I applied through college or university. The process took 1 day. I interviewed at Kantar (Bengaluru) in Dec 2011
Interview
IQ test and 2 interviews (HR and Business Head). First an IQ test is conducted comprising of logical questions. After that there is an interview with the HR and Business Head (for candidates who are selected based on the test). Post that an interview is conducted with the business head of the group to which the candidate is mapped. This could be a telephonic interview.
The process took 2 days. I interviewed at Kantar (Bengaluru) in Jan 2008
Interview
I had to give a quantitative + IQ test + case study then campus interview and subsequently telephonic interview with the Group business director. I was tested largely on my ability to think on new business ideas and proposition like feasibility of a new business at a particular location, application of core marketing concepts like the 4P's of marketing, logical reasoning questions like estimation of rice consumption in a city like Chennai... !!
You would require a blend of high level of quantitative aptitude and ability to think through the marketing problems in new and innovative ways :).