SCI : Measurement, Precision & Uncertainty - Case Notes

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Author - Dale Moore - Summer 1998

Background/Length I've used this case as a one-day activity. If you have other measurement activities; this could be modified to fit your activity and used as one part of a multi-period case.

Teaching Notes

I. Worksheet

Materials I hand out a worksheet with the following information on it. (Apologies to the biologists.) A PC (*.txt) version is available here.

A group of scientists was studying the effectiveness of a particular fertilizer in encouraging plant growth. They decided to measure plant mass after a chosen period of time and used two groups of plants, one given fertilizer and one that wasn't. (Otherwise, the plant groups were identical.) When the plants were harvested, they measured the masses of every plant, then reported one result for each group. Look at the possible reported values below, preparing to discuss the importance of uncertainty in measurement.

GROUP A vs. GROUP B
about 100 grams vs. about 100 grams
average 100 grams vs. average 100 grams
average 130 grams vs. average 130 grams
average 126 grams vs. average 132 grams
average 126.5 grams vs. average 131.5 grams
126.5 ± 0.4 grams vs. 131.5 ± 0.4 grams
126.5 ± 3.0 grams vs. 131.5 ± 3.0 grams
126.5 ± 2.5 grams vs. 131.5 ± 1.6 grams

Use After the students have had a chance to read the worksheet and discuss the sets of numbers, we go through each set (one at a time) in whole class discussion. I ask the students to tell me how useful the reported numbers are in comparing the two sets of plants and why. They end up defining (directly of indirectly) average, significant figures, standard deviation, etc. We address how standard deviation is calculated, and its utility in comparing two sets of data represented as two averages with standard deviations.

II. Measurement

Materials In addition to a bench-top balance (borrow from Chemistry by calling Dale Moore, or another chemist, at least one week in advance), you'll need something to measure. We've measured the mass and volume (by volume displacement using a large beaker of water) of oranges, then calculated density; we've also measured masses of shoes. I provided the oranges, bringing a couple bags; everyone measured the mass of their own shoe (including me).

Use I ask the students to report a single result; they work as a team to make measurements and calculate average and standard deviation. (Can work by groups or whole class or both.) Each student must be able to explain the calculation process, and those who have questions must first ask their classmates. This is as far as I've gotten, but it's been suggested that some sort of graphical representation might also be appropriate, especially a histogram for this type of measurement.