The aim of validation is not to show that a process works on ordinary days but that it holds the temperature band when pushed. The EU Good Distribution Practice guidelines likewise base container and packaging selection on the anticipated external temperature extremes and on the maximum transport time, including transit storage (Section 9.2).
| Variable | Hardest extreme |
|---|---|
| Ambient temperature | Hottest and coldest days of the shipping period |
| Duration | Longest route, with an allowance for delay |
| Door openings | Route with the most stops |
| Fill level | Minimum and maximum load |
| Preparation | Weakest conditioning the instruction allows |
Load has no single hard extreme. A lightly loaded box holds more air and swings more, so in hot weather the minimum load is often the hardest. Test both ends rather than assuming: WHO TRS 961 Annex 9 (Section 6.8.3) asks that qualification of a passive container state the minimum and maximum volume, weight and thermal mass it can carry.
Pushing every variable to its extreme at once is a mistake too. If your records never show the hottest day, the longest delay and the weakest preparation together, that combination only produces unnecessary cost and unnecessary rejections. The choice is the hardest combination that can appear together in your own records.
The decision file should hold the rationale for the chosen scenario, the route and seasonal data behind it, the acceptance criterion, and a separate instruction for situations outside it. A successful result covers only the conditions tested; if route, loading or packaging changes, change control decides whether the new setup falls outside the scope.