—FIGURE 3—
Net Gain in QALYs With Increased Life Expectancy but
With Periods of Diminished Quality of Life
(e.g., Chemotherapy to Treat Cancer)
1.0
Quality
of Life
Treatment starts here
QALYs gained
QALYs lost
0.0
Years of Life
—FIGURE 4—
The Time Trade-Off Method
1.0
Under time trade-off, individuals
suffering from a health impairment are
asked how many years of their expected
lifetime they are willing to exchange for
a shorter but healthy life (see Figure 4).
The answer to this question depends
heavily on personal values. Accordingly,
large variations in responses can be expected, with some individuals ready to
make aggressive swaps and others unwilling to give up even a few days. The
variability can be mitigated (albeit, not
perfectly) by determining the swap of
the “average person” in a group of people who suffer from the same condition.
Utilities then can be derived from the
equation:
time trade-off utility = 1 –
life expectancy.
Quality
of Life
Shorter but healthy life
Longer but impaired life
0.0
Years of Life
reduced pain (two years).
According to this measure, the past
6½ years in Werther’s life are equivalent to 5¼ years of perfect health (see
Figure 1).
It should be clear that the value of
any given treatment (say, the surgery
that increased Werther’s health from
30 percent to 90 percent) can be established by calculating the difference of
QALYs with and without surgery, as illustrated in Figures 2 and 3.
Other scenarios are also possible,
including interventions that enhance
the quality of life but reduce expected
lifetime, interventions that reduce both
quality of life and life expectancy, etc.
QALY Weights
How are preferences (or utilities) assigned to different states of health ( 1.0,
0.7, 0.3 and 0.9 in Werther’s case)? In
health economics, QALY weights are
typically measured using one of the following techniques:
Time trade-off ■
Rating scale ■
Standard gamble. ■
Imagine that there’s a group of 100
people with trade-offs, life expectancies
and preferences (or utilities) as shown
in Table 1.
In this group (and for one given condition), people are willing to give up 12. 1
percent = 299.9/2,587.9 of their lifetime
(44 days per year) in exchange for perfect health. That is, individuals value
their well-being (expressed as a QALY
weight) at 88. 4 percent. The same principle applies to other conditions, which
are then ranked in increasing order to
produce a list of numeric preferences as
follows:
0.00 < [u(condition)
1] < ... <
u(conditionn) < 1.00
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;-
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
of generality, higher sub-indices correspond to higher preferences values.
The rating scale method, also known
as the feeling thermometer, is similar:
Patients are asked to assess their current
state on a scale from 0.0 to 1.0, where 0.0
represents death, 1.0 perfect health, and
