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A-LevelMathematicsProbability and statistics9709/61 · May/June 2024 · Q711 Marks

Every July, as part of a research project, Rita collects data about sightings of a particular kind of bird. Each day in July she notes whether she sees this kind of bird or not, and she records the number X of days on which she sees it. She models the distribution of X by B(31, p), where p is the probability of seeing this kind of bird on a randomly chosen day in July. Data from previous years suggests that p = 0.3, but in 2022 Rita suspected that the value of p had been reduced. She decided to carry out a hypothesis test. In July 2022, she saw this kind of bird on 4 days.

  1. (a)Use the binomial distribution to test at the 5% significance level whether Rita's suspicion is justified.[5]
  2. (b)In July 2023, she noted the value of X and carried out another test at the 5% significance level using the same hypotheses. Calculate the probability of a Type I error.[2]
  3. (c)Rita models the number of sightings, Y, per year of a different, very rare, kind of bird by the distribution B(365, 0.01).
  4. (c)(i)Use a suitable approximating distribution to find P(Y = 4).[3]
  5. (c)(ii)Justify your approximating distribution in this context.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.

  1. [B1] H₀: p = 0.3 H₁: p < 0.3
  2. [M1] B(31, 0.3), P(X < 4) = 0.7^31 + 31×0.7^30×0.3 + 31C2×0.7^29×0.3^2 + 31C3×0.7^28×0.3^3 + 31C4×0.7^27×0.3^4 = 0.00001577 + 0.0002096 + 0.0013475 +0.0055826 + 0.016748
  3. [A1] = 0.0239 (3sf)
  4. [M1] '0.0239' < 0.05
  5. [A1FT] [Reject H₀] 'There is sufficient evidence (at 5% level) to support Rita's suspicion', or 'There is sufficient evidence to suggest the probability of seeing this type of bird has decreased'
  6. [B1FT] P(X≤ 5) = ['0.0239' + 31C5×0.7^26×0.3^5]= 0.0627 [which is > 0.05]
  7. [B1FT] P(Type I error) = '0.0239'
  8. [B1] [λ=] 3.65
  9. [M1] e^-3.65 x 3.65^4 / 4!
  10. [A1] = 0.192 (3sf)
  11. [B1] n = 365 > 50 np = 3.65 < 5 or p = 0.01 < 0.1

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About this question

Topic
Probability and statistics, A-Level Mathematics (syllabus 9709). Structured question, 11 marks.
Source
A-Level Mathematics, May/June 2024 Paper 6 Variant 1, question 7.

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