Williams performance chief Rob Smedley says the team is encouraged by the "significantly different" car it plans to unveil for 2016.
Williams finished third in the constructors' championship for the second year in a row but failed to close the gap to Mercedes in front, something Ferrari managed to do after leapfrogging the Grove outfit into second. Despite another strong season the team only visited the podium four times and saw its performance tail off in the final races.
Smedley explains that drop-off in form, which saw them fighting with Red Bull and Force India in the final races, came from the team switching its focus to 2016.
"The end of the season has been slightly lacklustre compared to last year but last year we developed the car right up until the very last moment," he said. "This year, having been more comfortable and consolidated in third, our thoughts switched to 2016 and 2017."
He also says the radical difference between the team's 2015 and 2016 cars prevented Williams from introducing any upgrades at the final races of the season.
"The 2016 car we have in the wind tunnel and the 2015 car are significantly different, so there's nothing we could do from an aerodynamic point of view. Other parts of R&D and the mechanical design we are trialling things all the time.
"We think that everything we are seeing at the minute is good. We're hitting targets. We are always looking, not just at numbers, but in other areas aerodynamic development as well, other significant areas. It's all ongoing, it's a process of getting us back towards the front. At the moment we are reasonably happy with what we are seeing.
"Of course, it's all relative. We could have a stellar development and Ferrari and Red Bull could have an even more stellar one."