If we are in a situation where we are unable to collect the data in the first place and you propose that theories or opinions are better, from what place did these putatively superior data or opinions come from? Because they clearly aren't coming from data.
Leveraging the experience of many? I wan't my website to be fast. I don't have deep understanding on web optimization and don't have yet enough data or time to build my own solution. I search around in the web, and go to http://developer.yahoo.com/performance/rules.html. I trust them (yahoo) because them are probably more competent and have MORE DATA than me. I implement some of the ideas, get some results (and if I'm smart, get a clue on why are good ideas and how it affect the performance).
Acting in data IMHO is only if have LOT of data. If not, acting in expertise of others look best to me..
If we are in a situation where we are unable to collect the data in the first place
that's mischaracterising my point. I didn't say anything about having no data. I talked about when the data you're able to get is unrepresentative.
from what place did these putatively superior data or opinions come from? Because they clearly aren't coming from data.
i think that's just playing word games with "data". We're talking about data concerning the problem at hand. Theories may come from data, but they don't come from data you obtain about the problem at hand. And knowledge can come from experience, which again is a form of data, but clearly not the sort the person was talking about. You can reason based on knowledge and principles.
Sure, it'd be better if you had good data, but when you can't have that sometimes the best you can do is to reason based on knowledge/experience/theoretical ideas.
This is not true.
It depends on how much data you can get and how representative it is.
A big mistake is to assume that an unrepresentative sample of data is representative, and to draw conclusions based on that assumption.
If you do not have a representative set of data (and you can't always get one) then data can be less-than "theory or opinions".