Evaluating methods and conclusions

Sample questions

Original practice questions written in the style of the exam.

1. Question 13 marks

   A survey found a significant positive correlation between the number of fast-food outlets in a town and the number of people with heart disease. A newspaper concluded that fast food causes heart disease.

   Explain why this conclusion may not be valid.

   Mark scheme

   • A correlation does not show a causal relationship (1)
   • Other factors may be involved, e.g. larger towns have both more outlets and more people / differences in age, smoking or exercise (1)
   • These variables were not controlled in a survey, so their effect cannot be separated from the effect of fast food (1)
2. Question 24 marks

   A student investigated the effect of temperature on the respiration rate of maggots using a simple respirometer. She measured the distance the coloured liquid moved in 5 minutes.

   Temperature / °C	15	20	25	30	35
   Distance moved / mm (single reading)	6	9	14	13	22

   She concluded that respiration rate increases with temperature. Evaluate her method and conclusion, and suggest two improvements.

   Mark scheme

   • The general trend supports the conclusion, but 30 °C does not fit the pattern (possible anomaly) (1)
   • Only one reading at each temperature, so anomalies cannot be identified and the results may not be reliable (1)
   • Improvement: carry out at least three repeats at each temperature and calculate a mean (1)
   • Improvement: use a control tube with glass beads / leave to equilibrate before measuring, to allow for temperature and pressure changes (1)