ABSTRACT
A depiction of a non-linear relationship that is either U-shaped, or the inverse, ∩-shaped. Such a curve might be best described with the inclusion of a quadratic term in a curvilinear regression model. Figure U.1a shows the U-shaped relationship between baseline haemoglobin levels and hazard ratio for incident Alzheimer disease (AD), and Figure 1b the reverse U-shaped relationship between baseline haemoglobin levels and rate of annual cognitive decline, among a group of older community-dwelling individuals around the Chicago, Illinois area (SHAH et al., 2011). Study participants were eligible for inclusion if there were no signs of dementia at the visit associated with the baseline haemoglobin measurement. Average length of follow-up for the 881 individuals included was 3.3 years. The curves show that higher and lower levels of haemoglobin were associated with higher risk of AD. See also J-shaped curve, exponential curve, S-shaped or sigmoid curve, bathtub curve.
