Some instructors I've taught with seem to view uncertainty in a lab setting as a bad thing. They want every student in lab to be able to collect data that clearly support the hypothesis in question, and then talk about the lab as having "failed" if some students didn't collect the "correct" data.
This mindset has always baffled me. <soapbox> How can we expect to train our students to do science if we never let them do science? One of the primary goals of any science lab should be, at least partially, learning how to implement the scientific method, which includes coming up with questions, hypotheses, and tests for those hypotheses, in addition to simply carrying out experiments and interpreting data. By restricting students to following well-known protocols that are guaranteed to produce specific results, we prevent students from practicing critical portions of the scientific method. After all, deciding how to approach a problem is as big a part of science as actually performing experiments. Having a lab where everything fails would clearly be frustrating for all involved, but as long as the general techniques are sound, why not give the students the freedom to try a few things that might not work? </soapbox>
The reason I'm writing about this is that today's lab was, for those with the data-focused lab mindset, a failure. Last week the students transferred three species of bacteria and one eukaryote (yeast) onto agar plates that were subsequently exposed to different environmental variables (e.g. temperature, UV light). Unfortunately, the media had some moisture on it when the students were doing the plating (it had only been mixed up that morning by our lab tech), so the bacteria ended up growing in smears instead of well defined colonies on some plates. I had intended to have a thermophilic bacterial species in lab (one that prefers to grow at higher temperatures than most bacteria), but that culture had been misplaced before lab, so the first lab section wasn't able to grow it. And, finally, something went wrong with our yeast cultures (or the yeast wouldn't grow on our media), because we got all of about five yeast colonies across over a hundred plates.
So, to summarize, the plates the students saw today weren't anything close to how they "should" have looked. However, the students didn't know this, and I never said that anything had gone wrong in my intro to today's lab. Instead I directed them to collect as much quantitative data as they could (without specifically telling them how to collect the data), and they went right to work trying to figure out what effects their manipulations had.
The groups came up with a variety of different methods to collect data. Some groups whose microorganisms had low survival rates (e.g. UV light) counted the total number of colonies growing on their plates. Most groups, however, found that their colonies had grown together into large masses (confluent growth), and thus colony counting was impossible; these groups set out to estimate the surface area the bacterial colonies had covered. A few groups created grids on clear plastic sheets to help them estimate the growth (counting the # of squares or line intersections with bacteria under them), though one group measured how much surface area they had initially covered with bacteria, and then measured how much surface area the bacteria were currently growing in. I could have provided each group with a detailed handout telling them exactly how to quantify bacterial growth on agar plates, but doing so would have done little except prevent the students from having to apply their scientific reasoning skills.
While the students were collecting data, they independently figured out that something went wrong with the yeast, and after a while they all came to conclusions regarding what their data told them about the variables they had tested. Each group presented their data to the class (each group tested only one environmental variable), and we wrapped up the lab with a short discussion of why they saw the effects they did, complete with some comparative data from species we hadn't used in lab. Sure, some groups saw odd peaks or spikes in their data, but everyone seemed to leave with the main ideas the lab was trying to introduce. Nobody complained that their yeast cultures didn't grow, and nobody seemed to mind that some of their bacteria weren't growing in discrete, easily countable colonies.