Historical data shows that the number of goals scored per match at European Football Championships has a mean of 1.93 and a variance of 1.4. A large random sample of n matches is taken. Find the least value of n such that the probability that the average number of goals scored per match exceeds 2 goals is less than 0.24.

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Worked solution

6

1. State the approximate distribution of the sample mean using the Central Limit Theorem.

Let XX be the number of goals scored per match.
We are given E(X)=1.93\mathrm{E}(X) = 1.93 and Var(X)=1.4\mathrm{Var}(X) = 1.4.
Let Xˉ\bar{X} be the sample mean number of goals in a sample of size nn.
Since nn is large, by the Central Limit Theorem, Xˉ\bar{X} is approximately normally distributed:
Xˉ∼N(1.93,1.4n)\bar{X} \sim \mathrm{N}\left(1.93, \frac{1.4}{n}\right)

2. Formulate the probability inequality given in the question.

The probability that the average number of goals exceeds 2 is less than 0.24:
P(Xˉ>2)<0.24\mathrm{P}(\bar{X} > 2) < 0.24

3. Standardise the normal variable and equate to the inverse normal value.

Standardising Xˉ\bar{X} to the standard normal variable ZZ:
P(Z>2−1.931.4n)<0.24\mathrm{P}\left(Z > \frac{2 - 1.93}{\sqrt{\frac{1.4}{n}}}\right) < 0.24
P(Z>0.07n1.4)<0.24\mathrm{P}\left(Z > \frac{0.07\sqrt{n}}{\sqrt{1.4}}\right) < 0.24
From standard normal tables or a calculator, the critical value zz where P(Z>z)=0.24\mathrm{P}(Z > z) = 0.24 is z≈0.7063z \approx 0.7063.
For the upper tail probability to be strictly less than 0.240.24, the standardised value must be greater than this critical value:
0.07n1.4>0.7063\frac{0.07\sqrt{n}}{\sqrt{1.4}} > 0.7063

4. Solve the inequality for $n$.

0.07n>0.7063×1.40.07\sqrt{n} > 0.7063 \times \sqrt{1.4}
0.07n>0.83570.07\sqrt{n} > 0.8357
n>11.938\sqrt{n} > 11.938
n>142.53n > 142.53

5. State the final integer value for $n$.

Since the sample size nn must be an integer, the least value of nn is 143143.

Answer

143143

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