Figure 3From: 25-hydroxyvitamin D deficiency, exacerbation frequency and human rhinovirus exacerbations in chronic obstructive pulmonary disease Adjusted partial residual plot of the relationship between 25-hydroxyvitamin D levels and (1) day length (hours), and (2) hours outdoors. The graphs show the effect of each variable on 25-hydroxyvitamin D independently of the other variable. The zero value is the annual average.Back to article page