Planarian Regeneration and the Limits of a Classroom Comparison
Ms. Alvarez's ninth-grade research class is testing whether a commercial ‘regeneration solution’ speeds planarian regrowth. Each student cuts 30 planarians transversely behind the pharynx and places the posterior halves individually into wells: 10 in spring water, 10 in pond water collected behind the school, and 10 in a 1:50 dilution of the commercial solution. Wells sit on a windowsill; room temperature ranges from 19 °C to 26 °C over the two-week study. Students photograph each worm every 48 h and measure blastema length with a ruler. One pond-water well turns green with algae on day 4. The class reports mean blastema length at day 14: spring water 1.8 mm, pond water 2.4 mm, commercial solution 2.6 mm. A student concludes the commercial product ‘works best’ and posts the result online. The teacher asks the class to defend or revise that conclusion using the actual design.
1. Identify two specific design flaws in the planarian study and explain how each could bias the comparison among treatments.
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Two flaws stand out. First, treatment is confounded with water source. Pond water brings algae, bacteria, and nutrients-the green well is visible evidence of biological contamination. If pond-water worms regenerate faster, the cause could be microbial food, oxygen, or other solutes, not the commercial product. Second, temperature and light are uncontrolled, and wells are not randomized across the windowsill. A well near the glass may be warmer, and regeneration is temperature-sensitive. With only 10 worms per group, one contaminated or unusually warm well can shift the mean. Measurement is also not blind, so a student expecting the product to win may round or measure differently. The 2.6 mm versus 1.8 mm difference is therefore a correlation with treatment, not evidence that the product caused faster regeneration.
2. Rewrite the student’s conclusion as a testable hypothesis and a prediction. Then describe one control condition the current design lacks.
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Hypothesis: At a constant 22 °C and under identical light, a 1:50 dilution of the commercial solution will produce greater mean blastema length at day 14 than spring water alone. Prediction: if the hypothesis is correct, the product group’s mean will exceed the spring-water mean by at least 0.5 mm, and the ranges should be reported, not just the means. Missing control: a vehicle or diluent control-spring water adjusted to the same pH, ion concentration, and preservatives as the product but without the active ingredient. Without that control, the product’s effect cannot be separated from its formulation. A no-cut control would also help establish baseline regrowth, but the vehicle control is the key missing comparison.
3. Explain how random assignment of wells to shelf positions and blind measurement would change the interpretation of the 2.6 mm versus 1.8 mm difference.
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Random assignment would spread wells with different temperature and light microenvironments across treatments, so shelf position would not systematically favor one group. Blind measurement-coding wells and having a second person measure without knowing treatment-would remove expectation bias. If the product advantage remains after both changes, the causal claim is stronger. If it shrinks or disappears, the original conclusion was likely an artifact of confounding or bias. Blinding does not fix contamination or the small sample size, so both design changes and replication at the well level are needed. Randomization and blinding improve internal validity but do not by themselves establish external validity for other planarian species or conditions.