The Outlier in the Catalase Lab
In a tenth-grade biology class, teams investigate catalase activity in potato tissue by measuring O2 bubbles produced per minute at pH 3, 5, 7, 9, and 11. Each team runs three 30-second trials. Team 4's pH 3 trial 3 produces 42 bubbles/min, while its other pH 3 trials produce 4 and 6 bubbles/min and the class mean is 5. The team asks to delete the 42 because 'it is an outlier.' Team 2 used tap water instead of the pH 7 buffer and reports results similar to pH 5. A bottle of 3% H2O2 opened last year has been used all day and sits near a sunny window; the classroom is 26 °C. You have one class period for analysis before students write their lab reports. The department's practical exam asks students to design a controlled investigation, not just interpret results.
1. The team asks to delete the 42-bubble value. How should you respond, and what decision rule should students use for handling anomalous data in this investigation?
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Start by separating 'outlier' from 'error.' An outlier is a value far from the pattern; it is not automatically a mistake. Deleting it without a documented cause is data fabrication, and it removes exactly the evidence students need to learn about reliability. I would ask the team to check their notebook: Was trial 3 run after the potato was reused? Did the H2O2 bottle sit in sunlight? Was the timer started late? Was the bubble stream measured at the same point? If a procedural deviation is documented-for example, the disc was left in the 26 °C water bath for 10 minutes instead of 2-they may exclude that trial, but they must report the exclusion and the reason. If no cause is found, they should retain the value, report the median alongside the mean, and discuss variability. The class rule: exclusion requires a pre-specified, evidence-based criterion; otherwise report all data, show spread, and treat the value as a prompt for investigation. I would have the team rerun pH 3 with fresh H2O2 and a standardized disc. This models how scientists handle messy data: transparency, replication, and uncertainty, not tidying.
2. Identify the confounds in this lab and design a controlled version that a team could complete in one 50-minute period. Include controls and the measurement strategy.
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Confounds: H2O2 concentration is unknown and likely lower because the bottle is old and warm; temperature varies near the window; pH 7 was tap water in Team 2, so buffering differs; potato catalase activity varies by disc surface area and tissue age; bubble counting is observer-dependent and bubbles coalesce; trial order and time since cutting affect activity.
A controlled version: use fresh 3% H2O2 kept in an ice bath; cut potato discs with a cork borer to a fixed diameter and thickness; blot dry; use 10 mL of buffer at each pH made the same day; equilibrate tubes in a 25 °C water bath for 5 min; add one disc and measure gas volume in an inverted graduated cylinder or with a pressure sensor for 60 s rather than counting bubbles; run three trials per pH with a fresh disc each trial; randomize pH order. Include a negative control of boiled potato (denatured catalase) and a baseline of buffer plus H2O2 without tissue. Have two students independently record volume. If equipment is limited, assign each team two pH values and pool class data with a common protocol. The 50-minute constraint means pre-cut discs and pre-aliquoted buffers; students still do the timing and measurement.
3. How will you assess inquiry practices in the practical exam rather than only the final graph? Provide a rubric with three criteria and describe one common scoring pitfall.
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Assess the practices students actually use: planning, carrying out, and analyzing. Criteria: (1) Planning-identifies the independent variable (pH), dependent variable (gas volume per minute), and at least three controlled variables, plus a negative control; (2) Conducting-follows the standardized protocol, records raw data with units and observations, and repeats trials; (3) Analyzing-uses all data or justifies exclusion, reports a measure of center and spread, and connects the pattern to enzyme structure and denaturation. Each is scored 0–3 with descriptors.
A common pitfall is scoring the neatness of the graph or the 'right' conclusion rather than the reasoning; another is giving credit for a control that is only named, not explained. I would use a task-specific checklist and have students write a short decision log explaining one procedural choice. To keep inter-rater agreement, I would score two anchor reports with the department and compare. The practical exam should include a brief oral defense-'Why did you use a boiled-potato control?'-so I can probe understanding that a written report may hide.