If you only need documentation for public auditors such a hall test can be done with relatively few interviews – about 50 will be sufficient.
Such interviews can be done with two different techniques:
§ Personal interviews or
§ Mailed questionnaires
In theory you are also able to make telephone interviews but in the telephone it is almost impossible to demonstrate different packaging designs. Your questions have to be closely related to the physical packaging design so your interview must be connected to either test samples of relevant packaging or photos/drawings/videos of packaging. Such demonstrations are possible in the two proposed techniques. Test samples will be best but not always possible.
The layout of the test shall be focusing on the critical level of packaging. In consequence your questions and test samples shall describe different designs around the critical area.
Remember the overall question is:
Can the design use just a little less material and still fulfil the user/consumer requirement?