Higher Biology 2025, Question 11: experimental design, controls and graph marks

01Why the introduction matters most here

For experimental design questions, the introductory passage is doing two jobs: it lets you identify the variables so tables and graphs make sense, and it holds the details you will need for evaluation questions later.

In this question, the first paragraph gives the purpose of the investigation, the second tells you the independent variable is sumithion concentration and the dependent variable is the hatching success of fish eggs, and the third gives the specifics of the setup.

02Reading the table

The table shows both variables with units. Hatching success is a percentage, which is why the numbers exceed the 40 eggs described in the passage, and sumithion concentration starts at 0.0 mg/L.

03Part (a): variables and controls

Sumithion concentration is what is being changed, so it is the independent variable.

For the control variable, you are asked for one other than the two already mentioned. Volume of water, number of eggs and the time the eggs are left have all appeared already, which leaves temperature, pH, salinity of the water, or species of fish.

Identifying the control group means taking the value from the table: 0.0. Zero is accepted, but quoting directly from the table is better practice. Then the describe part, note that this is in bold for a reason, needs an answer specific to this experiment: to show that it was sumithion that caused the decrease in hatching success.

04Part (b): two marks for the graph

Mark one is axes and scale. Sumithion concentration is independent, so it goes on the x-axis; hatching success is dependent, so it goes on the y-axis. Label both with the column headings and units. Your graph must fill at least half the grid, and you must not skip numbers on your axes.

Mark two is for points plotted correctly and joined. Small crosses are easier to place accurately than dots. Check every point against the table.

Your paper is scanned. Draw in sharp pencil if you like, then go over everything in pen.

05Part (c): the trend most people got wrong

Fewer than 25% of candidates answered this correctly. The tempting answer is "as sumithion concentration increased, hatching success decreased", but that ignores the plateau from 0.8 to 1.0 mg/L, so it earns nothing.

The correct answer is that as sumithion concentration increased, hatching success decreased and then remained level. You do not need to quote values.

06Part (d): read the whole question

This part is straight course-specification recall with several acceptable answers, but read the entire question so you do not give the one answer you have been told not to give.