I see many people prefer one product or service over another in same category but when we ask them what did you like or dislike in those products they don't really have any answer.
They say "I don't know why and I know product A is similar to B but I prefer product B, even I don't know what reason I could give."
So in this condition what we can do to know what they did like or dislike anything in a product or service?
There are questionnaires created especially to measure and compare the user experience of different products. Off the top of my head I know two, but I would guess there are more.
AttrakDif
The AttrakDiffTM questionnaire by Hassenzahl et al. (2003), developed together with User Interface Design GmbH, measures subjective assessments concerning pragmatic and hedonic qualities and the attractiveness of interactive products.
The questionnaire has the format of a semantic differential and uses 28 seven-step items on 4 dimensions which poles are opposite adjectives (e.g. complicated - simple, cheap - premium, conservative - innovative, ugly - attractive).
UEQ - User Experience Questionaire
The User Experience Questionnaire (UEQ) allows a quick assessment of the user experience of interactive products. The format of the questionnaire supports users to immediately express feelings, impressions, and attitudes that arise when they use a product.
Source: http://www.ueq-online.org/index.php/what-is-the-user-experience-questionnaire/
I think these questionaires could be especially helpful to get a better idea of why a specific product is liked more than another one.
The scientific method can be very helpful here. Speculate on what feature(s) people care most about (a hypothesis). Present a choice between products / services that have and don't have those features (an experiment).
If you're creative enough you can come up with a large enough list of what it might be that you feel overwhelmed trying to present them all. In that case do a binary search. For example, you can find the most important feature of 8 features using only 6 products (A-F) and 3 trials:
A(1,2,3,4) > B(5,6,7,8)
C(1,2) < D(3,4)
E(3) > F(4)
Turns out people really like feature 3 even if they can't explain why.
If you're wondering how you present a product with only one feature realize what this really means is that there is only one feature of the compared products that is different.
Users actions speak louder than their words. Have users try both products, tell them to think out loud, but most importantly, look at what they do and how they react to the interface/interaction.
You don't need them to tell you why they like product B more than A, find it out yourself. Read more here.
