Collecting Data as Evidence

By the end of this lesson, you will be able to collect quantitative data using SI units and qualitative data to serve as evidence in an investigation.

Scientists gather two kinds of information. Quantitative data is information you measure with numbers, like 25 degrees Celsius or 150 grams. Qualitative data is information you describe with your senses, like "the water turned cloudy" or "the soil smelled like rotten eggs." Good investigations usually need both, because numbers tell you how much and descriptions tell you what happened.

When you record numbers, you use the International System of Units, or SI, which is the worldwide set of measurement units that lets scientists everywhere understand each other. You measure length in meters, mass in grams or kilograms, volume in liters or milliliters, temperature in degrees Celsius, and time in seconds. Using the same units means a student in Amarillo and a student in Brownsville can compare results without confusion.

Imagine your class studies rainfall after a Texas thunderstorm. You set out a rain gauge and find that 38 millimeters of rain fell in 45 minutes. That is quantitative data. You also write that the sky looked green before the storm, the rain came down in heavy sheets, and the schoolyard grass was flattened. That is qualitative data. Together, these observations become evidence, which is data you use to support an explanation or answer a question. Neither type alone tells the whole story.

Collect data carefully. Read the measuring tool at eye level, record the unit every single time, and write descriptions of what you actually observe instead of what you expect or guess. Never change a measurement to make your results look neater, because honest data is what makes science trustworthy.

Try it: If you tested how fast ice melts and wrote down "melted completely in 240 seconds" and "the leftover water looked murky," which piece is quantitative and which is qualitative?