Part II—The Data
The following day Susan found Steve in the computer lab working on his research paper.
“Here’s a copy of my paper,” she announced as she walked up to him.
“Oh, cool,” he replied, looking up from the computer. “I’m really ready for a break, so let’s go get some coffee. No, first let me print out what I have so far. Will you read it and see what you think?”
“Of course, if you buy the coffee—and after you’ve had a look at mine!” Susan replied.
“Deal,” Steve answered with a smile. They collected Steve’s copy of his paper and went to the coffee shop next door.
When Steve got to their table with the two coffees, Susan had a pile of papers sitting in front of her.
“I hope I don’t have to read all of those!” Steve said as he handed Susan her coffee.
“No,” Susan said with a laugh, “this pile contains all of the information I have collected about plastics and whether they’re safe for food use, and it’s pretty convincing, I think. You can just read my paper to get the ‘boiled down’ version of the information.” Susan picked up the top paper and started handing it to Steve.
“Wait,” Steve said, holding his hand out so she couldn’t put the paper in front of him. “I’ve been reading too much lately, so save my eyes, and just tell me what you found. My first question is, what is the evidence that things come out of the plastics and into the water, or whatever it is you have in the plastic?”
“OK, here is what I found on that.” Susan showed him Table 1 (from Howdeshell, et al., 2003).
“Whoa, Susan, you’re supposed to be giving me the easy answer, not making me figure everything out on my own!”
“I’ll explain it to you. What they did was to get used and new animal cages, either polycarbonate or polysulfone, to test. They used glass casserole dishes and also polypropylene cages as negative controls since neither is manufactured using bisphenol A, we just call it BPA …"
“Wait, what’s this BPA stuff?”
“Oh, it’s the monomer that’s used to make polycarbonate and polysulfone, which are two common plastics,” Susan replied. “Figure 3 from the same article shows the structure of BPA, and also how it is incorporated into the two kinds of plastics.”
Susan looked at Steve. “OK, are you ready to go on and look at that table now?”
He nodded, intent on looking at the material in front of him.
“So,” Susan said, going back to Table 1, “they had these two different kinds of plastic cages, new and used. They rinsed them all, first with just tap water and then with highly purified water (HPLC grade). Then they added 250ml of that HPLC grade water to each cage and let them sit for a week. They then collected the water and measured the amount of BPA found in the water from each type of cage. You can see that they repeated the experiment several times.”
“Yes,” Steve answered, “but what are all these dashes? I don’t get that.”
“Oh, well, I think they were making adjustments in the experiment as they did more replicates. You know, they learned something and applied it as they repeated the experiment.”
“Then those aren’t really replicates, are they?” Steve asked.
Susan ignored his comment and went on. “First, they just compared glass and the used cages. They saw that glass didn’t leach out any BPA, so they didn’t repeat it the second time. But that time they added new polypropylene cages to compare.”
“I guess the third time they thought they’d do a more complete test,” Steve added. “Did they add more used cages?”
“Yes,” Susan replied. “I guess they wanted a larger sample number.”
“But those released a lot more BPA,” Steve observed. “Why? Do they explain that?”
“They said those cages looked more worn than the other ones—there were more marks on the plastic, like they had been damaged more during their use.”
Questions
- Are the replicates helpful? Why or why not?
- What do you conclude from these data?
- What other results would you like to see?
- What are the strengths and weaknesses in these data?
Image credits: Table 1 and Figure 3 from Kembral L. Howdeshell et al., “Bisphenol A Is Released from Used Polycarbonate Animal Cages into Water at Room Temperature,” Environmental Health Perspectives, 111:9, 2003. Reproduced with permission from Environmental Health Perspectives.
Originally published at http://www.sciencecases.org/water_bottle/case2.asp
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