A simple one question Yes/No survey was sent to about 250 people from a population of 481. 135 responses received showed about 80% “Yes” and 20% “No”. How do I calculate my 95% and 99% confidence level ranges? I ended up with roughly 70% to 90% being the range but have been challenged on its validity. A couple of online calculators gave similar results as mine. The sample was based on those who had provided an email address for general communication, so reasonably random.