Moving from the bulk reaction of yeast (catalase) and H2O2 to a procedure that can produce reasonable, reliable and precise data without just telling them, “This is the technique that we will use”, can be tricky. But it is a discussion full of quantitative considerations if that procedure is going to generate quantitative data that can support a claim.

Some of my learning goal targets that I keep in mind to guide my questions during discussing include: 1. Introducing the floating disk technique but making sure the students understand how it is working. 2. How do we explore variables systematically. (serial dilutions) 3 What is this replicability thing?, 4. Emphasizing the importance of exploratory work to help establish data that can inform design. 5. How big of sample do we need? What factors into determining sample size? 6. Identify and contrast systematic and random error.

With these thoughts guiding my questions we launch into a discussion about the mess I created earlier.

With practice over the years it is easy to have barrage of questions ready to go. Typically, I reframe/choose my next question based on student responses. In that way, we are all following along on the same reasoning path–or at least as much as 20+ individual agents can follow the same path.

What did we mix to create the mess? What did we get out? How is this related to the models we explored? How could we quantify what is going on? What are we going to try and figure out? What can we control? What do we need to know? What should we measure? How should we systematically measure it? How can we be sure to all generate data/information that can inform our exploration? How can I capture the products produced? How do I measure the products over time? What could/should I use for controls? What should we quantify if we want to make a claim? This last question can be particularly productive if out goal is to collaboratively develop an experimental protocol. I never know exactly where we will go but with the guiding questions in my mind and with practice on my part it doesn’t usually take too long before we get to a starting/exploratory protocol that we can test in class.

*How many disks should I drop to be confident that I have measured the rate of rise?*In the past, I had my students collect data on 10 disks of yeast per substrate concentration because I used this lab to introduce box plots. The choice was somewhat arbitrary but you need a sample of 10 or more if the box plot is going to provide relevant information. For example, a sample size of 4, split into 4 quartiles isn’t going to tell me much. In today’s AP Bio world I might use this lab as an opportunity to explore another way to estimate an appropriate sample size–using standard error. Here’s how that works.

**Pre-Determining Sample Size:**

Let’s combine these two equations and since, earlier we decided that plus or minus 0.5 seconds was probably enough precision we can just substitute that for the 95% CI.

Substitue 0.66 for the stdev.s that is estimated from our exploratory data:

Multiply both sides by the square root of n.

We are getting close, now. Square both sides and you end up with the sample size you’ll need to assure that you have a 95% confidence interval that is plus or minus 0.5 seconds around the mean of your sample.

Ah, finally. Looks like a sample size of 7 will assure that the 95% CI will fit between plus or minus 0.5 seconds around the mean. Of course if we wanted a 99% CI we could use 3 x SEM in the work. Or we could define a more precise CI interval of say 0.25 seconds around the mean. It is up to you. But with this type of work, you can make a strong argument as to why you chose the sample size you chose.

*How is the rate of the enzyme reaction affected by the concentration of the substrate?*They can work in groups, with their family, or by themselves but I want everyone to have a lab notebook entry of the methods, the questions, the design and the data they have collected along with graphs of the data. I’m not explicit about what that should look like at this point. I don’t want to be too helpful. I actually want mistakes so we can address them. If I’m too helpful at this point and tell them to make a scatterplot of just the means of the time to rise versus the substrate concentration then many will be will not know how to work in a novel situation in the future.