Response Conversion for improving comparability of International Physical Activity data.
BACKGROUND
Many questionnaires for measuring physical activity (PA) exist. This complicates the comparison of outcomes.


METHODS
In 8 European countries, PA was measured in random samples of 600 persons, using the IPAQ as a 'bridge' to historical sets of country-specific questions. We assume that a unidimensional scale of PA ability exists on which items and respondents can be placed, irrespective of country, culture, background factors, or measurement instrument. Response Conversion (RC) based on Item Response Theory (IRT) was used to estimate such a common PA scale, to compare PA levels between countries, and to create a conversion key. Comparisons were made with Eurobarometer (IPAQ) data.


RESULTS
Appropriateness of IRT was supported by the existence of a strong first dimension established by principal component analysis. The IRT analysis resulted in 1 common PA scale with a reasonable fit and face validity. However, evidence for cultural bias (Differential Item Functioning, DIF) was found in all IPAQ items. This result made actual comparison between countries difficult.


CONCLUSIONS
Response Conversion can improve comparability in the field of PA. RC needs common items that are culturally unbiased. Wide-scale use of RC awaits measures that are more culturally invariant (such as international accelerometer data).
Dept of Prevention and Healthcare, TNO Quality of Life, Leiden, Netherlands.
Marijke
M
Hopman-Rock
Elise
E
Dusseldorp
Astrid
A
Chorus
Gert
G
Jacobusse
Alfred
A
Ruetten
Stef
S
van Buuren
D000328:Adult; D000704:Analysis of Variance; D015982:Bias; D016009:Chi-Square Distribution; D003469:Culture; D003625:Data Collection / Q000379:methods*; D005060:Europe; D005260:Female; D006801:Humans; D038622:Internationality; D008297:Male; D008875:Middle Aged; D009043:Motor Activity / Q000502:physiology*; D011795:Surveys and Questionnaires