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- SUBJECTS AND METHODS
A systematic review of prevalence studies on urinary incontinence and storage symptoms in the UK since 1960 identified 25 publications from 21 studies ; response rates were 53–98%. Analysis of response bias was limited and inconsistent, with five studies reporting an under-representation by elderly women and young groups, and five reporting no difference in relation to age and sex. Increasing awareness of data protection issues is likely to reduce response rates further, making insights into nonresponse bias increasingly important.
Nonresponse bias may be a particular problem in surveys on sensitive subject areas such as urinary incontinence. The potentially embarrassing nature of the questions may discourage participation by some people, although the anonymity of the postal questionnaire may encourage participation by others. Questionnaires which contain sensitive questions are less likely to be returned , but the evidence that nonresponse bias occurs is conflicting [3,4].
The question of nonresponse bias has been investigated in depth in the context of general health and several specific diseases and conditions, but little has been done in the area of incontinence and urinary symptoms. The pattern of symptom reporting over time taken to return a questionnaire  and differences between subsequent mailings  have been used as indicators of nonresponse bias. Another approach is to follow-up a sample of nonresponders until they are all traced, and then assess whether and how they differ from the responders . The aim of the present study was to identify the nature and extent of nonresponse bias in a postal survey on urinary symptoms, by following a sample of nonresponders.
- Top of page
- SUBJECTS AND METHODS
Nonresponders are by definition difficult to trace and interview, and in studies which have used this approach [7,12] many have always remained untraced. In the present large study on 1050 nonresponders, the interviewers were very successful in tracing them and only 1% remained unaccounted for.
At around the time of mailing 6.9% of the survey sample were identified as ineligible. After tracing, 12.2% of the nonresponder sample were identified as ineligible, mainly because they had moved. The high rate of ineligibility in the youngest men (40–49 years) may have been a result of less frequent visits to the GP , which is when general practices are usually informed of address changes. The high ineligibility rates in the very elderly men and women (≥ 80 years) are the result of the high morbidity, mortality and rates of admission to residential care in this group. If the sample details of this age group are adjusted for the levels of ineligibility found in the nonresponders, the response rates to the postal questionnaire increase from 63.5% to 67.5% in men, and from 58.5% to 62.5% in women.
Although almost the whole sample was successfully traced, only half (49%) of the eligible nonresponders answered the interviewer's questions, which is not unexpected as they had previously not responded to three mailings of the questionnaire. Overall, the main reason for not speaking to the interviewer was a general unwillingness or lack of interest, reasons commonly given for not participating in surveys . Very few had a hostile attitude to the survey. However, in many the reason was associated in some way with poor health, and the percentage increased markedly with age. The nonresponders who spoke to the interviewer were asked why they had not previously returned the postal questionnaire, and the pattern of reasons given was similar, with the main one being general unwillingness or ‘never got round to it’. Poor health as a reason increased with age. This is consistent with reports of high nonresponse rates caused by illness or frailty in surveys in the very elderly .
Between responders and nonresponders overall there was little difference in the reporting of urinary symptoms, and this was further supported by comparing the early and late responders. However, there was greater reporting of some urinary symptoms by the elderly nonresponders and differences in the two measures of general health. Current poor health was more common among elderly nonresponders, but a long-term health problem was more common among responders.
Reports on nonresponse have shown poorer health in responders  and nonresponders , but many studies have shown small or no differences depending on who and what is being investigated [17–20]. Two studies on prostatism used follow-up questionnaires to collect information on samples of those not participating [21,22]. Both studies reported fewer urinary symptoms in the non-participants, which had sizeable effects on the adjusted prevalence rates. However, the studies compared men who had agreed or not agreed to participate in studies which involved substantial commitment and intrusive clinical procedures, so it is not surprising that men with symptoms were more interested and willing to take part. Moller et al. reported less incontinence and strong urgency when 529 late responders to a postal questionnaire were compared with 2860 early responders, while a smaller study  showed no difference in reporting of incontinence in 38 nonresponders who returned the third mailing of a questionnaire. The evidence that withdrawal rates in longitudinal studies are greater in people with no urinary symptoms is conflicting [25–27], and where there were differences these were not large.
The present study compared data from a self-completed postal questionnaire with data collected by an interviewer. In addition, the nonresponders answered the questions later than the responders, and there may have been onset or progression of symptoms. There is reported evidence about the mode of administration of questionnaires on urinary symptoms, with two studies [28,29] showing less reporting of urinary symptoms in a telephone interview than in an earlier self-completed postal questionnaire. A third study, investigating a few ‘visually impaired or illiterate men’, found no difference . The chances of an effect of mode of administration are probably greater in the elderly group. Although instructed not to, the interviewers may have helped some of the very elderly and frail nonresponders to answer the questions.
Concern about the possible sensitive nature of the questionnaire content proved unfounded. The actual refusal rate to the postal questionnaire was not large, and when a reason was given for not taking part, it was usually a general unwillingness to take part in surveys. Only very few complained about the actual contents of the questionnaire. The general acceptance of the questionnaire is indicated by the response rate of 63.3%. Response rates to postal surveys depend among other things upon the nature of the sampling frame. The present large county-wide sample was selected from the lists of over 400 GPs, and the response rate compares well with other large postal surveys on health which have been conducted recently in this country using the same sampling procedure .
In conclusion, we found little evidence of nonresponse bias in the reporting of urinary symptoms in the overall sample, which gives confidence in the population prevalence rates for urinary symptoms that we calculated for people aged ≥ 40 years. Although the prevalence rates among the very elderly are possibly underestimated, the proportion of very elderly people within the population is very small.