PONE-D-19-15672

The Prevalence of and Demographic Factors Associated with Radiographic Knee Osteoarthritis in Korean Adults Aged ≥ 50 years: The 2010–2013 Korea National Health and Nutrition Examination Survey

PLOS ONE

Dear Dr. Kim,

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Reviewer #1: No

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: No

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Reviewer #1: Yes

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Reviewer #1: No

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5. Review Comments to the Author

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Reviewer #1: Please see my Major comments below:

1. This reader does not agree with the authors’ statement “Evidence for the sociodemographic factors affecting radiographic knee OA, other than traditional factors such as age, female sex, and obesity, is limited.”

a. First, this statement is vague in providing the rationale for this current study.

b. Second, many of the factors in the current analysis have been examined previously in NHANES in the US, as well as studies in China, and other countries worldwide. For example, factors such as income, education, occupation, geographical area of residence in addition to the more “traditional” factors such as sex, age, and BMI have been examined as evidenced by the authors citing of previous work in their Discussion section. In fact, many factors were also examined in the 2009 Korean NHANES by Lee KM et al. Yonsei Med J 56(1):124-131, 2015.

c. Thus, the novelty of and rationale for this study need to be better defined.

2. Methods section is not clearly written with respect to statistical analysis and variables of interest.

a. First, it is unclear the methods used to account for sampling weights?

b. Second, please clarify the statements “To compare the weighted prevalence of radiographic knee OA by sociodemographic factors, chi-squared tests and analysis of covariance were performed. The prevalence of radiographic knee OA was analyzed using age, sex, area of residence, education level, number of family members, household income, occupation, and obesity [body mass index (BMI) ��25 kg/m2] as confounding variables.”

I may be mistaken, but my understanding of analysis of covariance (ANCOVA) is that it is used to test the main and possible interaction effects of categorical variables on a continuous dependent variable, controlling for other continuous variables. It is unclear what this main dependent variable is? Was Radiographic OA as measured with KL grade used as a continuous/ordinal variable in order to justify use of ANCOVA? If so, this cannot be a measure of prevalence? Therefore, it is unclear how prevalence of radiographic knee OA (which I am assuming is a proportion) was defined. If proportion of those with KL >=2 vs. those <2; or >=3 vs those <3; or those =4 vs. <4, was defined and what analyses were used to adjust for “confounders” in the prevalence of ROA?

c. My understanding of a confounding variable is that it is an “extraneous” risk factor for ROA other than the exposure of interest and needed to be controlled for in the analysis. We usually control for confounding in relation to an exposure of interest. It seems to me that age, sex, area of residence, education, # family members, household income, occupation, BMI >= 25, are not necessarily confounding variables in this research project, and that they are used more like predictors of, or covariates in a prediction model with ROA (defined one of 3 ways: as KL >=2 vs. <2, >=3 vs. <3, or =4 vs. <4) as the outcome?

3. Methods and Tables should stand alone? In other words, a reader should not have to study the tables to understand how variables of interest were defined in a study. For example, I believe that categories of age, education, income, occupation, etc.. should be defined in the Methods section. Also, how was rural vs. urban defined?

4. Results section:

a. Please clarify what “irrespective of severity” means.

b. As for the Tables such as Table 3, what does “other variables” mean? Is Table 4 based on multivariable logistic regression? If so, need to mention these variables are mutually adjusted? is used in the models. Is it used as a continuous variable or as categorical variable? How was rural vs. urban defined?

c. Given separate models for different definitions of ROA and separate models where different sets of covariates were used, it would be clearer to understand what the authors mean by prevalence(s) of radiographic knee OA or “after adjustment” in the Results.

d. Since prevalence and incidence of OA are very different in men compared with women, and factors associated with OA may be very different in the two groups, we tend to stratify by men and women in analyses? Why did the authors NOT conduct the analyses stratified by men and women? Would these factors, whether sociodemographic or lifestyle factors differ in men and women, including that of occupation?

5. In the ABSTRACT, the last sentence in the Results section related to the use of Logistic regression analyses should be under the Methods section? Again, because the authors mentioned that ROA was defined as KL >=2 vs. < 2, then several times mentioned “regardless of OA severity”—this was a bit confusing because data for “severity” were not presented in abstract, as the Abstract is a stand-alone document I read before I read the results in the main paper.

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Reviewer #1: No

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The Prevalence of and Demographic Factors Associated with Radiographic Knee Osteoarthritis in Korean Adults Aged ≥ 50 years: The 2010–2013 Korea National Health and Nutrition Examination Survey

PONE-D-19-15672R1

Dear Dr. Kim,

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With kind regards,

Young Dae Kwon, M.D., Ph.D.

Academic Editor

PLOS ONE