I would like to include two covariates that are Snow and Salt for deterioration modeling of bridges. I generated the Cumulative Martingale residual for the two and the shapes are attached. I Tried several other functions such as log, square root, power two and three, but it looks that none worked so far. I would be grateful if anyone could give me a hint on how to proceed with this. [. .
The histogram of the data looks like a bell curve as shown below. The vertical access in the histograms is a percentage and there are about 4000 bridges in the data set. about 90% of the bridges are failed - not in a physical way but some defined performance criteria. The loess fit for snow and salt is shown below.