I am doing a Customer Discovery phase (as per Customer Development model by Steven G. Blank). In this step I need to have to identify the following:
I have launched a survey and have a few responses. Question is - how many responses are enough for concluding that this is a viable market? I understand it depends on the market, product and demographic size, but how do you guesstimate this if you don't have such data?
It depends on the product, so the best approach is to know the demand of the product, how many people use/buy the product. Once you get that number you will be able to estimate.
If you know that there is 1000 individuals having the "problem" and you get 100 surveys you will know what the 10% of these people think about the "problem".
Out of that 10% (100 individuals) you can get the average like: 1 out of 10 think that bla bla bla.
You must know the starting number (how many individuals have the problem). If the number is low your percentage of surveys will be high and vise versa, but there will be cases where you will need to have a minimum that works for you, is realistic, and doesn't affect or misrepresent the truth.
Individuals with the problem Surveys
1 to 1000 1 to 100 -> 100%
1000 to 10.000 100 to 1000 -> 10% This would be your minimum
Let's say 100.000 individuals have the "problem". It may not be viable for you to survey the 100.000 individuals (100%) so, you may try to survey 10% (10.000 individuals). But, still this may not be viable, so what to do in this case? Reform the survey so the number of individuals having the problem is more flexible to your capabilities or hire more people for the surveys :)
Review what makes a statistically valid sample and what is margin of error and possibly use a sample size calculator and adjust accordingly (tip: the number is likely smaller than you'd think).
More important is to back up statistical claims with some methodology vs winging it - you may chose the wrong metric, but at least there was some thought behind it.
If you are attempting to generate stats for funding, many times the VCs will set the bar at what they consider an "acceptable population" so all bets on what the "correct" number is off.