# In Review

The results of my 4-trial fractional factorial experiment (main effects) showed that extraction time is the most important variable, and that temperature and C/W ratio are less important. In the interest of completeness this experiment will not simply vary extraction time, but rather seek to efficiently find the maximum.

# Lessons from the Screening Experiment

I am accustomed to making my coffee with about 22 fluid ounces of water, which makes two nice cups for my morning. During the screening I used 22 fl oz as a baseline, and increased or decreased the amount of coffee accordingly. The result was that some experiments required 48.8 g of coffee. You can’t possibly imagine how much coffee that is. It almost completely fills a coffee cup with just the beans!

In subsequent experiments I will limit the amount of coffee beans to something more modest, perhaps about 20 g, and vary the amount of water. With this more cautious approach I may live to see the final results.

# The Design

NIST offered three designs for response surface objective (RSO) experiments. Specifically there are two interpretations of the Box-Wilson central composite design, each of which requires 20 total runs. An alternative design reduces the number of experiments to 15, which is the reason I chose it.

Specifically, the Box-Behnken design provides an RSO approach for 3 factors in 15 experiments. That is the same number I originally suggested through my naïve approach, but without all the baggage. Effectively, my original 5 values for each factor get reduced to three, and those are shuffled about in a way that is well-suited to estimating their effects. In my case, the specific experiments would be:

 Response Surface Objective Actual Values Statistics Jargon Trial r (g/ml) t (sec) T (F) R T T 1 0.035 10 195 -1 -1 0 2 0.075 10 195 +1 -1 0 3 0.035 300 195 -1 +1 0 4 0.075 300 195 +1 +1 0 5 0.035 55 185 -1 0 -1 6 0.075 55 185 +1 0 -1 7 0.035 55 205 -1 0 +1 8 0.075 55 205 +1 0 +1 9 0.055 10 185 0 -1 -1 10 0.055 300 185 0 +1 -1 11 0.055 10 205 0 -1 +1 12 0.055 300 205 0 +1 +1 13, 14, 15 0.055 55 195 0 0 0

This set will take me about three weeks. With luck by the time you read this I will have trickled out previous articles, and you will have results in two weeks or less. Sorry for the suspense.

Oh yes, I’m interested in other people’s Q functions too, so if you want to run my exact experiments, please post your data and I will post your results. Public or anonymous, at your discretion.