1040-5488/15/9201-e27/0 VOL. 92, NO. 1, P. e27 OPTOMETRY AND VISION SCIENCE Copyright * 2014 American Academy of Optometry

CORRESPONDENCE

Letter to the Editor: Reliability of Muscle Blood Flow Measurements in Orbicularis Oculi I was interested to read the article by Thorud et al.1 published in the September 2014 issue of Optometry and Vision Science. The authors aimed to test reproducibility and stability of muscle blood flow recordings in orbicularis oculi using photoplethysmography.1 As the authors pointed out, 12 subjects were tested twice within 1 to 5 weeks. The short-term repeatability of muscle blood flow measurements was considered good, but they found a greater within-subject maximum variability compared with betweensubject average variability.1 Why didn’t the authors use the wellknown intraclass correlation coefficient or weighted kappa for reliability analysis?2Y6 Regarding reliability or agreement, it is important to consider an individual-based instead of group-based approach using appropriate tests.2Y6 Having either less or greater average variability does not mean good or poor agreement. The reason is that in reliability assessment, we should consider individual results and not the global average. That is why we should report intraclasscorrelation coefficient single measures instead of average measures to correctly assess the reliability. In other words, the possibility of getting exactly the same average of a variable between or within subjects (interobserver or intraobserver) with no reliability at all is really high.2,3 As the authors pointed out in their conclusion, investigators should be aware of the effect of time, possibly attributed to confounding factors such as environmental changes and mental stress, when comparing photoplethysmography muscle blood flow recordings. Such a conclusion can be a misleading message entirely owing to an inappropriate use of statistical tests to evaluate reliability and also misinterpretation of the finding.2Y6 Siamak Sabour, MD, PhD Tehran, Iran

REFERENCES 1. Thorud HM, Helland M, Aaras A, Kvikstad TM, Lindberg LG, Horgen G. Reliability of muscle blood flow measurements in orbicularis oculi. Optom Vis Sci 2014;91:215Y21. 2. Jeckel JF, Katz DL, Elmore JG, Wild DM. Epidemiology, Biostatistics and Preventive Medicine, 3rd ed. Philadelphia, PA: Saunders/Elsevier; 2007. 3. Rothman KJ, Greenland S, Lash TL. Modern Epidemiology, 4th ed. Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2010. 4. Sabour S, Ghassemi F. Accuracy, validity, and reliability of the infrared optical head tracker (IOHT). Invest Ophthalmol Vis Sci 2012;53:4776. 5. Sabour S, Ghassemi F. Validity of selfreported eye disease and treatment: is that correct?! Br J Ophthalmol 2012;96:1359. 6. Sabour S, Ghassemi F. Reliability and validity of conjunctival ultraviolet autofluorescence measurement. Br J Ophthalmol 2012;96:1271.

calculated as the ratio between twice the SD of the difference of the two measurements in each test person and the total mean, and stability was calculated as the ratio between the SD of the difference between the maximum and the minimum value for each subject in the six test runs (SDmm) and the SD of the measurements of the six subjects at each visit (SDv).3 One should note that we used a strict stability test, something that maybe should have been pointed out in the conclusion, not only in the discussion part of the article. Finally, we consider the statistical tests used for testing reproducibility and stability of muscle blood flow recordings over time appropriate.4,5 Hanne-Mari Schiøtz Thorud, PhD Magne Helland, MSc Arne Aara˚s, MD Tor Martin Kvikstad, MSc Kongsberg, Norway Lars Go¨ran Lindberg, PhD Linko¨ping, Sweden Gunnar Horgen, PhD Kongsberg, Norway

Authors’ Response We thank Dr. Sabour for his interest in our article ‘‘Reliability of Muscle Blood Flow Measurements in Orbicularis Oculi.’’1 He claims that we used inappropriate statistical tests to evaluate reliability. We think that most of this criticism relies on our somewhat wrong use of the expression reliability in the title of the article. Reliability may be defined as precision2 and is not entirely equal to what we tested in our study, namely, reproducibility and stability of muscle blood flow recordings over time. Dr. Sabour further claims that our statistical tests had a group-based instead of a more appropriate individual-based approach. Because of the time aspect in the study (measuring increasing muscle blood flow over a 10-minute period), we choose to show group data for each minute of recording, instead of, for example, BlandAltman charts with averaged blood flow values for each test person. However, we claim that our data had an individual-based approach because the agreement index was

REFERENCES 1. Thorud HM, Helland M, Aara˚ s A, Kvikstad TM, Lindberg LG, Horgen G. Reliability of muscle blood flow measurements in orbicularis oculi. Optom Vis Sci 2014;91:e215Y21. 2. Sabour S, Ghassemi F. Accuracy, validity, and reliability of the infrared optical head tracker (IOHT). Invest Ophthalmol Vis Sci 2012;53:4776. 3. Aara˚s A, Veierod MB, Larsen S, Ortengren R, Ro O. Reproducibility and stability of normalized EMG measurements on musculus trapezius. Ergonomics 1996;39:171Y85. 4. McAlinden C, Khadka J, Pesudovs K. Statistical methods for conducting agreement (comparison of clinical tests) and precision (repeatability or reproducibility) studies in optometry and ophthalmology. Ophthalmic Physiol Opt 2011;31:330Y8. 5. Weir JP. Quantifying test-retest reliability using the intraclass correlation coefficient and the SEM. J Strength Cond Res 2005; 19:231Y40.

Optometry and Vision Science, Vol. 92, No. 1, January 2015

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Letter to the editor: Reliability of muscle blood flow measurements in orbicularis oculi.

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