A written questionnaire delivered to respondents by mail. Respondents have to answer the questionnaire and send it back, thanks to a pre-paid return envelope. A letter of Introduction is attached to it along with the instructions. An incentive can be added to get a better response rate.
Also see: Segmentation Studies
A survey that is mobile-friendly, ready to be taken on a smartphone device. It helps to reach a larger sample size as not everyone is sitting behind a laptop and people are more accessible through their smartphones.