8. The collection of conditions data in the 2001 NHS focused on long-term conditions, with information about recent illness (requiring action in the two weeks prior to interview) not collected except in relation to persons with long-term asthma or diabetes. Information regarding conditions in this guide, therefore, refers exclusively to long-term conditions.
9. The tables in the following section examine selected health conditions for changes in questionnaire methodology and give the estimates and standardised rates/percentages for comparison. The focus of this questionnaire comparison is on the use of direct questions or prompt cards to elicit responses.
10. The issue of context effects can be important and also needs to be considered. Context effects are the influences on response by accompanying or associated questions. For example, in the 1995 NHS, after the questions about long-term conditions, respondents were asked about recent actions they had taken for their health (e.g. use of medication, consulted a doctor, had days away from work) and the medical conditions involved. This provided an opportunity for respondents to be reminded about a condition which they had, but had forgotten to previously mention (e.g. because it was controlled through use of medication) and for them to identify it as a long-term condition (in which case earlier responses were amended accordingly). In contrast, the 2001 NHS respondents were asked about recent actions taken for illness but, with the exception of the NHPA conditions, were not asked to associate those actions with a particular condition. Under the 1995 approach, for example, a respondent may be reminded about dermatitis from questions about the use of skin ointments or creams, while in 2001 this trigger was not available. While the result of this context effect may overall have been to increase the 1995 prevalence estimates relative to 2001, other changes introduced in 2001 (e.g. direct questions or mention on prompt cards) may have more than compensated for this effect for some conditions.
11. A further factor which may affect comparability is that the reported prevalence of illness is complex and dynamic, and directly a function of respondent knowledge and attitudes, which in turn may be influenced by the availability of health services and health information such as public education and awareness. For example, a public education program has been running in Australia over a number of years aimed to raise public awareness and acceptance of mental health disorders. One consequence may be that respondents are more willing to talk about, and more willing to report feelings of anxiety, depression or other mental and behavioural problems now than they might have been willing to report before the awareness campaign began.
12. The criteria used to determine the usefulness of a particular time-series are based on both the conceptual consistency between surveys and the plausibility of the trend produced. The assessments are provisional because each is based on a subjective decision. This limitation is unavoidable given that it is impossible to quantify the effect of changing the questionnaire independently from the actual change in prevalence (and diagnosis/awareness) of conditions over time. For each selected condition, assessments are made for two intersurvey comparisons (1989-90 to 1995 and 1995 to 2001) and categorised with the following notations:
- A - Acceptable. No significant changes between surveys;
- AL - Acceptable with limitations. Difficult to quantify the impact of questionnaire changes. Use with caution;
- NA - Not acceptable - significant intersurvey differences or unexplainable major change in prevalence preclude the reliable use of data for time-series applications;
13. In addition to the assessments for each condition, notes on the questionnaire design that may have influenced the level of response for the various conditions are also provided. Estimates and standardised rates are presented with 95% confidence intervals (based on the relative standard error of the estimate). Where relevant, comments (including information from other sources) are provided to assist in making assessments on the reliability of the time-series.
3.6 CODING, CLASSIFICATION AND SCOPE OF LONG-TERM HEALTH CONDITIONS FOR ASSESSMENT
14. Conditions reported in the 2001 NHS were coded to a relatively fine level of detail and classified within the World Health Organisation International Classification of Disease 9th Revision (ICD-9), ICD-10 and the International Classification of Primary Care (ICPC). The 1989-90 and 1995 surveys have conditions coded more broadly and are classified to the ICD-9 only. Therefore, the time-series assessments are limited to only those disease entities defined in the ICD-9. For more information on the classifications used, see Appendix 3, 4 and 5 of 2001 NHS Users' Guide, Appendices.