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Creating a better iced tea with design of experiments

iced tea in a glass and a small bowl with tea leaves

What's the best way to make delicious iced tea with fruit juice? (Photo courtesy of Caroll Co)

My wife is a huge fan of tea – to the extent that our kitchen has two shelves dedicated to it. For a recent family gathering, she wanted to clear out some tea by making a large batch of iced tea (i.e., to allow more room in the cupboard to buy more tea). She had served iced tea at previous gatherings to rave reviews, but wanted to try something different by making iced tea with added fruit juice after trying such a concoction at a café.

Her own attempts at fruit-juiced iced tea had mixed results, so she was nervous about making a large batch if it wasn’t going to turn out well. We were also in a time crunch – we had only one day to figure this out. What better way to spend a day off than running a designed experiment?

The Factors

For this experiment, we used what we had on hand at home to create an iced tea that (hopefully) tastes good enough to serve to guests. While I couldn’t promise my wife that we’d find a usable recipe, I was confident enough that I convinced her to be the official taste-tester. The factors we considered:

  • Tea type: black tea or oolong
  • Steep method: hot water vs. cold water
  • Steep time: short (5 minutes hot/4 hours cold) or long (10 minutes hot/8 hours cold)
  • Amount of tea: 2 tsp per cup or 3 tsp per cup
  • Juice: cranberry or apple
  • Juice proportion: 25% or 50%
  • Added sugar: 1 tsp or 2 tsp per cup of liquid
  • The first four factors are based on the brewing of the tea, while the last three are related to the tea itself. This means that if I make bigger batches of tea, I can split these batches up and vary the last three factors. This would make the first four factors hard-to-change, and the last three easy-to-change (since I can just vary those on a measured cup). On the other hand, there’s nothing stopping me from varying all seven factors for each cup, so how to decide what to do?

    Whole Plots and Run Size

    We knew that the final tasting would be done with a Styrofoam cup, so in theory there’s almost no limit as to how many cups she could taste. However, even taking a few sips at a time, we didn’t want to overwhelm her with too many choices. We decided on 16 cups as a reasonable number to use.

    We also realized that in order to make a batch of tea in a teapot and let it cool posed a limitation in the number of containers/space we had available, so we went with eight batches of tea – i.e., 8 whole plots. Each of these batches would be used for 2 cups of iced tea, for a total of 8*2 = 16 cups (i.e., runs).

    The Response

    We wanted to measure the taste, but how could we differentiate between the 16 cups of tea? We could try a forced ranking, but there may be subtle differences that would make it difficult to distinguish, and large gaps may exist between the good-tasting and bad-tasting teas. The nice thing is that she can take small sips, so it’s easy enough to taste everything more than once before deciding. In the end, we laid out a tape measure on the table, with my wife placing each cup somewhere on the tape measure relative to how it tasted compared to the others. The worst would be 0, best would be 10, and all other scores based on their final placement.

    The Experiment

    We decided each batch of tea would be made with 2 cups of water. All the teas were chilled in the fridge so they had the same starting temperature. Each Styrofoam cup would end up with one cup of liquid (tea and juice mix).

    Below is what my Factors Table looks like (note the first four factors are hard to change) in the Custom Designer.

    I created the design using main effects only, and in the Design Generation outline, I specified 8 whole plots (batches of tea), and 16 runs (cups of tea).


    I created the design using main effects only, and in the Design Generation outline, I specified 8 whole plots (batches of tea), and 16 runs (cups of tea).


    Check back tomorrow for my next post, in which I'll reveal the results of this experiment.

    Is there any special way you like to make iced tea? Leave me a comment below and let me know.

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    Warren Foley wrote:

    Very nice and simple application of DOE and the learnings have real significant practical application. Everybody in my region of the country loves good iced tea!


    Ryan Lekivetz wrote:

    We've gained a real appreciation for sweet tea since moving to North Carolina. Thanks for the kind words, and thanks for reading!


    giorgio marrubini wrote:

    Thank you Ryan for sharing this nice use of DoE!

    I loved reading it and will try soon a similar study, so please give us also the rest of the story.


    Ryan Lekivetz wrote:

    Part 2 with the results should be posted in the next few days. I look forward to hearing how your study goes too!


    Michael Clayton wrote:

    Blindfolded replicates would be interesting, and randomization of test order between replicates.

    We can see many ways to turn this into a major neighborhood event, videotaping for u-tube.


    Teresa Obis wrote:

    Thank you Ryan for sharing. Nice DoE.

    Are you sure the factor tee amount, juice proporton and sugar are continuous? I am not sure. With two values you can not say that this is continuous.

    How will change result if you consider that factors as categorical.


    Ryan Lekivetz wrote:

    Great suggestion! In hindsight, at the very least it would have been nice to have a few replicate runs for some pure error.


    Ryan Lekivetz wrote:

    Good catch. In the two-level case, the continuous and categorical enter the design and model in the same way with the -1/+1 coding. By treating them as continuous, I may want to explore the range a bit more in an augmented design either by changing the range or adding higher-order terms.