Before you choose the method that you’ll use to collect data, you need to think about the size of population you are studying (e.g. the group of visitors to your museum or online exhibition), the characteristics of that population and the amount of data (the sample) that will be representative enough so you can report what you learn with confidence.
...
1. Define the population.
Population = the whole group at the centre of the research question.
Who are you trying to learn from? This is your population. You’ve already done this in Phase one  you should know who your key stakeholder(s) are. Ask yourself: can  or should  you survey the whole population?
...
3. Agree your confidence interval and confidence level.
Looking back at external validity above, it’s important now to think about your confidence intervalConfidence intervals, confidence levels and margins of error: how do we use them?
Primarily quantitative research (numbers)
Share your perceptions about the representativeness of the sample and robustness of the findings
Help others to interpret the data
Guide you to use a tool to determine the sample for your research
Confidence means probability. If you were to repeat the same study, what scale of difference would there be if you compared the results? For example, would they be within five above or five below? Ten? This is your confidence interval. Five below would indicate a 95% confidence level. Ten below would indicate a 10% 90% confidence level.
What level of risk or uncertainty am I willing to accept?
You should ask yourself what confidence interval you would expect and/or find acceptable. You can use this both to report on your findings and, even more importantly, plan your sampling approach. If you are happy that the results are not easily generalisable, then you can proceed with a lower confidence level. 95% is a fairly common and accepted confidence level. E.g. you are 95% sure that if you repeated the survey, the results would lie within these parameters.
Higher confidence interval demands a bigger sample. Resource limitations and some degree of uncertainty means that a lower confidence level may be appropriate.
What is a margin of error?
Helps you ascertain the sample size
Looks similar to the confidence level but different
Expressed as
⧮ (or similar)
Percentage (e.g. 9%)
± 9
4. Calculate your sample.
Sample = a small part of the whole population intended to show what the group is like or experiences
Sampling = gathering information or data from a subset of a larger population, rather than from everyone
You now need to work out how you will sample the (target) population/stakeholder. This is based on an understanding that you can’t hear from everyone, so you have to try to get a sample that is representative enough so that you can report confidently on what you have learned. How can the sample be representative of the whole population?
...
Panel  

 
Sample sizes in qualitative research Qualitative methods like interviews result are likely to result in rich, qualitative data. Numbers of responses, or people that you interview, matter less in this context. For a small population size, you might interview everyone involved. This will give you a very complete insight into the perspectives of the population. For a big population size, it’s unlikely that you will have time or the money to interview 10% of the population. Rather, you should aim for a smaller sample that is representative of the overall group. If your overall group is very diverse, think about whose perspectives you need most (e.g. educators) and how many you would need to interview to get their perspectives. You should also think about how many people it would take to interview before you start seeing the same patterns or trends  this is called saturation. After this point, there is less value in interviewing more people because you do not learn anything new. The sample therefore depends on how homogenous (how much it is the same) or heterogenous (how much it is different) your population is. If you have a very heterogenous or diverse population, you need to interview more people. If your population is homogenous, you might need to interview a small group. Therefore there is no fixed right or wrong with interview sample sizes, though some sources suggest that between 10  30 interviews are usually sufficient. The sample will also depend of course on how much time you have available to conduct interviews. You have to be realistic and consider what you want to know and what you can do to get this perspective. If you need more data than you think you can get through interviews, consider combining them with another method, like a survey. Tips:

Tip.
LookPanel  

 
Tips:
Such tools ask you to consider:
The calculator then works out your margin of error, which you should ideally report with your findings. Based on observation in the nonacademic cultural sector, such margins of error are rarely reported. Examples 
...
Expand  

 

Expand  

 

...