How to grow MORE potatoes in a raised bed

I had a little extra garden space and wanted to try something new for the first time, so I bought a set of 6 seed potatoes on half price. If doing an experimental plant in the garden, the ones on clearance are less of a risk, even if they do get in the ground a little late :)

I planted them directly into the ground, but quickly learned that they grow well in containers where you can build the soil up around the stalks as they grow. The idea is that the more the plant is under ground, the more roots it can grow, and the more potatoes you will get. 

It was too late to transfer them into containers, so I took an old plastic pot that I had bought a shrub in and cut out the bottom of it. I then set it over the plants and filled it with dirt as the plants grew.

To make the experiment officially scientific, I left some of the potato plants alone to simply grow in the raised bed. At the end of the growing season, we'll see which type of growing conditions the potatoes like best!

Any predictions???