There may be subgroups of patients with particular problems. Older patients may have different needs and concerns from younger ones, and may also interpret questions differently. There may be major cultural differences, and a questionnaire developed in one part of the world may be inappropriate in another. For example, respondents from Mediterranean countries can be less willing to answer questions about sexual activity than are those from northern Europe. Subjects from Oriental cultures may respond to some items very differently from Europeans.
The field study should be designed with a sufficiently large sample size to be able to detect major differences in responses according to gender, age group or culture. However, it is difficult to ensure that similar patients have been recruited into each subgroup. For example, in a multi-country field study there might be country-specific differences in the initial health of the recruited patients. Some countries might enter patients with earlier stage disease, and treatment or management of patients may differ in subtle ways. Thus observed differences in, for example, group mean scores could be attributable to "sampling bias" in patient recruitment or management, and it is extremely difficult to ascribe any observed mean differences to cultural variation in the response to questions.
Methods of analysis might include comparison of subgroup means and SDs. However, to eliminate the possibility of sampling bias, differential item functioning (DIF) is an attractive approach for multi-item scales. DIF analysis, which is described in Chapter 6, allows for the underlying level of QoL for each patient, and examines whether the individual item responses are consistent with the patients' QoL.
There may be different semantic interpretation of words by different cultures. Nordic and northern European countries interpret "anger" as something which is bad and to be avoided; in Mediterranean countries, "anger" is not only acceptable, but there is something wrong with a person who avoids expressing anger. Thus the interpretation of an answer to the question "Do you feel angry?" would need to take into account the cultural background of the respondent.
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