Calculating sample size — how many responses are enough?
The most common question before every survey: "how many responses do I need?" Short answer: significantly fewer than most think. With a population of 100,000 people, 384 responses suffice for a 95-percent confidence interval at ±5 percent accuracy — at 1 million people this number barely changes.
The formula is manageable: n = (z²·p·(1-p)) / e². With z=1.96 (95% confidence), p=0.5 (worst case) and e=0.05 (±5% tolerance), the result is around 384. For more accuracy (±3%) you need around 1067. For internal trends, 100 responses are often enough — perfect for quick decisions.
More important than the raw number is sample quality. 384 responses from random newsletter subscribers carry less weight than 100 responses from a stratified sample by region, age and customer-since date. A live confidence interval in the dashboard helps interpretation — when the confidence width stabilizes, the survey is statistically robust even without hitting the threshold.