Practical Skills II: Analysis

• Calculations using experimental data — how to process raw readings (means, derived quantities like area, logs) before you can even plot anything.
• Plotting graphs — choosing scales, labelling axes correctly, drawing lines/curves of best fit, and reading off the y-intercept.
• Using units correctly — the 6 SI base units and how derived units (N, J, Pa) are built from them.
• Trends & patterns — recognising linear, directly proportional, inversely proportional, and rate-of-change relationships.
• Interpreting graphs — turning proportional relationships into equations, spotting inverse-square laws, and calculating gradients properly.
• Precision & accuracy — knowing the difference (this trips up almost everyone) plus sensitivity vs resolution.
• Reducing errors — parallax error, fiducial markers, zero errors, and how to fix each.
• Suggesting improvements — data loggers, cameras, computer modelling, and making a method reproducible.
• Uncertainties — absolute, fractional, percentage; how to find them for readings, measurements, repeated data, and digital displays.
• Combining uncertainties — the ADD rule for +/− and the ADD-percentages rule for ×/÷, plus the MULTIPLY rule for powers.

Collecting data is only half the practical. The moment you've got your raw readings, you need to process them — and crucially, you need to have planned your table so there's room to do this.

When you repeat a reading (say, three current readings for the same length of wire), you don't just pick one — you average them. But there's a subtlety examiners love to test:

• Ignore any anomalous readings when calculating the mean (a reading that's obviously way off due to a mistake, not natural variation).
• The mean must be given to the same number of significant figures as the readings that made it. If your readings were all to 3 s.f. (like 1.34, 1.35), your mean must also be 3 s.f. — don't suddenly give 5 decimal places just because your calculator did.

Sometimes the quantity you actually need for your graph isn't something you can point a ruler or ammeter at. You measure something else, then use an equation to get there. Two classic examples:

• Area of a wire: you can't measure area directly, so you measure the radius r with a micrometer, then calculate Area = πr² for each value of r.
• Extension in Hooke's Law: you can't measure "extension" with a ruler in one go — you measure the initial length and final length separately, then calculate Extension = Final length − Initial length.
• Logarithms: sometimes you need ln(value) or log(value) for a straight-line graph (see Topic 4). This always needs its own column in the table — one column for the raw reading, the next column for its log.