Observations, measurements and variables

Learning objectives

—Precise observations and measurementsMake observations and measurements with appropriate precision, record them methodically and present them appropriately.—Independent, dependent and control variablesIdentify independent, dependent and control variables.

A useful experimental record states what was directly observed or measured, uses precision supported by the instrument, and presents values so their meaning and units are unambiguous.

Evidence type	Record	Avoid
qualitative observation	exact colour, precipitate, effervescence, state or other visible change	explaining the cause in place of the observation
scale reading	read at eye level, identify the scale interval, and record to the required supported precision	adding unsupported decimal places
change in a quantity	final reading minus initial reading, with a sign or stated rise/fall where needed	reversing readings or dropping the unit
repeated or ordered data	use a table with one row per condition or trial	prose that hides which value belongs to which condition

Condition	Initial temperature / $^{\circ}\mathrm{C}$	Highest temperature / $^{\circ}\mathrm{C}$	Temperature rise / $^{\circ}\mathrm{C}$
trial 1	16.0	32.4	16.4

Put the unit in the heading rather than repeating it in every data cell. Use a consistent number of decimal places for measurements made with the same instrument; calculate a difference from unrounded recorded readings and report it at matching precision.

Precision is the fineness and consistency of recording, not a guarantee that a value is close to the true value. Do not write more digits than the scale or stated requirement supports.

In a fair investigation, the independent variable is deliberately changed, the dependent variable is measured or observed, and control variables are kept constant so they do not provide alternative causes for the result.

Variable type	Diagnostic question	Example when comparing reactions of carboxylic acids with magnesium
independent	what does the investigator deliberately change?	identity of the carboxylic acid
dependent	what result is measured or observed?	reaction time, gas volume in a fixed time, or another stated rate measure
control	what else could affect that result and must stay constant?	acid concentration and volume; magnesium mass and surface area; temperature

First write the investigation as “effect of ___ on ___”: the first blank identifies the independent variable and the second the dependent variable. Then select control variables by asking which other factors could change the dependent result.

Name a measurable quantity and how it is held constant—for example, “use $25.0\,\mathrm{cm^3}$ of acid each time”—rather than writing only “same acid”. Keep the apparatus arrangement consistent when it could affect the measurement.

A control variable is not the same as a control experiment. Do not call every unchanged feature a control variable: include conditions that could materially affect the dependent result.

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