Practical and Experimental Skills
Variables and fair tests
Every experiment has an independent variable (what you change), a dependent variable (what you measure) and controlled variables (kept the same). A fair test changes only the independent variable. A common trap is failing to identify the control variables or confusing the independent and dependent variable. Always state what you will measure and how, with units.
Reliability and accuracy
Repeat readings and calculate a mean to improve reliability. Accuracy is how close a reading is to the true value; precision is how close repeated readings are to each other. Use appropriate apparatus: a measuring cylinder for volume, a thermometer for temperature, a stopwatch for time. A frequent trap is describing a result as 'accurate' when only precision has been shown.
Recording and presenting data
Present data in tables with headings and units; draw line graphs for continuous data and bar charts for categories. Label axes, include units and use a suitable scale. When drawing conclusions, refer to the data, not opinion. Identify anomalies and explain them rather than ignoring them.
Worked example
A student tests how light affects photosynthesis by counting bubbles at different distances from a lamp. Distance is the independent variable, bubble rate the dependent variable, and temperature and carbon dioxide must be controlled. Rule of thumb: change one thing, measure one thing, keep everything else the same.
Practical trap
When using a microscope or measuring, record the magnification or units clearly and repeat for reliability.