Alice W. Leung

Missing Bushing Investigation

Engineering, Root Cause Analysis, Working Memory, Contextual Inquiry, Usability Testing

Creator/Investigator: A. Leung

Project Overview


Users forget to load the same component at a certain step for every batch of assemblies. This is a repeated reject that affects yield loss during production.

The challenge is to reduce/eliminate the amount of rejects caused by users forgetting to load components.

My Role

I was the investigator and engineer for this root cause analysis/process improvement project.

With my knowledge gained from HCI principles, I was able to look at the problem from users' perspective. This was, in fact, more of an interaction design problem.


I reduced the amount of working memory load for my users by re-arranging process steps and combining components in the same section.

I have successfully eliminated this repeated failure, which resulted in a cost savings of around $54,000 per year.

Disclaimer: Certain content of products, tools, or process are proprietary information, and therefore not shown on this page. Most content and terms in this project are generalized to protect company rights.

Bushing Assembly

Project Overview


During catheter assembly process, multiple components are assembled together. However, some components must be put together first before adding glue to bond the final device together.

Failed Devices due to missing components

Different operators, on occasion, repeatedly forget to load the same component into the device before bonding the final device together. This led to unnecessarily yield loss within each batch of catheter devices.

Previous Solution Attempts

Visual verification step was previously added in the instructions to remind people to load that component into device before gluing the device.

Contextual Inquiry

Contextual Inquiry

Understanding how users perform the process

We observed how people put together the components into a final catheter device, and there is one component that are used in two different locations during assembly a plastic bushing (Bushing #1 and Bushing #2).

  • There is a sub-assembly section to put all the components (including Bushing #1) together.
  • Bushing #2 is packaged separately from other components.
  • Bushing #2 is introduced later in the instructions, which means people are working in a different set of process steps before assembling Bushing #2.

Quotes from users and engineers:

I know I need to put this in, but I don't know why I keep forgetting...

We added instructions to check for it, why are people still forgetting??

Root Cause Analysis

Root Cause Analysis

Synthesizing the insights from contextual inquiry
Capacity of attention (aka working memory)

Information can easily be lost because our working memory has low capacity and it is volatile. If items in working memory don’t get combined or rehearsed, they are at risk of having the focus shifted away from them.

This volatility applies to goals and object details - losing items from working memory corresponds to forgetting or losing track of something you were doing.

Design implications

We should help operators remember essential information from one moment to the next. Try to avoid requiring operators to remember what they have done to have to go back to the previous step, because their attention is focused on their primary goal and progress toward it.

Conceptual Design

Conceptual Design (Poka-Yoke)

Re-arrange process steps

Keep all components including both Bushing #1 and Bushing #2 as one subassembly.

This volatility applies to goals and object details - losing items from working memory corresponds to forgetting or losing track of something you were doing.

Add visual aid (Poka-yoke, aka Foolproof)
  • Use visual aid as a stopper to prevent Bushing #2 from falling off until the subassembly is assembled together.
  • It is intended to be removed before bonding final devices together.
  • It cannot be bonded if visual aid is still on the subassembly.

Prototype & Testing

Prototype & Usability Testing

A visual stopper

A plastic tubing that has the same outside diameter (OD) with a slit along the length of the tubing to snap onto the cylinder of the subassembly.

Test the prototype

We tested whether or not this will eliminate the need for constant working memory because the idea of this design is to make it impossible for people to forget.


We were successful at solving this issue without any re-occurrence after a few weeks of testing. This fix was permanently implemented into the process.

Business Impact

Business Impact

Psychological Impact

Operators are happy because they won’t blame themselves for the rejected devices solely due to being forgetful. Assembly process is improved because it has considered the human factors of putting components together.

Cost Savings

The cost savings associated with this project was estimated to be $54,000 per year.

Recent Works