by Jeff Sauro | September 17, 2004 ::
RSS
Converting a Proportion to a Sigma Value
A sigma value is a description of how far a sample or point of data is away from its mean, expressed in standard deviations usually with the Greek letter σ or lower case s. A data point with a higher sigma value will have a higher standard deviation, meaning it is further away from the mean.Approximating the Proportion to a Normal Distribution
An easy way to exploit the Central Limit Theorem solution and obtain a sigma value from the task proportion is by using the Excel function NORMSINV. NORMSINV approximates the area under a normal bell-shaped curve. Take the unsuccessful proportion .1399--the proportion defective and insert it into the following equation in Excel.=NORMSINV(1-.1399)
Note: My sigma values DO NOT contain
a 1.5σ adjustment
or "shift."
Task Completion Benchmark
A common benchmark for the percent of users that should complete a task is 90% or 90 out of 100 users that attempt a task should be able to complete it. There are at least two problems with this benchmark:The Bottom Line
In a sense, task completion then is a good preliminary test for detecting egregious usability problems or for first time or novice users. I'd continue to use 90% as a goal for novice(never or rarely completed the task) and use 99%+ for experienced(complete the task weekly) users.
View All Articles |
Subscribe to RSS
|
Follow on Twitter |
Get Email Updates
| July 16, 2010 | PP wrote: |
| It was really helpfull as i was not clear about the sigma level concept, thanks! |
| May 20, 2010 | Judy wrote: |
| I need values that are above and below a mean based upon a target and upper and lower spec limits oon both side of the cureve. Do you have an example for this? |
| April 8, 2010 | Atul wrote: |
| good to implement |
| September 14, 2009 | prabhakar wrote: |
| can i know the difference if my spred is closer and spred is larger , in both cases result is in specified limits. |
| November 28, 2008 | Zafar wrote: |
| Hi, i found this realy a good and informative. I shall be thankful if u would help me in the following: i. if want to find the chance of success between two values ii. if expected value is greater than mean iii. if expected value is less than mean afer looking for corresponding value from Z table, how it will be interpreted. Thanks Waiting for an early response from ur side |
| June 26, 2008 | Irfan wrote: |
| Let's say I have set up a standard to deliver pizza in 25 min. from the order to delivery. If I meet that target 68% of the time, is my process running at 1 Sigma. i.e. DPMO - 690,000 times I missed the target and did not meet my 25 min target or less thatn 25 min. Please explain this concept with example. |
| March 28, 2008 | lavish wrote: |
| value of 1sigma 2sigma 3sigma 4sigma 5sigma 6sigma |
The Five Most Influential Papers in Usability
What five users can tell you that 5000 cannot
6 things you didn’t know about Heuristic Evaluations
Why you only need to test with five users (explained)
Does better usability increase customer loyalty?
Can you use the SUS for websites?
If 1 of 5 users has a problem in a usability test will it impact 1% or 20% of all users?
Featured Product
Copyright © 2004-2010 Measuring Usability LLC
