ACT Science — procedures and measurements: instruments, units, error, trials and sample size

ACT describes the Scientific Investigation category of the Science section, 18 to 32 percent of the questions, as understanding experimental tools, procedures and design, and comparing, extending and modifying experiments, for example by predicting the results of additional trials. Questions about controls and variables are only part of it. The rest asks how a measurement was actually made: which instrument, which unit, in which order, how often, on how many subjects, and what the numbers in a procedure add up to.

This material trains that practical side with twelve short laboratory descriptions. You choose a graduated cylinder over a beaker when a volume must be precise, and a spring scale reading in newtons when a force is wanted. You decide which difference a balance that reads to 0.1 gram cannot detect. You put the steps of weighing a sample in the order that makes the tare button work for you. You compute a dissolved solid's mass by difference from three recorded masses, an average of four timed trials, and an average rate of gas production over a whole run rather than one part of it.

Several questions separate two kinds of error that students often blur. Random error scatters readings up and down and is reduced by repeated trials and averaging; systematic error shifts every reading the same way, is untouched by averaging, and is removed by calibration against standards of known value. One question shows why a constant offset in a thermometer spoils each temperature but not the change between two readings. Others ask why a study of 100 plants per group is more reliable than one of 5, and where the result of an added trial should fall when it lies inside the range already tested.

Each explanation works the reasoning or the arithmetic step by step and says exactly why each wrong option fails, so that a missed question teaches the rule behind it. The flashcards summarize the instruments and what they measure, taring, mass by difference, resolution, random and systematic error, calibration, sample size, added trials and average rates.

This material offers a quiz and flashcards. It is independent practice based on ACT's public description of the ACT Science section; all procedures and data are original, and nothing here is produced or endorsed by ACT.

• Choose the instrument and unit that fit a quantity, and judge what an instrument's resolution can detect
• Order the steps of a measurement procedure correctly, including taring and calibration
• Compute mass by difference, averages of trials and average rates from a described procedure
• Distinguish random from systematic error and know what averaging and calibration each fix
• Explain why larger samples are more reliable and predict the result of an added trial inside the tested range

Practice material written by Zestly, based on ACT's published description of the ACT Science section (reporting categories Interpretation of Data, Scientific Investigation, and Evaluating Scientific Arguments and Models with Evidence; act.org, retrieved September 2026). All data sets, figures and questions are original.