One of the common questions we get here at PublicVoice is “how many people do I need to send my survey to?”. Sampling can be a complex business and there isn’t a one size fits all answer. The main thing is to ensure that your sample is big enough to accurately represent your population (the people you are trying to understand as a result of doing the survey).
Here are some key things to think about when putting together your sample:
What is your population?
This is the amount of people that fit into the demographic that you are interested. For example, if you are trying to work out how popular your product or service is in Auckland then your population would be all residents of Auckland.
Don’t worry if you don’t know the exact number; it is common for populations to be estimated.
How accurate do you want to be?
There are two things to consider when deciding on the accuracy of your sampling – Margin of Error and Confidence Level.
A commonly used margin of error is 5%. So, for example, if 90% of survey respondents said they understood the new Council plan, a 5% margin of error adds 5% onto each side of your figure. Therefore you can be confident that between 85 and 95 percent of people understood the Council plan.
Your confidence level is essentially ‘how sure’ you can be. It is expressed as a percentage and represents how often the true percentage of the population who would pick an answer lies within your results.
What do you think the response rate will be?
Unfortunately, not everyone will want to complete your survey no matter how hard you try and no matter how many times they are reminded. Lots of things can increase your response rate, including an interesting survey topic, straightforward questionnaire, respondent incentives and building up a relationship before surveying begins. However, even with the most enticing survey imaginable, a response rate of 20-30% can be considered a success.
The good news is that PublicVoice is here to help. Our well established Citizens’ Panel is a great way of getting the sample that you need quickly and efficiently. Get in touch if you’d like to know more about how we can be of assistance.
In the below table are some guidelines to help you estimate the number of completed survey responses you will need.
| Population | Margin of Error | Confidence Level | ||||
|---|---|---|---|---|---|---|
| 10% | 5% | 1% | 90% | 95% | 99% | |
| 100 | 50 | 80 | 99 | 74 | 80 | 88 |
| 500 | 81 | 218 | 476 | 176 | 218 | 286 |
| 1,000 | 88 | 278 | 906 | 215 | 278 | 400 |
| 10,000 | 96 | 370 | 4,900 | 264 | 370 | 623 |
| 100,000 | 96 | 383 | 8,763 | 270 | 383 | 660 |
| 1,000,000+ | 97 | 384 | 9,513 | 271 | 384 | 664 |