Usability, Customer Experience & Statistics

Sample Size Calculator for a Completion Rate

Jeff Sauro • January 4, 2008

Use this calculator to understand how sample size changes will affect the confidence interval around a completion rate. For more context, see the Sample Size article.

Known Users: Most Likely Completion Rates


Known Minimum Completion Rate: Unknown Users


The calculations are derived from the Adjusted-Wald binomial confidence interval.

About Jeff Sauro

Jeff Sauro is the founding principal of MeasuringU, a company providing statistics and usability consulting to Fortune 1000 companies.
He is the author of over 20 journal articles and 5 books on statistics and the user-experience.
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Posted Comments

There are 10 Comments

October 22, 2010 | James Terrell wrote:

Very Useful.
I would also like to have Confidence as an input rather than restrict it to 95%.
Also, an article on the math behind the tables would be entertaining 

July 8, 2009 | Amir Raza wrote:

how we compute sample size for median(IQR) comparison between groups 

June 24, 2009 | Ofori Boateng wrote:

I am about looking to determine my sample size of how many managers I should select to from six companies for my survey so that my sample size and frame will be valid. Unfortunately I do not have the total number of managers that are working for these phone companies in Ghana and it is becoming increasing difficult for me to find the numbers. Is there a way for me to determine this unknown sample size. The tpye of managers I need for my survey are those in operations research, financial operations, and makerting department. Please try and help me here.

Thank you.

Ofori Boateng 

June 6, 2009 | Thomas Smith wrote:

) "Why is population shape of concern when estimating a mean? What does sample size have to do?" 

June 6, 2009 | Thomas Smith wrote:

The mean of a data sample, size n, forms an approximately normal distribution. ... Sample size n is about 30 to be sure that the central limit theorem applies. 

June 6, 2009 | Thomas Smith wrote:

In 1992, the FAA conducted 86,991 pre-employment drug tests on job applicants who were to be engaged in safety and security-related jobs, and found that 1,143 were positive. (a) Construct a 95 percent confidence interval for the population proportion of positive drug tests. (b) Why is the normality assumption not a problem, despite the very small value of p? 

June 6, 2009 | Thomas Smith wrote:

A random sample of 10 miniature Tootsie Rolls was taken from a bag. Each piece was weighed on a very accurate scale. The results in grams were 3.087 3.131 3.241 3.241 3.270 3.353 3.400 3.411 3.437 3.477 (a) Construct a 90 percent confidence interval for the true mean weight. (b) What sample size would be necessary to estimate the true weight with an error of 0.03 grams with 90 percent confidence? (c) Discuss the factors which might cause variation in the weight of Tootsie Rolls during manufacture. 

April 16, 2009 | Brad Woodruff wrote:

What is the best way to calculate sample size for an incidence rate, such as a mortality rate? 

February 10, 2009 | theresa wood wrote:

3. A researcher is interested in estimating the average salary of fire fighters in a large city. He wants to be 95% confident that his estimate is correct. If the standard deviation is $1050, how large a sample is needed to get the desired information and to be accurate within $200? 

May 17, 2008 | Chris wrote:

How do you calculate a confidence interval when the population standard deviation is unknown? 

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