Pick the formula that matches your population
Cochran is the general formula for a large or unknown population and takes your expected proportion into account. Yamane and Slovin are simplified formulas for a known finite population at a fixed confidence level.
The calculator runs all three side by side so you can see how much your choice of formula actually changes the number — often less than researchers fear.
Apply the finite population correction
When your population is small — one faculty, one estate, one company — the uncorrected Cochran figure overstates what you need. The correction pulls the number down, sometimes sharply.
Report both figures in your methodology so the reader sees your reasoning.
Add for attrition, not for comfort
Field work loses respondents. Spoiled questionnaires, unreachable participants and incomplete responses are normal. Add a stated attrition allowance and say what rate you assumed and why.
Padding without justification looks arbitrary; a stated 10–20% allowance based on comparable studies does not.
Sample size is not the same as power
A precision-based sample size answers 'how tight is my estimate'. A power analysis answers 'can I detect an effect of this size'. If your study tests a hypothesis rather than estimates a proportion, read the power notes on the calculator page and report the effect size you designed for.