# Hitting the Bullseye with Accuracy and Precision

Estimation & Measurement

Following up on the students’ studies of accracy and precision in measurement, we created our own sets of data and then observed the differences in accuracy and precision between the different procedures.

We practiced estimating and measuring length, volume, and weight.  We considered the length of a piece of rope, the volume of a bowl, and the weight of a bag of glass beads. We first estimated blindly, using our own knowledge of centimeters, milliliters, and grams. Then we estimated again, but this time using a reference object for comparison: a string of known length, an 8-ounce juice cup, and a reference weight. Finally, we used a ruler, a graduated beaker, and a spring scale to measure the objects. For each round the students graphed the data obtained by the whole class. To no one’s surprise, the measured data was both the most accurate and the most precise, but the calibrated estimates were pretty accurate as well, once we averaged the whole class’s data. It was interesting to see that having lots of measurements actually increases the overall accuracy of the average measurement! We also talked about the errors that may have occurred in our measurements, and what type of errors they were — random errors, systematic errors, or the occasional blunder.  Fortunately there were not too many of those — the students did good and careful work today, and came up with measurements that were both accurate and precise.  Bullseye!

Estimación & Medición

Hoy practicamos la estimación y medimos longitud, peso y volumen. Medimos el largo de una cuerda, el volumen de un tazón y el peso de una bolsa con mostacillas de vidrio. Primero estimamos de manera ciega solo usando nuestra experiencia, en centímetros, milímetro y gramos. Luego volvimos a estimar, pero esta vez usando un objeto de referencia para poder comparar: un pedazo de cuerda de tamaño conocido, una taza de 8 onzas de jugo y una peso de referencia.

Finalmente, usamos una regla, un vaso precipitado y una balanza para realizar las mediciones de cada objeto. Para cada una de las estimaciones y mediciones graficamos los datos obtenidos por toda la clase, y así pudimos observar cual set de datos fue el más preciso y acertado. Obviamente, los datos de las mediciones fueron los más acertados, pero los datos estimados usando las referencias estuvieron bastante cercanos a la realidad (una vez que promediamos los datos de toda la clase).  Fue muy interesante observar que entre más datos se tiene, más precisos son las mediciones promedio. También conversamos sobre cómo la precisión (o reproducibilidad) de una medición depende básicamente de la herramienta con que se mide; pero que también es muy importante que el científico que realiza la medición lo haga con mucho cuidado, porque puede obtener datos inexactos e imprecisos.

Author

## Dr. Catherine Sukow

Dr. Sukow's interest in science education began when she was a teenager, with an extended visit to San Francisco's Exploratorium. In college, she had summer jobs in a similar, smaller, museum. She focused her Master's research at NCSU on the structure of metal silicides on silicon, and her Ph. D. work at Brandeis on the structure of crossbridged actin bundles. While volunteering in her childrens' schools, she was reminded how much fun it is to teach science, and is happy to be teaching now with Science from Scientists. In her spare time, she also enjoys yoga, choral and solo singing, and attempting a variety of international cuisines.

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