Fundamental Measurement: The Need Fulfilment Quality of Life (N-QOL) Measure
The quality adjusted life year (QALY) has serious problems related to its failure to adhere to measurement theory. If a QALY is to be meaningful, the utility score that translates time spent to an equivalent time spent in so-called perfect health must have ratio properties (i.e., it must support multiplication). Multiattribute utility scores (e.g. those generated by the EQ-5D-5L) fail to meet this standard. The multiattribute instruments produce ordinal scores that lack a true zero and they generate negative values. The manifest deficiencies of multiattribute utility instruments render them unfit, not only as a measure of therapy response but also in generating QALY claims. After 30 years of belief in their use, utilities and QALYs are clearly analytical dead ends.
The purpose of this commentary is to demonstrate a coherent way forward in health technology assessment by focusing, not on clinical attributes as surrogates for quality of life, but on measures that are based on a conceptual model describing patient value in terms of need-fulfilment. Building on an extensive, yet often overlooked literature, need-based measures that fit Rasch Measurement Theory criteria are converted from ordinal scores to interval scores to evaluate response to therapy. These measures meet the requirements of single attribute fundamental measurement which is the standard in the physical sciences. It is proposed that a translation from a Rasch interval scale (defined by logits) can be transformed to a bounded ratio scale. Need based Quality of Life (N-QOL) scales bounded by 0 (where no needs are fulfilled) to 1 (where all needs are fulfilled) form such scales. The N-QOL supports the full range of arithmetic operations. Multiattribute utilities and mathematically invalid QALYs can be put to one side as unfortunate historical curiosities in favor of a disease or target population specific N-QOL scale. Such a scale has the required properties to evaluate disease specific response to therapy This can also support N-QOL adjusted life years with a need- fulfillment life year (NALY) metric with ratio properties.
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