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by iAmAPencilYo
543 days ago
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Further down, they explain the measurement errors involved: "
If you buy the same product twice, how much will chemical levels vary? When we bought two samples of the same product, plastic chemical levels differed on average by 59%, calculated as Relative Percent Difference (RPD). To test whether completely identical samples would show different levels of chemicals, we sent about 10% of our products in triplicate. This means we sent three copies of the product from the same batch – with matching lot number and expiration date – bought at the same store on the same day. We found that the triplicate samples differed less – on average by 33%. Our lab’s quality control methodology lists 20% RPD as an acceptable margin of measurement error for duplicate samples, meaning if you tested the exact same sample twice, you could see up to a 20% difference purely due to measurement noise. Taking that into account, the RPD for two samples of the same product (not necessarily from the same lot) ranges from 39-59%. For samples with the same lot number and expiration date, the RPD narrows to 13-33%. Within-product variability appears high, possibly because we are dealing with very small chemical concentrations measured in nanograms." |
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Perhaps plots would be better/less alarming than easy-to-cherry-pick tables, but I'm not expert on conveying this sort of data either...