Explain the basic concept of error analysis to your students. Error analysis is the amount of uncertainties that can be applied to a specific measurement and how varying circumstances will change a measurement depending on when and how it's taken.
Choose something to measure to demonstrate error analysis. This should be a measurement that can vary from day to day, such as a person's height.
Demonstrate the measuring of someone's height to your middle school students. Measure to the nearest foot and inch.
Show your students what happens to the height when it is measured in centimeters, then converted to feet and inches. The measurement will be more exact, thus demonstrating error analysis in terms of human error.
Ask your volunteer to stand up straighter with the chin held high. Measure height again. Once again, it will have changed. This shows your students another factor of error analysis: varying circumstances.
Give your middle school students a math assignment to measure various items, such as the length of their feet, a plant in the room or a piece of chalk. Ask them to think of possible factors that might affect the error analysis for each item.