I would think whether you choose two methods of raw data collection (questionnaire and interview) or one (questionnaire with open ended responses) you would still want to triangulate your data-by comparing data back to either the existing theory in the field or previous existing empirical research done in the field. Unless of course you were collecting masses and masses of data-such as conducting many many interviews and surveys, so much so that your data is readily quantifiable and replicable- meaning that it stands alone. Hope this makes sense. If my words seem a bit wrong, it has just been a very long week!!!