(c) Observations, measurements and variables
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
Measure, record and present observations precisely
A useful observation records what is detected; a measurement adds a numerical value and unit. Appropriate precision means using an instrument whose scale can distinguish the changes that matter and recording only the detail that instrument supports.
| Evidence move | Methodical decision |
|---|---|
| observe | describe visible changes with agreed categories, or select a measurable quantity instead of relying on vague words such as 'more' |
| measure | choose a suitable instrument, zero it where required, read its scale consistently and include the correct unit |
| record | enter each result immediately; put the changed condition in the first column and repeat readings in separate columns |
| process | keep raw readings, calculate a mean when repeats are available and mark rather than silently delete an anomalous value |
| present | use a table for exact values, a line graph for a continuous relationship, or a bar chart for separate categories |
If leaf-rise time is measured with a stopwatch reading to 0.1 s, record results such as 12.4 s rather than 12 s or 12.437 s. A results table can show light distance, three rise times and a mean; a line graph is appropriate because distance varies continuously.
Extra decimal places do not create extra precision. They are justified only by the measuring instrument and method. Also keep units in table headings or axis labels rather than mixing them unpredictably into individual data cells.
Identify variables by their role in the investigation
A variable is identified by the role it plays in a particular investigation: the independent variable is deliberately changed, the dependent variable is measured as the response, and control variables are kept constant so they do not alter that response.
| Role | Question to ask | Photosynthesis-rate example |
|---|---|---|
| independent variable | what is deliberately changed between conditions? | distance of a lamp from pondweed |
| dependent variable | what response is measured, with a method and unit? | oxygen volume produced per minute |
| control variables | what other factors could change the response and therefore must remain constant? | pondweed species and length, light colour, temperature, carbon dioxide availability and measurement time |
Read an aim in the form 'the effect of X on Y': X is usually the independent variable and Y the dependent variable. Then use biological knowledge to identify plausible alternative causes of Y and control the ones the method can reasonably hold constant. A variable may be biotic, such as species or starting mass, or abiotic, such as temperature, pH or light intensity.
A control variable is not the same as a control group. A control variable is a condition kept constant across all groups; a control group or reference condition is one comparison group. Time is the dependent variable only when time itself is measured as the response—otherwise it may be controlled.