Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-L] pseudo-pundits +- systematic error



That is a fairly large variance. When buying inexpensive tiles you need to
get them from the same lot. And boxes will often have a number on the side
telling you how much it varies. We had that problem when we had
underestimated a tile job and had to buy some more. As we installed the
tiles we realized that they were no longer lining up properly. Fortunately
they were on a porch in an inconspicuous spot. I then discovered the
numbers on the side +2... which differed.

So students actually got 2 lessons about the real world. One of them was to
be sure you always check things before you purchase. "Measure twice and cut
once"

John M. Clement
Houston, TX


I stumbled upon an unexpected distribution when I had my
students measure the corridor outside of my room. The
surface is a pebble-and-epoxy floor made of large tiles with
regularly spaced expansion joints. I decided that my
students should decide how precisely they could measure the
tiles with a meter stick, and see how the uncertainty
propagated when they used their average measurement for one
tile to estimate the length, width, and area of the hallway.

I measured one tile to get a sense of what kinds of numbers I
should expect them to get. It was very close to exactly 75
cm, so I decided that each tile was probably manufactured to
be exactly 75 cm (or as nearly exact as manufacturing
tolerances allow), and used that number for my calculations.

When my students obtained their data by measuring multiple
tiles, it turned out that the dimensions of the tiles varied
by about +/- 2 cm.
Evidently, the expansion joints were measured and hand cut
after the floor was laid, and I happened to choose one that
was exactly a convenient round number.

Because it turned out that we had a distribution of tiles
centered around 75.5 cm, the length of the corridor outside
my room as measured with the tiles came out to approximately
36 m +/- 1 m.