e. A set of observations has a mean of 9.0 and standard deviation 4.0. Another set of 20 observations has a mean of 10.0. Given the standard deviation of the combined set of 35 observations is 3.0, find the variance of the second set.

Answers

Answer 1

From the combined standard deviation, with the mean, and standard

deviation of the data sets, the required variance can be found.

[tex]\underline{\mathrm{The \ variance \ of \ the \ second \ set \ is} \ 4\dfrac{5}{28}}[/tex]

Reasons:

The given data information are;

Mean of the first set of observation, [tex]\overline x_1[/tex] = 9.0

Standard deviation of the the first set, σ₁ = 4.0

Sample size of second observation, n₂ = 20

Mean of second sample, [tex]\overline x_2[/tex] = 10

Standard deviation of the combined set, σ₁₂ = 3.0

Sample size of combined set of observation, n₂ = 35

Solution:

The number of elements in the second sample, n₁ = 35 - 20 = 15

The standard deviation of the combined set is given by the formula;

[tex]{\sigma_{12}} = \mathbf{\sqrt{\dfrac{n_1 \cdot \left ( \sigma^2_{1}-d_1^2 \right ) + n_2 \cdot \left ( \sigma^2_{2}-d_2^2 \right )}{n_{1}+n_{2}}}}[/tex]

Where;

[tex]\overline x_{12} =\mathbf{\dfrac{n_1 \cdot \overline x_1 +n_2 \cdot \overline x_2 }{n_1 + n_2}}[/tex]

Therefore;

[tex]\overline x_{12} =\dfrac{15 \times 9.0 +20 \times10.0 }{15 + 20} = \dfrac{67}{7}[/tex]

d₁ = [tex]\mathbf{\overline x_{12}}[/tex] - [tex]\mathbf{\overline x_1}[/tex]

[tex]d_1 = \dfrac{67}{7} - 9.0 = \dfrac{4}{7}[/tex]

d₂ = [tex]\mathbf{\overline x_{12}}[/tex] - [tex]\mathbf{\overline x_2}[/tex]

[tex]d_2 = \dfrac{67}{7} - 10.0 = -\dfrac{3}{7}[/tex]

Therefore;

[tex]3=\sqrt{\dfrac{15 \times \left ( 4.0^2-\left(\dfrac{4}{7}\right) ^2 \right ) + 20 \cdot \left ( \sigma^2_{2}-\left(-\dfrac{3}{7}\right) ^2 \right )}{15+20}}[/tex]

Solving gives;

[tex]Variance, \ \sigma^2_2 = \mathbf{\dfrac{{117} }{28}}[/tex]

[tex]\mathrm{The \ variance \ of \ the \ second \ set, \ \sigma_2^2} = \dfrac{{117} }{28} = 4\dfrac{5}{28}[/tex]

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