Letter to the editor: Phosphorus as predictive factor for erectile dysfunction in middle aged men: A cross sectional study in Korea; Methodological issues to avoid prediction fallacy

To the editor: We read the paper written by Min et al. [1] and published in Investigative and Clinical Urology in 2016. The authors aimed to study the effect of serum phosphorus on erectile dysfunction (ED) and the relationship with other clinical variables. It was concluded that phosphorus is a significant predictor of ED and a strong factor that can be modified in middle age. However, although this was a valuable investigation and its findings were very interesting, some methodological issues should be considered. First, Min et al. [1] evaluated the predictive performance of serum phosphorus on ED in a cross-sectional study, whereas longitudinal studies are most important for making assumptions for clinical prediction models [2]. In other words, the temporality assumption (the dependent variable has to occur after the independent variable) must be ensured in the prediction model. Thus, prediction models resulting from cross-sectional designs can be misleading [2,3]. Second, considering the predictive performance of serum phosphorus on ED to be significant is an optimistic interpretation. The internal and external validation of the Letter to the editor: Phosphorus as predictive factor for erectile dysfunction in middle aged men: A cross sectional study in Korea; Methodological issues to avoid prediction fallacy