Data quality
As expected from previous studies, responses to
each item were distributed across the full range of
the scale, with no evidence of ceiling or floor effects
for any item for the total data set (Table 1).
Among the 24 centres there was some variation
and five items had marginally skewed distributions
with few responses (0.7) for
Domains 1, 2 and 4 i.e. physical health 0.82, psychological
0.81, environment 0.80, but marginal
for social relationships 0.68. Across sites, results
were consistently high with most of the a’s in
Domains 1 and 2 above 0.75, and in the range of
0.51–0.77 for Domain 3, and 0.65–0.87 for Domain
4. As the Domain 3 a calculation is based on
only three items, but on six to eight items in the
other domains, lower values in a majority of centres
were not unexpected because the statistic is
sensitive to numbers. Alpha analyses carried out
by systematically removing then replacing each
item showed that all 26 items made a significant
contribution to the variance in the WHOQOLBREF.
Discriminant validity
Discriminant validity is an important characteristic
of any measure, and in 14 of the 24 centres,
members of the general population were selected
so that results from well people could be compared
with those who were sick, in an initial examination
of this property through a test of ‘extreme’ groups.
A comparison of domain scores from sick and well
respondents shows that for the majority of countries,
discriminant validity was significant for each
domain in the total population (Table 3). Discriminant
validity was best demonstrated in the
physical domain, followed by the psychological,
social and environment domains. The results of
the hierarchical multiple regression to assess the
impact of gender and age on domain scores from
sick and well people showed that gender and age
together, only explained 2.7% of the overall variance
(adjusted R2
), although this effect is signifi-
cant (F ¼ 96.3 (2,7007) p < 0.0001).
Construct validity
Domain concepts are more clearly represented and
their scores are more easily interpreted when items
are clearly identified with the intended domain.
Analysis of correlations showed that in the total