I have data that describe the number of days that a car should be tested before using it. There are different predictors which measure the complexity of the car (more complexity, more days will be need).
The main goal of my multiple regression model is to predict the number of days for new data.
I used a robust regression model but I think there is one thing I forgot to consider here:
The data are sampled over the last 3 years. I guess that there could be a time trend in terms of efficiency (ususally every year one tries to reduce the costs of testing and a car with same complexity would get in 2015 only 90 percent of test days as in 2014).
How can I capture the time trend?