Williams spent much of the second half of the season focusing on a "significantly different" 2016 car, which head of vehicle performance Rob Smedley says explains the team's dip in form at the final two races.
Williams secured third in the constructors' championship with one race remaining this year, but was comfortably clear of Red Bull in fourth for most of the season. Smedley said his team took the decision to switch its focus to 2016 as early as possible, meaning the 2015 car lacked development while the 2016 project took a new direction.
"After projecting forward that we would have a comfortable third in the championship and second place wasn't going to be possible, we halted development on this car quite a long time ago," Smedley said. "When we were developing the car, as I'm sure you are aware, the correlation with the wind tunnel was very good and when we were putting development on the car then that transpired in lap time.
"The end of the season has been slightly lacklustre compared to last year, but last year we developed the car right to the very last moment. This year, having consolidated and been comfortable in third, our focus had to switch to 2016 and 2017."
Smedley said the team saw no benefit in testing 2016 wings and bodywork on the 2015 car but did test other parts.
"The car we have in the wind tunnel for 2016 and our end-of-season 2015 car are significantly different. From an aerodynamic point of view we couldn't test any 2016 parts on the 2015 car, but other parts from R&D and mechanical design we have tried. But usually you need a package of upgrades to see any significant lap time improved, so all of our significant lap time improvement at the end of the year was back in the wind tunnel in Grove."
Asked if he was confident Williams will move up the grid in 2016, Smedley added: "It's all relative, isn't it? We could have a stellar winter of development and Ferrari and Red Bull could have an even more stellar one.
"Everything that we are seeing at the moment is good and we are hitting targets and always looking not just at headline numbers but also at very significant areas of aerodynamic development as well. It's an ongoing process of getting us back to the front and at the minute we are reasonably happy with what we are seeing."