Clearly the shorter the PROM the quicker it will be completed, resulting in less respondent burden and possibly higher response rates. The DHP-1which has 32 items has an approximate completion time of 9-12 minutes in comparison to 5-7 minutes for the DHP-18 with 18 items. A further consideration is that longer questionnaires take up more space, which can be an important factor when a number of different questionnaires are administered at the same time. However, the decision is unlikely to be dependent on respondent burden alone but, also the precision in which the scale measures the specific domains.
Precision is associated with length – the more items the greater the coverage, representation and precision in the measurement of the specific domain. Although completing the DHP-1 comprising 32 items, takes approximately 10-12 minutes, it offers a greater coverage of the three domains and therefore, greater degree of measurement precision. Conversely, the shorter the scale, the coarser the coverage of the domain and lesser precision. The level of measurement precision of the scale will therefore, be a significant factor when matching the scale to a particular application.
Small group differences and individual measurement
Detecting small differences between groups and individual assessment e.g. screening of individual patients, requires a high level of score reliability (≥ 0.90) to achieve statistical power together with narrow confidence intervals around score estimates. As measurement reliability tends to increase with scale length, shorter scales might be less adequate in detecting small differences. It should be noted however, that the DHP-1 andDHP-18 have not yet been evaluated for individual assessment.
Large surveys and samples
The DHP-1 and DHP-18 can be considered for use with large and representative surveys, samples and group comparisons as large and representative samples (N>500)(ref) enable greater precision and statistical power to be achieved more than by increasing measurement reliability.
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