My ninth grade students enjoy labs, but my colleagues say I do too many and the students aren’t learning anything. How many labs should I do each week?
—Carolyn, Billings, Montana
I’m curious as to what you mean by “labs.” Some teachers use the word lab to describe a variety of activities from investigations and experiments to cookbook demonstrations, small-group discussions, simulations, group writing assignments, laptop activities—anything students do in groups in science class. While all of these activities can be useful learning strategies, let’s assume you are referring to inquiry-based investigations and experiments.
NSTA’s position statement on Scientific Inquiry states, “Scientific inquiry is a powerful way of understanding science content. Students learn how to ask questions and use evidence to answer them. In the process of learning the strategies of scientific inquiry, students learn to conduct an investigation and collect evidence from a variety of sources, develop an explanation from the data, and communicate and defend their conclusions.”

Although you do not have to justify your choice of learning activities (or their frequency) to your colleagues, you may want to reflect on what you’re doing for your own professional piece of mind. While considering your activities, it may be helpful to parse the above position statement. Do your labs help students to

• understand science content—the processes and “big ideas” as well as facts and concepts;
• ask questions (not just answer ones that someone else asks);
• design and conduct various types of investigations, depending on the questions;
• collect and organize their evidence (data);
• analyze the evidence to develop an explanation; or
• communicate and defend their conclusions?

This is a lot to expect of students; they need guidance and modeling tailored to their level of experience. I had the opportunity to work with a middle school teacher who scaffolded the inquiry process for her students. She kept the unit’s “big idea” posted in the classroom and made sure to refer to it during every activity (lab or otherwise) to keep the students focused on the content. When she asked students for questions to investigate, she added a few of her own as a model. She guided the students through a discussion of how the experiment was designed and how the design related to the question (after experiencing various types of investigations, they took over more of the design process). She monitored them as they did the procedure and collected data, and she assisted or intervened when necessary. She worked with the students as they reviewed their data and determined  if their evidence answered the questions, and discussed why it did or did not. During the process, the students recorded the data and their conclusions in their notebooks. The teacher recognized this was a time-consuming process, but she was confident they were learning (and the assessment results supported this conclusion).
I really don’t have a numeric answer to your question. Regarding the number of activities, for scientific inquiry the quality of the activities is more important than the quantity. Doing an activity for the sake of doing an activity without any follow-up or reflection may lead to the second concern about what the students are actually learning and whether they truly understand the concepts. I attended a workshop with a middle school teacher who remarked, “I keep my students so busy they don’t have time to think.” I still wonder what—if anything—they learned.
Image from http://www.flickr.com/photos/jimmiehomeschoolmom/3423116