Today, during B Lunch, we had a help session on Standard NMHT.4.a, which is, “I am able to differentiate between accuracy and precision in data sets.” Whitney, Alexis and Alex showed up for the session.

I started out by showing two targets. The first target had shots from two accurate rifles:

accurate rifles

Accuracy means “centered on the target” for a rifle, so both these rifles are accurate, even though 4 of the 5 circled bullet holes are barely on the target. The second had shots from two precise rifles:

precise rifles

The rifle that is off to the upper right is precise, because precise for a rifle means “close shot grouping.”

Then we talked about what it means for measurements to be precise and accurate. Precise measurements agree closely with each other. In other words, the range of values in a group of measurements is small. Accurate measurements agree with a true or accepted value. In other words, the average of a group of measurements that is accurate agrees with a true or accepted value for the quantity that is being measured.

Next, we looked at 4 sets of seven measurements and worked out a description for each, in terms of accuracy and precision:

accurate and precise

These measurements are accurate, because the average of their values is the same as the true value of 10. They are also precise, because the range, 0.2 is small, compared to the average (about 2%).

accurate, not precise

These measurements are accurate, because their average agrees with the true value, but they are not precise, because the range, 8, is large, compared to the average (about 80%).

precise, not accurate

These measurements are not accurate, because the average does not agree with the true value, but they are precise, because the range is small, compared with the average.

not accurate, not precise

These measurements are neither accurate nor precise, because the range is large and the average does not agree with the true value.

After this, the students were invited to take a short assessment to demonstrate proficiency on Standard NMHT.4.a

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