| Exam Board | OCR |
|---|---|
| Module | S2 (Statistics 2) |
| Year | 2010 |
| Session | January |
| Marks | 3 |
| Paper | Download PDF ↗ |
| Mark scheme | Download PDF ↗ |
| Topic | Modelling and Hypothesis Testing |
| Type | Sampling methods critique |
| Difficulty | Easy -1.2 This is a straightforward sampling methodology question requiring students to identify that the method gives unequal selection probabilities (students 0-199 are twice as likely to be selected) and suggest using rejection/re-sampling. It tests basic understanding of random sampling principles with minimal calculation or complex reasoning. |
| Spec | 2.01c Sampling techniques: simple random, opportunity, etc2.01d Select/critique sampling: in context |
A college has 400 students. A journalist wants to carry out a survey about food preferences and she obtains a sample of 30 pupils from the college by the following method.
• Obtain a list of all the students.
• Number the students, with numbers running sequentially from 0 to 399.
• Select 30 random integers in the range 000 to 999 inclusive. If a random integer is in the range 0 to 399, then the student with that number is selected. If the number is greater than 399, then 400 is subtracted from the number (if necessary more than once) until an answer in the range 0 to 399 is selected, and the student with that number is selected.
\begin{enumerate}[label=(\roman*)]
\item Explain why this method is unsatisfactory. [2]
\item Explain how it could be improved. [1]
\end{enumerate}
\hfill \mbox{\textit{OCR S2 2010 Q2 [3]}}