How should I solve this question in 9709/71

May/June 2015 Paper-71
Question 2
Sami claims that he can read minds. He asks each of 50 people to choose one of the 5 letters A, B, C, D or E. He then tells each person which letter he believes they have chosen. He gets 13 correct. Sami says “This shows that I can read minds, because 13 is more than I would have got right if I were just guessing.”
(i) State null and alternative hypotheses for a test of Sami’s claim. [1]
(ii) Test at the 10% signiﬁcance level whether Sami’s claim is justiﬁed. [5]

Reply 1

Gregorius

14

Original post by Abinaya P L

May/June 2015 Paper-71
Question 2
Sami claims that he can read minds. He asks each of 50 people to choose one of the 5 letters A, B, C, D or E. He then tells each person which letter he believes they have chosen. He gets 13 correct. Sami says “This shows that I can read minds, because 13 is more than I would have got right if I were just guessing.”
(i) State null and alternative hypotheses for a test of Sami’s claim. [1]
(ii) Test at the 10% signiﬁcance level whether Sami’s claim is justiﬁed. [5]

So, first of all, tell us what you have tried; the style in this forum is to help people along from where they've got stuck after they've had a go!

If you are completely stuck right at the beginning, think about what the probability is for Sami making a correct guess for any particular individual, and therefore write down the probability of him making an incorrect guess for that individual. What sort of trial is this usually called? Now string fifty of these guesses together; what is the probability distribution that is involved?

Reply 2

Abinaya P L

OP

Original post by Gregorius

So, first of all, tell us what you have tried; the style in this forum is to help people along from where they've got stuck after they've had a go!

If you are completely stuck right at the beginning, think about what the probability is for Sami making a correct guess for any particular individual, and therefore write down the probability of him making an incorrect guess for that individual. What sort of trial is this usually called? Now string fifty of these guesses together; what is the probability distribution that is involved?