OCR Further Statistics 2023 June — Question 5 10 marks

Exam BoardOCR
ModuleFurther Statistics (Further Statistics)
Year2023
SessionJune
Marks10
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Mark schemeDownload PDF ↗
TopicWilcoxon tests
TypeWilcoxon rank-sum test (Mann-Whitney U test)
DifficultyChallenging +1.2 Part (a) is a standard application of the Wilcoxon rank-sum test with small samples requiring ranking, calculating test statistic, and comparing to critical values - routine for Further Statistics students. Part (b) adds a nice twist requiring understanding of how changing one value affects the test outcome, demanding careful analysis of rank boundaries, but the logic is straightforward once the method is understood. This is moderately above average difficulty due to the two-part structure and the conceptual element in (b), but well within reach of competent Further Maths students.
Spec5.07b Sign test: and Wilcoxon signed-rank5.07e Test medians

5 An historian has reason to believe that the average age at which men got married in the seventeenth century was higher in urban areas compared to rural areas. The historian collected data from a random sample of 8 men in an urban area and a random sample of 6 men in a rural area, all of whom were married in the seventeenth century. The results were as follows, given in the form years/months.
Urban:\(18 / 3\)\(18 / 5\)\(19 / 9\)\(20 / 7\)\(25 / 6\)\(34 / 6\)\(41 / 8\)\(46 / 3\)
Rural:\(18 / 0\)\(18 / 1\)\(18 / 4\)\(19 / 11\)\(22 / 2\)\(28 / 11\)
  1. Use an appropriate non-parametric method to test at the \(5 \%\) significance level whether the average age at marriage of men is higher in urban areas than in rural areas.
  2. When checking the data, the historian found that the age of one of the men, Mr X, which had been recorded as 28/11, had been wrongly recorded. When corrected, the result of the test in part (a) was unchanged. Determine the youngest age that Mr X could have been, given that it was not the same, in years and months, as that of any of the other men in the sample.

Question 5:
AnswerMarks Guidance
5(a) H : distributions of ages for urban and rural areas
0
are identical; H : distributions differ as to median
1
Or H : m = 0, H : m > 0, where m is the median
0 d 1 d d
of population differences of age of marriage for
men in urban and rural areas
or H : population median ages in urban & rural areas
0
are equal, H : median age higher in urban areas
1
Rankings of rural ages are 1, 2, 4, 7, 9, 11
R = 34 and m(m + n + 1) – R = 56
m m
W = 34
CV 31
34 > 31 so do not reject H , or 0.0876 > 0.05
0
AnswerMarks
Insufficient evidence that average ages are higherB1
M1
B1
A1
B1
M1ft
A1ft
AnswerMarks
[7]1.1
3.1b
1.1
1.1
1.1
1.1
AnswerMarks
2.2bIf “distributions identical”, allow any type of average.
If not “distributions identical” then must use median.
Allow use of m and m, provided they are defined as median, or entirely
u r
verbal (see below) but need some context
Find rankings of rural ages within whole sample, can be implied by 34
Consider m(m + n + 1) – R (if omitted, can get all other marks)
m
Correct W, allow p = 0.0876 from N(45, 60) with cc
Correct CV used, allow 31 or 31.76 from N(45, 60)
Comparison and correct first conclusion, needs correct method for R .
m
FT on their 34, but not 56, and on 29 instead of 31 but no other CV except
FT on wrong m or n (e.g. m = 5, CV 23 which gets B1M1B0A1B0M1A1)
Contextualised, not over-assertive, not “significant evidence that average
ages are the same”. Same ft. Allow “different”.
If from valid method their W  31, so that conclusion changes, FT for (a) (“significant evidence that average ages are different”) and consult PE if (b) is problematic
AnswerMarks Guidance
(b)1 + 2 + 4 + 7 + 8 + 10 = 32 M1
A13.1b
1.1Attempt to change total to 32 (or 30 if CV 29 used), e.g. 8 seen
Find age that makes rank sum of 32 (or 31 if 29 used – can’t get 30??)
(M1A1 can be implied by answer of 20/0, allow 19/12)
AnswerMarks Guidance
ORT&I: Test at least two new ages and find R for each
mM1
19/10 or 20/0 or 19/12 statedA1
11th becomes 7th so least age of Mr X is 19/10A1
[3]3.2a (old 11th becomes new 7th, old 7th becomes new 8th)
Condone absence of working.
AnswerMarks
AH : median in urban & rural areas are equal, H : median higher in urban areas (no context – needs “ages” as a minimum)
0 1B0
BH : average ages in urban & rural areas are equal, H : average age higher in urban areas (minimum context, but “average” not median”)
0 1B0
CH : median ages in urban & rural areas are equal, H : median age higher in urban areas (condone omission of “population”)
0 1B1
DH : median of differences in ages is 0, H : greater than 0 (BOD for their meaning “urban minus rural” rather than “rural minus urban”)
0 1B1
EH : m = m, H : m > m where m and m are the median ages of marriage for men in urban and rural districts respectively
0 u r 1 u r u rB1
Question 5:
5 | (a) | H : distributions of ages for urban and rural areas
0
are identical; H : distributions differ as to median
1
Or H : m = 0, H : m > 0, where m is the median
0 d 1 d d
of population differences of age of marriage for
men in urban and rural areas
or H : population median ages in urban & rural areas
0
are equal, H : median age higher in urban areas
1
Rankings of rural ages are 1, 2, 4, 7, 9, 11
R = 34 and m(m + n + 1) – R = 56
m m
W = 34
CV 31
34 > 31 so do not reject H , or 0.0876 > 0.05
0
Insufficient evidence that average ages are higher | B1
M1
B1
A1
B1
M1ft
A1ft
[7] | 1.1
3.1b
1.1
1.1
1.1
1.1
2.2b | If “distributions identical”, allow any type of average.
If not “distributions identical” then must use median.
Allow use of m and m, provided they are defined as median, or entirely
u r
verbal (see below) but need some context
Find rankings of rural ages within whole sample, can be implied by 34
Consider m(m + n + 1) – R (if omitted, can get all other marks)
m
Correct W, allow p = 0.0876 from N(45, 60) with cc
Correct CV used, allow 31 or 31.76 from N(45, 60)
Comparison and correct first conclusion, needs correct method for R .
m
FT on their 34, but not 56, and on 29 instead of 31 but no other CV except
FT on wrong m or n (e.g. m = 5, CV 23 which gets B1M1B0A1B0M1A1)
Contextualised, not over-assertive, not “significant evidence that average
ages are the same”. Same ft. Allow “different”.
If from valid method their W  31, so that conclusion changes, FT for (a) (“significant evidence that average ages are different”) and consult PE if (b) is problematic
(b) | 1 + 2 + 4 + 7 + 8 + 10 = 32 | M1
A1 | 3.1b
1.1 | Attempt to change total to 32 (or 30 if CV 29 used), e.g. 8 seen
Find age that makes rank sum of 32 (or 31 if 29 used – can’t get 30??)
(M1A1 can be implied by answer of 20/0, allow 19/12)
OR | T&I: Test at least two new ages and find R for each
m | M1
19/10 or 20/0 or 19/12 stated | A1
11th becomes 7th so least age of Mr X is 19/10 | A1
[3] | 3.2a | (old 11th becomes new 7th, old 7th becomes new 8th)
Condone absence of working.
A | H : median in urban & rural areas are equal, H : median higher in urban areas (no context – needs “ages” as a minimum)
0 1 | B0
B | H : average ages in urban & rural areas are equal, H : average age higher in urban areas (minimum context, but “average” not median”)
0 1 | B0
C | H : median ages in urban & rural areas are equal, H : median age higher in urban areas (condone omission of “population”)
0 1 | B1
D | H : median of differences in ages is 0, H : greater than 0 (BOD for their meaning “urban minus rural” rather than “rural minus urban”)
0 1 | B1
E | H : m = m, H : m > m where m and m are the median ages of marriage for men in urban and rural districts respectively
0 u r 1 u r u r | B1
5 An historian has reason to believe that the average age at which men got married in the seventeenth century was higher in urban areas compared to rural areas. The historian collected data from a random sample of 8 men in an urban area and a random sample of 6 men in a rural area, all of whom were married in the seventeenth century. The results were as follows, given in the form years/months.

\begin{center}
\begin{tabular}{ l l l l l l l l l }
Urban: & $18 / 3$ & $18 / 5$ & $19 / 9$ & $20 / 7$ & $25 / 6$ & $34 / 6$ & $41 / 8$ & $46 / 3$ \\
Rural: & $18 / 0$ & $18 / 1$ & $18 / 4$ & $19 / 11$ & $22 / 2$ & $28 / 11$ &  &  \\
\end{tabular}
\end{center}
\begin{enumerate}[label=(\alph*)]
\item Use an appropriate non-parametric method to test at the $5 \%$ significance level whether the average age at marriage of men is higher in urban areas than in rural areas.
\item When checking the data, the historian found that the age of one of the men, Mr X, which had been recorded as 28/11, had been wrongly recorded. When corrected, the result of the test in part (a) was unchanged.

Determine the youngest age that Mr X could have been, given that it was not the same, in years and months, as that of any of the other men in the sample.
\end{enumerate}

\hfill \mbox{\textit{OCR Further Statistics 2023 Q5 [10]}}