Sample Size Calculator


Before using the sample size calculator, you need to understand confidence interval and confidence level. If you are not familiar with these terms, click here.  To learn more about the factors that affect the size of confidence intervals, click here.  If the population is large (more than 50,000), you can leave the population cell blank.

This calculator requires Internet Explorer 3.0 or later or Netscape 3.0 or later or a compatible browser. Leave the population box blank, if the population is very large or unknown.

Determine Sample Size

Confidence Level: 95% 99%
Confidence Interval:
Sample size needed:

Find Confidence Interval

Confidence Level: 95% 99%
Sample Size:
Confidence Interval:


Real Life Example

Question   If you carry out a survey, on many pilots on one fleet who have completed a manual load sheet. Over what length of time would you need to collect the data to make this an accurate survey - or to put it another way, how many loadsheets do we need to collect? 

What would be the % error in the results? 

Could you then apply that same error of probability to another fleet in your airline? Would it be accurate to say for example to say that if we found a 20% error in loadsheets with one fleet, then the same could be applied to another fleet?

1. We want to calculate the number of loadsheets to collect - this is the "sample size". Imagine that the pilots are filling in a few loadsheets a day each, and there are say 100 pilots. then the number collected is {100 * a few * 365 = many} load sheets per year. So we can use 'large population' statistics. [Hint: so we leave the population cell of the calculator blank since the population is so large]
2. Using the top calculator [to determine sample size], enter the population and confidence level (most statisticians use 95%), and the confidence interval (accuracy level) - let's use 5 [i.e. plus or minus 5 percentage points]. Press the calculate button, and the sample size (which is the number of loadsheets needed to be checked) is  . . . . .wait for it  . . . .punch in the numbers . . . .  384.  So for survey of 384 load sheets, you can then say that you are "95% sure" that the "true percentage of inaccurate load sheets is plus or minus 5 percentage points. If you reckon that 20% of your load sheets are inaccurate for whatever reason, then you are saying that you are "95% sure" that the " true percentage of inaccurate load sheets is between 15-25% [i.e. 20%-5% and 20%+5%]
3. For an accuracy level of plus or minus 3 percentage points [which would be 17-23% for the example above], the number of load sheets needed is 1067 [try the calculation for yourself]. Take that many load sheets to work out the inaccuracies, and you can then say that you are 95% sure that the true percentage of inaccurate load sheets is between 17% and 23%, and that you're bloody sick of looking at loadsheets. 
4. It would only be accurate to transfer the results if the other aircraft had very similar loadsheets and computations and pilots. For example, I guess that a Navajo Chieftain and a Cessna 402
would be fairly similar.

Sample Size Terminology





Sample Size calculator | Confidence Level calculator

Factors that Affect Confidence Intervals


Sample Size





Population Size

