The answer is “enough to represent your market.” This number varies significantly according to the type of market research you’re conducting.
For Web surveys this is a pretty straightforward question to answer. You will use quantitative methods to determine a sample size.
There are standard ways to calculate statistical validity, and a very easy-to-use calculator and descrition of the underlying statistics can be found here. You can use it to determine how many people you need to respond to a quantitative study in order to get results that reflect your target population.
You will need to know the population size of your audience – the total number of people in the group your sample represents. Even if you don’t know the exact population size, this is not really a problem. The mathematics of probability allows you to make a pretty good guess as long as you have some kind of basic idea. The number of participants needed is not linear. As your audience gets much larger, your sample size doesn’t increase very much.
For example, to get a result that is accurate 95% of the time within 5%, you need:
Qualitative Studies – In-depth Interviews and Focus Groups
But enough statistics homework for now. 🙂 The question that is less straightforward to answer, and so is asked all the time: “how many participants do I need for a qualitative study?” such as focus groups or a series of in-depth interviews.
The first thing to do when answering this question is to figure out segmentation. How many types of participants do you need to represent? This can include verticals, countries, roles, years of experience, customers vs. prospects vs. competitor customers vs. partners, and so on. The most common segmentation when working with Corporate IT is to have two groups: “technology decision-makers” and “economic buyers”.
Once you figure this out, and map any overlap between these areas (for example, partners may also be end-users), you’re ready to go.
In my experience, with corporate IT you need about 8-10 participants of each “type,” with a minimum of 10 participants, to produce a valid study. The only exception is studies with competitor’s customers, where you typically need more participants.
Note that most market research companies, including Dimensional Research, base pricing for qualitative work on the number of participants, so doing a good job right-sizing your project will give you the most bang for your buck.