Enough of pigs, today I'm going to write about plasticine spheres. Actually I've just been marking the first pig pracs and the results are remarkably good. One student even asked what they could write as improvements because the results were so good. Anyway I've been looking for a simple example to demonstrate errors and it's not so easy to find one that can be used at the start of the course when all student can measure is length. I was keen to find an example where the thing being measured was uncertain rather then the uncertainty being a result of the way it was measured. I came up with the idea of measuring the diameter of balls of plasticine with a vernier calliper. The measurements are not the same for several reasons. Firstly its difficult to roll a perfect sphere so the diameter will be different depending on the orientation of the ball. Secondly if you re-roll the spheres several times they won't be the same and thirdly, it's difficult to place the calliper on the correct place on the ball. Anyway, I tried measuring a ball about 18 times (exactly 18 times) and plotted the distribution of the diameters. It turned out really well. Not quite a normal distribution but then again I did only measure the diameter 18 times. The mean happens to be very close to the peak, that's very satisfying especially when I didn't fix the data at all. To plot the distribution I grouped the diameters every 0.05cm, 3.40 →3.45, 3.45 → 3.50 etc.

Having just read a couple more pig reports I realise that I am going to have to include an explanation of the lab with my 4psow, otherwise I might fall foul of the In animal experimentation policy. Just remembered where I had heard of pigs and wings before. How could I forget.