I am a biologist that is using GDP data as part of my next research project and I am finding large gaps in the data, both in terms of years and countries. I am wondering what the best practice is for compiling and comparing GDP? In particular how wrong would it be to take a span of 5 years and average across countries with multiple years of data, while for other countries only using single year of data? I am interested in being able to compare multiple countries and less interested in change over time. Any ideas or literature recommendations would be greatly appreciated.