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Low 10-year reproducibility of glycaemic index and glycaemic load in a prospective cohort study: methodological and statistical issue to avoid misinterpretation

Published online by Cambridge University Press:  27 December 2018

Sohrab Iranpour  and
Siamak Sabour Open the ORCID record for Siamak Sabour [Opens in a new window]
Sohrab Iranpour
Affiliation:
Department of Community Medicine, School of Medicine, Ardabil University of Medical Sciences, Ardabil, 56189-85991, Iran Social Determinants of Health Research Center, Ardabil University of Medical Sciences, Ardabil, 56189-85991, Iran
Siamak Sabour*
Affiliation:
Department of Clinical Epidemiology, School of Health and Safety, Shahid Beheshti University of Medical Sciences, Tehran, 5511-198353, Iran Safety Promotions and Injury Prevention Research Centre, Shahid Beheshti University of Medical Sciences, Tehran, 19839-63113, Iran
*
email [email protected]
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Abstract

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Type
Letter to the Editor
Information
British Journal of Nutrition , Volume 121 , Issue 7 , 14 April 2019 , pp. 839 - 840
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Copyright
© The Authors 2018 

We were interested to read the paper by Mullie et al.( Reference Mullie, Deforche and Mertens 1 ) published in July 2018 issue of British Journal of Nutrition. The aim of the authors was to investigate the reproducibility of glycaemic index (GI) and glycaemic load (GL) using data obtained from a prospective cohort study with two nutritional assessments in 2002 and in 2012. For GI and GL, the reproducibility and cross-classification of quintiles between the two measurement periods were tested with Pearson’s correlation coefficient( Reference Mullie, Deforche and Mertens 1 ).

It is important to bear in mind that in all fields of research, validity (accuracy) and reliability (precision) are two important methodological issues that are completely different, and these issues are assessed using appropriate tests( Reference Sabour 2 ).

Authors investigated the association of dietary GI and dietary GL in 2012 with dietary GL and dietary GI in 2002, respectively( Reference Mullie, Deforche and Mertens 1 ). It refers to validity and has nothing to do with reliability( Reference Bland and Altman 3 ). Also, authors investigated the association of dietary GI and dietary GL in 2012 with dietary GI and dietary GL in 2002, respectively (that refers to reproducibility) using Pearson’s correlation coefficient( Reference Mullie, Deforche and Mertens 1 ). The Pearson correlation coefficients between 2002 and 2012 were 0·27 for GI and 0·41 for GL. For quantitative variables, applying Pearson’s correlation and paired t test are among common mistakes in reliability (reproducibility) analysis( Reference Sabour 2 ).

Pearson’s correlation assumes that the relationships between variables are linear. So, it just measures linearity but fails to detect non-reliability (departure from the 45° line). Applying Pearson’s correlation, high correlation coefficients are possible with far from reproducibility (reliability)( Reference Sabour 2 ). Therefore, Pearson correlation coefficient is not an appropriate statistical test for reproducibility analysis. Briefly, weighted κ are good estimates for quantitative variables, intra-class correlation coefficient or Bland–Altman plot, and for qualitative variables with more than two categories( Reference Sabour 2 – Reference Sabour 6 ).

The authors mentioned that low 10-year reproducibility should be taken into account in prospective cohort studies with only one nutritional assessment at baseline.

In terms of reproducibility (reliability), such a sweeping conclusion may provide a misleading message. In conclusion, for reliability analysis, appropriate tests as well as correct interpretation should be applied. Otherwise, misinterpretation cannot be avoided.

Acknowledgements

This research received no specific grant from any funding agency, commercial or not-for-profit sectors.

S. I. wrote the paper. S. I. and S. S. share responsibility for final content.

There are no conflicts of interest.

References

1. Mullie, P, Deforche, B, Mertens, E, et al. (2018) Low 10-year reproducibility of glycaemic index and glycaemic load in a prospective cohort study. Br J Nutr 120, 227–230.Google Scholar
2. Sabour, S (2016) Reliability of immunocytochemistry and fluorescence in situ hybridization on fine‐needle aspiration cytology samples of breast cancers: methodological issues. Diagn Cytopathol 44, 1128–1129.Google Scholar
3. Bland, JM & Altman, DG (2010) Statistical methods for assessing agreement between two methods of clinical measurement. Int J Nurs Stud 47, 931–936.Google Scholar
4. Iranpour, S, Pordanjani, SR & Sabour, S (2018) Reliability of cartilage digestion and FDA-EB fluorescence staining for the detection of chondrocyte viability in osteochondral grafts; statistical issue to avoid misdiagnosis and misinterpretation. Cell Tissue Bank 19, 833–834.Google Scholar
5. Sabour, S & Dastjerdi, EV (2013) Reliability of four different computerized cephalometric analysis programs: a methodological error. Eur J Orthod 35, 848.Google Scholar
6. Sabour, S (2015) Reliability of automatic vibratory equipment for ultrasonic strain measurement of the median nerve: common mistake. Ultrasound Med Biol 41, 1119–1120.Google Scholar