Every chronic quality problem hides a clue in plain sight: somewhere, under similar conditions, the same process is producing a good result. Good vs. Bad Analysis — sometimes called "Is/Is Not" comparison — starts from that observation and works backward to the cause.
The core idea
Instead of asking "why did this fail?", the technique asks a sharper pair of questions side by side:
- Where, when and under what conditions does the problem occur (the "Bad" cases)?
- Where, when and under what conditions does the same process not produce the problem (the "Good" cases)?
The difference between those two columns — and only that difference — is where the cause lives. Everything that is identical between "Good" and "Bad" can be set aside; it cannot explain why one outcome differs from the other.
How LearnEX teams apply it
In our Yellow Belt and Green Belt cohorts, participants are coached to build a simple comparison table across dimensions such as:
- Machine / line / station — does the problem track a specific asset?
- Shift / operator / time of day — is there a human or temporal pattern?
- Material batch / supplier / lot — does an input variable move with the defect?
- Method / setting / parameter — has something changed in how the work is done?
Each row is filled in for both the "Good" instances and the "Bad" instances. Patterns that are consistent across both columns are eliminated immediately — a discipline that prevents teams from chasing comfortable but irrelevant theories.
Why it works as a first step
Good vs. Bad Analysis is deliberately low-tech. It requires no special software and can be run on a whiteboard in under an hour — which is exactly its strength. It:
- Narrows the search space dramatically before any statistical tool is applied, saving weeks of unfocused data collection
- Forces specificity. "It happens sometimes" becomes "it happens on Line 2, second shift, with Lot batches received after the supplier change"
- Builds team alignment early, because the comparison is visual, collaborative and easy to challenge in the room
Where it fits in the bigger picture
This technique is one of more than 350 problem-solving tools in the LearnEX library, and it typically sits at the Analyze stage of a DMAIC project — right after the problem has been clearly defined and measured, and just before root-cause hypotheses are tested with more rigorous statistical methods.
Used well, Good vs. Bad Analysis often gets a team 70% of the way to the true cause before a single control chart is drawn — which is exactly why it's one of the first tools we teach.
Curious how this fits into a complete certification pathway? Explore our [Six Sigma Certification Programs](/programs/six-sigma-certification).