Or, I could use the original two years and study the top 40. Either way, more songs would give better results. Taking a look at years that are farther apart could be instrumental, as well. Try a forty-year time difference, and perhaps the trends would be more distinct. Looking at other genres of music could prove interesting as well.
Popular music is what the most people like to listen to at the same time.
Other types of music, and music that did not hit number one would undoubtedly give different research results.