Compare survival records without confusing counts and proportions
Use denominators and keep survival, reproductive output and causal evidence separate.
Exam scope and assessment notes
Core and Extended · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall inherited features, phenotype, diffusion, photosynthesis and transpiration. Diagrams are original schematics; comparison records are invented and use only the stated assumptions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What denominator gives the survival proportion?
What you will learn
- Compare survival proportions in a stated case
- Judge what population records do and do not establish
Original schematic and invented comparison records. Features, environment, inheritance information and measurement limits are stated separately.
| Feature or record | Interpretation |
|---|---|
| Initial individuals | A 40 / B 80 |
| Survivors | A 30 / B 48 |
| Survival | A 75% / B 60% |
| Offspring | A 60 / B 96; denominator must be stated |
Compare survival proportions in a stated case
Original records track two same-species groups through the same stated observation period. Group A starts with forty individuals and thirty survive; group B starts with eighty and forty-eight survive. A raw count comparison gives B more survivors, but its starting group is twice as large. Calculate survivors divided by the initial group before comparing the fraction surviving. Otherwise a difference in starting number can be mistaken for a difference in survival proportion.
Group A has 30/40 = 0.75, or seventy-five per cent survival. Group B has 48/80 = 0.60, or sixty per cent. The difference is fifteen percentage points. Relative to B’s proportion, A’s proportion is twenty-five per cent higher: (0.75 − 0.60)/0.60 × 100. Percentage points compare percentages by subtraction, whereas relative increase uses a reference denominator. These statements describe the record, not a guaranteed survival advantage in every population.
Judge what population records do and do not establish
Survival is not the same measurement as reproduction. A later observation of the surviving groups records invented offspring totals of sixty for A and ninety-six for B. Dividing by the original groups gives 1.5 and 1.2 offspring per starting individual; dividing by survivors gives 2.0 in both groups. Both denominators answer valid but different questions. State the denominator explicitly and do not quietly switch between output per initial individual and output per survivor.
The case information may supply an inherited feature and a proposed mechanism, but the totals alone do not prove that feature caused the outcome. Record sampling, age, environment, health, other differences and repeated observations would matter in a real investigation. Nor do the counts identify an allele frequency or prove a new mutation. Use them to practise describing evidence with the correct population boundary, then identify the additional information needed for a stronger adaptive explanation.
Pause and explain
What is the difference between 75% and 60%?
Worked example
Calculate survival proportions and explain why the raw survivor counts reverse their order.
Show the worked solution
- Divide 30 by 40 and 48 by 80 to obtain 75% and 60%.
- B has more survivors because its starting group is larger; compare proportions rather than totals for this question.
Answer A has the higher observed survival proportion, 75% versus 60%, despite B’s higher raw count. The difference is 15 percentage points, with causal interpretation still limited.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find group A’s survival percentage from 30 survivors out of 40 initial individuals.
Give me a hint
Use survivors divided by the initial group.
Compare my reasoning
- 30/40 = 0.75.
- Multiply by 100 to express the proportion as a percentage.
Group A has 75% observed survival in this invented record.
Look for these in your work
- Correct denominator
- Percentage and record scope
Compare A and B as survival proportions, a percentage-point difference and a relative increase over B.
Give me a hint
B has 48 survivors out of 80; retain B as the relative reference.
Compare my reasoning
- A = 75% and B = 60%, so the difference is 15 percentage points.
- 15/60 × 100 = 25% relative increase over B.
A’s observed survival is 75% versus B’s 60%; 15 percentage points higher, or 25% higher relative to B. This does not prove the feature caused the difference.
Look for these in your work
- Point versus relative comparison
- Causal boundary
A has 60 offspring and B 96. Compare output per initial individual and per survivor using the supplied starting and survival counts.
Give me a hint
These two denominators describe different outcomes.
Compare my reasoning
- Per initial individual: 60/40 = 1.5 and 96/80 = 1.2.
- Per survivor: 60/30 = 2.0 and 96/48 = 2.0.
A and B produce 1.5 and 1.2 offspring per initial individual, but both produce 2.0 per survivor. State which denominator is being used; the record does not establish individual causal effects.
Look for these in your work
- Both denominator choices
- Correct group averages
Common mistakes
- Ranking survival from raw counts alone
- Changing the denominator without saying so
- Equating survival with reproduction or allele frequency
What can you explain now?
What is the difference between 75% and 60%?
Compare with the explanation
15 percentage points
Subtracting percentages gives a percentage-point difference.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Without your notes, compare survival proportions in a stated case and judge what population records do and do not establish. Explain the mechanism and keep the stated environment and evidence limits clear.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.