5 Why root cause analysis use in OOS Investigation.

In Pharmaceutical Industries most of the investigators are using 5-Why root cause analysis tools to identify the exact root cause of the events.

For example, OOS observed in Assay test. 

Result:79.0% (Limit- 95.0 to 105.0%) 

Remeasurment performed.

Same vial:79.0%

Refilled vial:79.0%
Re-dilution from stock:99.5%.
From above Re-measurement testing root cause is identified as sample dilution error.

5-Why root cause analysis for above case study. 

1-Why--Problem
OOS (79.0% result) observed in assay test
2-Why 79.0% result. 
Analyst had done wrong sample dilution (prooven in hypothesis testing)
3-Why analyst had done wrong sample dilution.
During investigation it has been observed that analyst had 2 pipettes (4ml and 5ml) in his working tray. Hence analyst had used 4ml pipette for sample dilution instead of 5ml pipette, resulting 20% lower Assay.
4-Why analyst had 2 pipettes in his tray.
Analyst have 2 different products for analysis and he had kept all preparations in a same tray.
5-Why.....Not applicable.

So from above investigation exact root cause is identified in 4th-Why only.
Based on root cause, proper and effective CAPA should be taken.