I have had a similar discussion over this issue recently. Historically, data is the plural of datum and was expressed as 'data were collected'. However, due to hyper-correctness this has evolved over time to data being considered and expressed as a singular, i.e. 'data was collected'. The oxford dictionary suggests either use, for historical or recent correctness. I found an article a while ago in the guardian who concluded the latter use of data as a singular, so this is what I have done in my thesis. As long as you can justify your expression and, most importantly, use that consistency throughout, either can work.