Recent social media posts have shocked Dutch society to its core, prompting public outrage and cries for tribunals. What had happened? Well, a few customers of the largest supermarket chain in the Netherlands (the Albert Heijn or AH for short) were interested to learn whether the “1 kg” bag of potatoes (i.e., about 2.2 pounds) really contains 1 kg of potatoes. Lo and behold, when they put the bags on their kitchen scales they registered a weight that was far below the 1 kg mark. The conclusion is as obvious as it is chilling: the AH is ripping off the hard-working Dutch people and withholding them their rightful portion of potatoes.
Before being swept up in a frenzy of righteous anger and joining the chorus for the public “potatoeing” of the scoundrels who perpetrated this heinous crime, let’s pause and take a moment to reflect. It actually seems highly unlikely that something is structurally (or even incidentally) amiss with the weight of the AH potato bags. Firstly, when the bags are machine-filled they will immediately be weighted, and bags that happen to have too few or too many potatoes will be taken out of production. It seems deeply implausible that the factory setting for the weight of a 1 kg bag would purposfully be set too low. Such cheating would be relatively easy to detect empirically (by just weighting a number of bags), and it would constitute fraud. Also, many people across several organizations would need to be in cahoots to keep this quiet. Secondly, the Dutch populace may sleep sound at night knowing that the weight of a product is subject to strict regulations, anchored in Dutch law (see also the “Metrologiewet“; see also this post) and EU law (see here and here).
The EU quality control regulation for product weight is as follows. For a bag of 1000 grams, a quantity of 1.5% or 15 grams constitutes the “tolerable negative error” or TNE. Now we have the following three requirements:
- The average weight across the 1 kg bags of potatoes shall not be less than 1 kg.
- No more than 2.5% of the 1 kg bags of potatoes may have a weight that is below 985 g (i.e., 1000 – 1 TNE).
- Not a single 1 kg bag of potatoes may weigh under 970 g (i.e., 1000 g – 2 TNE).
According to the third requirement, a single 1 kg bag that weighs less than 970 g would be legitmate cause for a potatoe rebellion. However, as the chairperson of the Dutch Society “Skepsis” I am naturally skeptical to reports of a single anomaly. Therefore I decided to do some systematic quality control testing myself. First and foremost, I bought 20 bags of 1 kg “vastkokende” potatoes that came in a sealed plastic bag. I will confirm the quality of my measurement apparatus (i.e., my kitchen scale) by measuring each bag three times. Moreover, I have bought a second kitchen scale and will also measure each bag three times with that scale. So we have: (a) 20 bags of 1 kg AH potatoes; (b) two kitchen scales; (c) six measurements per bag, three per scale.
I will collect the data today. Next week I will present the results as analyzed with JASP! Below is a photo of the measurent situation: 20 bags, two scales, pen and paper, and cat Amy, who is not allowed on the table.
Authors
Eric-Jan Wagenmakers (CEO) and the JASP Services BV Team
