While composing a song, do songwriters write the lyrics first, or the music?

For several months I conducted a poll on the site to find out how songwriters start the songwriting process. This graph shows the results. What I learned from this is that most songwriters start with lyrics first. Followed by writing a chord progression. In third place was starting with a melody.

Here are the poll results. http://learnhowtowritesongs.com/popular-ways-start-songs/