Graph and Calculator for Confidence Intervals for Task Times February 6, 2006
Visualizing your task time data is an essential step in understanding its distribution and computing accurate confidence intervals. This calculator creates a dot-plot of your task times, transforms the raw data to adjust for non-normality and computes the intervals.

Usable Statistics June 14, 2005
Do you need to feel more confident about using statistics? Dismayed by overly complicated "introduction" courses that focus on theory and not application? Do the "basic" books assume you know where to look for your answer? The first module in this series is on using confidence intervals in usability testing.

Restoring Confidence in Usability Results October 18, 2004
Adding confidence intervals to completion rates in usability tests will temper both excessive skepticism and overstated usability findings. Confidence intervals make testing more efficient by quickly revealing unusable tasks with very small samples.

Percentile to Z-Score Calculator December 4, 2007
Enter the area under the normal curve (a proportion between 0 & 1) and get the Z-critical value, one-sided or two-sided.

How Do You Calculate a Z-Score/ Sigma Level? June 14, 2004
The basics of z-scores are discussed plus an example of raw usability data converted into z-scores including three of Nielsen's five usability attributes.

Calculating a Sigma Level from Task Success September 17, 2004
Often the most reported measures of usability is task success. How does task success translate into a quality sigma value that can be compared to other reported sigma values?

Usable Statistics June 14, 2005
Do you need to feel more confident about using statistics? Dismayed by overly complicated "introduction" courses that focus on theory and not application? Do the "basic" books assume you know where to look for your answer? The first module in this series is on using confidence intervals in usability testing.

What's a Z-Score and Why Use it in Usability Testing? September 17, 2004
This common statistical way of describing data can be used in usability testing to standardize disparate data types to allow easy comparison between products or versions and providing a universal way of assessing quality.

Calculating Sample Size for Task Completion (Discrete-Binary Method) September 17, 2004

Calculating Sample Size for Task Times (Continuous Method) September 17, 2004

Deriving a Problem Discovery Sample Size Side-bar to The Risks of Discounted Qualitative Studies: March 8, 2004
Shows the history and computation of deriving a sample size for discovering problems in an interface.

Task Times in Formative Usability Tests June 6, 2008
Time-on-task can be used as a valuable diagnosis and comparative tool during formative evaluations.