×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# Tricks for sample sizing given very low failure rate

## Tricks for sample sizing given very low failure rate

(OP)
Got a weird question.  In trying to minimize cost, we are trying to pre certify a counting machine.  Customer requirements are that machine must count all of the data with 99.8% accuracy over its entire life.  Fairly strict!

Since we are talking only successes and failures this nets prohibitively large testing sizes (to get the test resolution to even say something with 99.8% confidence I think I need test sizes in the millions(darn that non continuous data)) using any of the standard discrete distributions.

Does anyone have any tricks for using smaller sample sizes?  Distributions that we have to use are strictly binomial or discrete in nature.

Or maybe some interesting math?

### RE: Tricks for sample sizing given very low failure rate

I buy machines like that so have been testing them for about ten years.  It seems to us that if they work they keep working with equal accuracy until a part wears out.

What seems to be a much greater problem for us is part size / weight variation.

To get 99.8% you need to have parts with maybe less than 1% variation if you are weigh counting.

The only way we get 99.9+ is with individual counting using a feeder passing an electric eye.   The problem there is with feeder design to prevent stacking or shadowing.

Before you spend a whole lot of time I’d make sure you knew just what the actual tolerances were going to be on the parts being counted.

Also for us the life of the machine is how long it counts accurately.  When it stops counting accurately it has reached the end of its usable life and needs to be rebuilt.   This isn't necessarily that big a deal.  One of our very small machines has a flipper gate to switch from one container to another.  That flipper gate wears out about every other year.  It is about \$30 and 20 minutes and then it is “rebuilt”.

Not sure how clear all this is.  Give me a call if you like and we can chat about it.

Tom Walz

Thomas J. Walz
Carbide Processors, Inc.
www.carbideprocessors.com

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

#### Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a partâ€™s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!