## 4 Discussion  
### Key Results  
The present results obtained in patients with nasal polyposis suggest that self-assessments of olfaction on numerical scales can be translated fairly accurately into nominal diagnostic categories of anosmia, hyposmia, or normosmia, but they cannot be expected to reliably provide fine-scale information about olfactory function that could be used as a surrogate for quantitative olfactory tests. Thus, the present independently recruited cohort replicated the observation that in patients with nasal polyposis, self-assessment of their olfactory function provides fairly reliable information about their sense of smell [1,21], but the reported high correlation of NRS ratings and olfactory test scores was not reproduced. This overall usefulness of the self-ratings as a source of information about main categories of olfactory function is consistent with previous assessments of olfactory self-reports, in which a simpler 4-point scale detected anosmia at positive predictive values of \(>\)58% in a mixed cohort of subjects [20]. The observed poor correlation between self-assessments and measured olfactory function, which contrasts with some previous reports, underscores the need to exercise caution when assessing correlations between self-assessments of olfaction and olfactory test results. Global analyses may suggest a strong correlation, but this is due to overall group differences between the olfactory diagnoses, which may suggest a correlative relationship to be much larger than it actually is. This has been performed in some of the reports of apparently strong correlations of self-assessments with sensory test scores and has probably contributed to the disagreeing study results about the utility of self-assessments as a substitute for olfactory testing. To avoid these pitfalls, it is a recommended practice (e.g., [59]) to cross-check numerical correlation calculations first with an analysis of the shape of the distributions of each variable and second with a scatter plot. However, there is often little correlation within the main diagnostic subgroups, which has been similarly observed previously [3]. Therefore, the self-ratings do not qualify as a good replacement for olfactory functional test scores. Thus, when the olfactory diagnostic subgroups were assessed for the correlation of self-ratings with test scores, i.e., for the possibility of using the former as a surrogate for the latter, significant correlations were found sparsely and mainly in patients with normosmia, with an additional correlation in hyposmic subjects and with the first rating scale for anosmia, although this diagnosis implies test results in the range of chance. However, \"functional\" anosmia, which is the precise meaning of olfactory diagnosis [60], does not indicate a complete lack of the sense of smell, and it would not be surprising to observe differences in individual subtest scores along a low range of possible scores, occasionally exceeding chance but not summing-up above the limit of hyposmia. The finding may also be interpreted as a lower validity of the precisely labelled scale for which every point required a clear decision in contrast to the continuous rating scale #2. The usefulness of self-assessment as a surrogate measure for scaled olfactory test scores was further supported by the significant negative correlations of the assessments with the general quality of life score of the SNOT-20 questionnaire, which is a weighted sum of nine questions in which worse symptoms get a higher rating than items that cause no problem for the patient. This agrees with the general perception that olfaction contributes positively to quality of life, which has been supported by numerous studies [61; 62; 63; 64; 65; 66]. However, while the general association of a worse quality of life with more reduced perception of one own's sense of smell was preserved, the PCA projected the quality-of-life ratings to be almost perpendicular to the olfactory test scores, which was consistent with their almost nonexistent correlation.