-like”.

Key team members never intended the ICECAP-O scores to be multiplied by life expectancy (in the way, say, an EQ-5D score is). Whilst we have recognised that people would like to do this, technically this is a fudge and comes down to definitions and the maths:

Death necessarily implies no capabilities but no capabilities (the bottom ICECAP-O state) does not imply death. But more fundamentally, the estimated ICECAP scores are interval scaled, NOT ratio-scaled (for reference, read the BWS book): we used a linear transformation to preserve the relative differences between states but the anchoring at zero would not be accepted by a math psych person: they would say defining the bottom to be the zero doesn’t make it so.

Since different individuals technically had different zeros (the BWS or any discrete choice estimates have arbitrary zero) – death – multiplying a technically averaged interval scale score (our published tariff) by a ratio scaled one (life expectancy) to compare across groups/interventions is wrong: If there is heterogeneity in where “death” is on our latent capability scale (which we can’t/didn’t quantify – unlike the traditional QALY models estimated in the proper way) then comparisons across groups that don’t have the same “zero” gives incorrect answers. We can compare “mean losses of capability from full capability” which is why I personally (though I don’t speak for the wider team here) prefer the measure to be used as an alternative measure of deprivation, like the IMD in UK or SEIFA in Australia.