Planning fair comparisons and reliable measurements

What you’ll learn

• Define measurable variables and identify the experimental unit.
• Choose controls, independent repeats and specific ways to reduce uncertainty.
• Present results with appropriate axes, units and precision.

Understand the idea

A testable biological hypothesis links a named independent variable to a measurable dependent variable. State how each will be measured and over what interval. Control other factors that could plausibly change the response. For a seedling investigation, species, initial size, water availability, light and temperature may matter. A control treatment supplies a meaningful baseline; it is different from a controlled variable, which is held constant across treatments.

Independent biological replicates capture variation among organisms or independently treated containers. Measuring one seedling repeatedly improves knowledge of that seedling but does not create more independent seedlings. If a treatment is applied to a whole tray, seedlings within it share conditions; the tray may be the experimental unit. Random allocation reduces selection bias, while repeating treatments across independent trays helps distinguish treatment effects from tray effects.

Use a table with quantities and units in the headings, and record readings to the instrument's justified precision. A graph of two continuous variables normally puts the independent variable on the horizontal axis; separate treatment categories are often better shown as bars or plotted group summaries. Repeated scatter concerns precision, whereas a common calibration offset concerns accuracy. Link improvements to an identified cause, and apply the school's supervision and risk assessment to apparatus and living material.

Key formulae

$$\overline{x}=\frac{\sum x_i}{n}$$

$$u_{\Delta x}=u_{x_1}+u_{x_2}\quad\text{(maximum absolute uncertainty)}$$

$$u_{\%}=\frac{u_{\mathrm{absolute}}}{|\mathrm{measured\ value}|}\times100$$

Worked example

A class wants to test whether increasing salt concentration reduces mean root growth in seedlings over three days. The draft places all untreated seedlings in one tray and all treated seedlings in one other tray. Improve the design and state the response to measure.

1. Measure the change in root length over the same three-day interval, rather than comparing only final lengths of initially different seedlings.
2. Use several stated salt concentrations, including an untreated baseline, while controlling species, initial stage, water volume, light and temperature.
3. Randomly allocate comparable seedlings among several independently treated trays at each concentration. Summarise independent tray results so that a single unusual tray cannot be mistaken for a treatment effect.

Answer: A range of salt treatments with independent trays, random allocation and controlled conditions; measure root-length increase over a fixed interval.