Atkinson & shiffrin multistore memory?

I have a psychology essment question regards multistore memory. I know that Atkinson & Shiffrin proposed the 3 stores of memory: sensory, STM and LTM, but can anyone tell me what METHOD they used? I am doing a distance learning course and the notes I have don't actually tell me what method was used for them to put forward their proposal. I've googled etc but I cannot find the answer anywhere! I have to state the reasons WHY, METHOD, RESULTS & CONCLUSION and I thought I understood it but my course papers touch briefly on Atkinson & Shiffrin but also mention Murdock, Miller and Peterson & Peterson. It's not clear to me what actual test Atkinson & Shiffrin conducted to come up with the multistore memory theory. Can anyone help?