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I've treated different labs differently - obviously! I've filled small bins with 2-3 lab setups. When students are ready for a particular lab, they watch a labcast video, complete any pre-lab assignments, grab a setup, then perform the lab. While this works most of the time, I learned quickly the flaws of this plan.
Students who work ahead:
Large sample size is important in science. Access to statistically significant (or insignificant) data is perhaps even more important than students designing experiments, in my humble opinion. The reason I haven't moved completely to inquiry labs is because I need each group to replicate trials of the same procedure to pool the data for statistical analysis. This presents a new challenge in an asynchronous course. Students who work ahead only have access to a small sample size. I've solved this problem by publishing data from previous years. Whether a student collects data early or late, their data will be compiled with other years of data.
Labs are messy; some materials can't be neatly stuffed into bins. Some labs require time consuming set ups, like water baths. Some require perishables which have to be ordered weeks in advance, while others need to used within three days of arrival.
I shifted to a synchronous approach for some labs. The resource intensive and perishable-heavy labs just require too much work to plan for different lab dates. I still offer some flexibility. I make some labs available for a few days. This gives students the time to catch up. I don't want students performing labs before they are relevant. My labs are strategically situated at specific parts of the learning cycle. It's important to allow some time for students to complete the prerequisite steps before performing each lab.
I'm dissatisfied with my system because I still have to rush some students through steps and prevent others from going ahead. I dislike encouraging students to skip steps in order to complete a lab.
It's clear to me that what's best for student learning, and not my convenience, is to make labs available only when students are ready for them. If my mastery learning cycles are designed in a pedagogically sound way, then rushing or prohibiting students from moving ahead is counterproductive. I also still believe that students must work with large sample sizes and inquiry is an opportunity for students to engage in critical thinking and take ownership of their learning. My solution is to have students design all/most of their experiments and stipulate they must have large sample sizes in their experiments.
This will require some adjustments. I'll have to alter the labcast videos to introduce the challenge, show some of the available materials and offer suggestions. This will take a great deal of organization and resources. I'll have to develop an effective technique to predict when students will be ready for particular experiments and use that information to strategically order materials.