BMJ 1998;317:601 ( 29 August )
Letters
Variations in population health status
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Non-response rates were not reported
Author's reply
Non-response rates were not reported
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EDITOR 601 -- BMJ_files/mdash.gif)
Kind et al reported the results of a survey which
aimed to measure the health of a representative sample of the
population of the United Kingdom by using the EuroQoL EQ-5D
questionnaire.1
The authors concluded that the instrument "is a practical way
of measuring the health of a population and of detecting
differences in subgroups of the population" and that "the
results are indicative of the average health status in the
general population of the United Kingdom ... living in the
community." They do not, however, report the non-response rates
of their survey.
We are informed that the respondents were representative of the general
population in respect of age, sex, and social class. They may
well have a similar distribution of these characteristics to
the general population, but this does not make their views of
their health status representative. Without knowing the non-response
figures and the reasons for non-response, one can have no
confidence in their conclusion.
The authors' sample consisted of 6080 addresses, at each of which
one adult aged 18 or over was sampled. Of the selected
addresses, 12% were unproductive as they were non-residential,
empty, or the locations were untraceable. The final sample
comprised "3395 subjects." If it was intended to sample one
adult from each of the supposedly remaining
5351 addresses, then, by my calculations,
3395 represents a response rate of 63%. What happened to the
other 1956, and who were they? The explanation is unlikely
to be non-contact by the interviewer of at least someone in the
household, given that figures from the Office for National
Statistics for 1993 (the year the survey was undertaken) show
that 63% of British households were occupied by two or more
adults.2
These figures imply that the response rate was poor and that this was
mainly because of people's refusal to take part. A poor
response rate could lead to unrepresentative findings. Maybe
there is another explanation for the number of adults
interviewed; if so, the authors should have provided
it.
In addition, it is not possible to deduce the item non-response rates
from their tables, as they only reported the numbers of people
who reported problems. I assume that item response was good
because this was an interview, not a postal, survey (and the
responses in figure 1 can be totalled to make 3381). This
information should have been provided, however, and commented
on, for potential users of the instrument.
Ann Bowling, Professor of
health services research.
University College
London Medical School, Centre for Health Informatics and Multiprofessional
Education (CHIME), Whittington Hospital Campus, London N19 5NF
- Kind P, Dolan P, Gudex C, Williams A. Variations
in population health status: results from a United Kingdom national
questionnaire survey. BMJ 1998; 316: 736-741[Abstract/Free Full Text].
(7 March.)
- Foster K, Jackson B, Thomas M, Hunter P, Bennett
N. General household survey 1993. London: HMSO , 1995.
Author's reply
EDITORBowling makes a helpful point in asking for
information on the response rate in our national survey. Of the
initial 6080 addresses identified from the postcode
address file, 756 were found to be non-viablefor example, empty or derelict buildings,
buildings not yet completed, business addresses, or simply
untraceable locations. At the remaining 5324 viable
addresses, completed interviews were achieved with
3395 respondents, yielding a 64% response rate. Reasons
for non-response included a broken appointment, illness, no
contact at the selected address or with the selected person,
and refusal to provide any information (including a reason for
this refusal).
All voluntary surveys are subject to non-response, and there can be no
set definition of what constitutes a poor response rate or,
indeed, a satisfactory one. There is clearly a relation between
the proportion of responders who contribute to a study and the
extent to which the data they generate can safely be regarded
as being representative of the study population as a whole.
Much depends on the magnitude of the non-response and, most
importantly, on the degree to which the disposition (not) to
respond is correlated with important survey variablesalmost by definition an area of some
uncertainty. Our sample was representative of the general
population in terms of the distribution of respondents by age,
sex, educational attainment, and housing tenure. Geographical
distribution also closely matched local populations defined by
regional health authorities and standard economic
regions.
Findings like these can never provide a complete guarantee against
non-response bias, but they do confirm our belief that the
survey results are likely to be broadly representative of the
population as a whole. Those with a residual concern about this
might wish to consult table 5.24 in the health survey for
England,1
which indicates rates of self reported health problems on the
EQ-5D that are not dissimilar to those reported in our study.
Paul Kind, Senior research
fellow.
Centre for Health Economics,
University of York, York YO1 5DD
- In: Prescott-Clarke P, Primatesta P, eds.
Health survey for England, 1996. London: Stationery Office, 1998.
©
BMJ 1998
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