| Exam Board | Edexcel |
|---|---|
| Module | S1 (Statistics 1) |
| Year | 2016 |
| Session | January |
| Marks | 9 |
| Paper | Download PDF ↗ |
| Mark scheme | Download PDF ↗ |
| Topic | Measures of Location and Spread |
| Type | Adding data values |
| Difficulty | Moderate -0.8 This is a straightforward S1 statistics question testing standard formulas and basic conceptual understanding. Part (a) requires direct application of mean and standard deviation formulas, (b) tests recall of skewness rules, while (c) and (d) require simple reasoning about how adding data affects summary statistics—all routine exercises with no problem-solving or novel insight required. |
| Spec | 2.02f Measures of average and spread2.02g Calculate mean and standard deviation2.02h Recognize outliers |
| END |
| Answer | Marks | Guidance |
|---|---|---|
| \(\mu\) or \(\bar{x} = \frac{8360}{10} = 836\) | B1 | |
| \(\sigma = \sqrt{\frac{\sum(x-\bar{x})^2}{10}} = \sqrt{6384}\) or \(4\sqrt{399} = 79.89993...\) = awrt \(79.9\) | M1, A1 (3) | M1 for \(\frac{63840}{10}\) with or without \(\sqrt{}\). NB \(\sum x^2 = 7052800\) but must see \(\sigma^2 = \frac{7052800}{10} - (836)^2\) for M1. A1 for awrt 79.9; accept \(s =\) awrt 84.2. Correct answer only M1A1 |
| Answer | Marks | Guidance |
|---|---|---|
| mean \(>\) median so positive skew | B1 | For correct comparison of mean and median (allow just \(836 > 815\)) |
| dB1 (2) | Dependent on 1st B1 for positive (skew) only. If mean \(< 815\) award B0B1 for comparison and statement of negative skew |
| Answer | Marks | Guidance |
|---|---|---|
| \(\frac{776 + 896}{2} = 836\) which is same as \(\bar{x}\), or one is 60 above \(\bar{x}\), one 60 below | B1 | For suitable calculation showing mean of two rabbits (or all 12) is same |
| So no change in the mean | dB1 (2) | Dependent on suitable calculation or reason. If they only say differences are same (but not 1 above and 1 below) and state no change then B0B1 |
| Answer | Marks | Guidance |
|---|---|---|
| \((896 - 836)^2 = (776 - 836)^2 = 60^2 = 3600 < 6384\) the average of \(\sum(x-\bar{x})^2\) | B1 | For suitable calculation showing 60 or 3600 and comparing with 79.9 or 6384 respectively |
| Or \(\sum(x-\bar{x})^2 \to 63840 + 2 \times 60^2 = 71040\) and \(\frac{71040}{12} = 5920 < \frac{63840}{10}\) | ||
| So standard deviation will reduce | dB1 (2) | Dependent on 1st B1 for stating s.d. "reduces" |
## Question 6:
### Part (a)
| $\mu$ or $\bar{x} = \frac{8360}{10} = 836$ | B1 | |
| $\sigma = \sqrt{\frac{\sum(x-\bar{x})^2}{10}} = \sqrt{6384}$ or $4\sqrt{399} = 79.89993...$ = awrt $79.9$ | M1, A1 (3) | M1 for $\frac{63840}{10}$ with or without $\sqrt{}$. NB $\sum x^2 = 7052800$ but must see $\sigma^2 = \frac{7052800}{10} - (836)^2$ for M1. A1 for awrt 79.9; accept $s =$ awrt 84.2. Correct answer only M1A1 |
### Part (b)
| mean $>$ median so positive skew | B1 | For correct comparison of mean and median (allow just $836 > 815$) |
| | dB1 (2) | Dependent on 1st B1 for positive (skew) only. If mean $< 815$ award B0B1 for comparison and statement of negative skew |
### Part (c)
| $\frac{776 + 896}{2} = 836$ which is same as $\bar{x}$, or one is 60 above $\bar{x}$, one 60 below | B1 | For suitable calculation showing mean of two rabbits (or all 12) is same |
| So no change in the mean | dB1 (2) | Dependent on suitable calculation or reason. If they only say differences are same (but not 1 above and 1 below) and state no change then B0B1 |
### Part (d)
| $(896 - 836)^2 = (776 - 836)^2 = 60^2 = 3600 < 6384$ the average of $\sum(x-\bar{x})^2$ | B1 | For suitable calculation showing 60 or 3600 and comparing with 79.9 or 6384 respectively |
| Or $\sum(x-\bar{x})^2 \to 63840 + 2 \times 60^2 = 71040$ and $\frac{71040}{12} = 5920 < \frac{63840}{10}$ | | |
| So standard deviation will reduce | dB1 (2) | Dependent on 1st B1 for stating s.d. "reduces" |
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6. Yujie is investigating the weights of 10 young rabbits. She records the weight, $x$ grams, of each rabbit and the results are summarised below.
$$\sum x = 8360 \quad \text { and } \quad \sum ( x - \bar { x } ) ^ { 2 } = 63840$$
\begin{enumerate}[label=(\alph*)]
\item Calculate the mean and the standard deviation of the weights of these rabbits.
Given that the median weight of these rabbits is 815 grams,
\item describe, giving a reason, the skewness of these data.
Two more rabbits weighing 776 grams and 896 grams are added to make a group of 12 rabbits.
\item State, giving a reason, how the inclusion of these two rabbits would affect the mean.
\item By considering the change in $\sum ( x - \bar { x } ) ^ { 2 }$, state what effect the inclusion of these two rabbits would have on the standard deviation.
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\hfill \mbox{\textit{Edexcel S1 2016 Q6 [9]}}