Formula
1. Combined Mean
`bar x_12 = ( N_1 * bar (x)_1 + N_2 * bar (x)_2 )/( N_1 + N_2 )`
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2. Combined Standard deviation (Method-1)
`sigma_(12) = sqrt(( N_1 * (sigma_1^2 + d_1^2) + N_2 * (sigma_2^2 + d_2^2) )/( N_1 + N_2 ))`
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2. Combined Standard deviation (Method-2)
`sigma_(12) = sqrt(((N_1 - 1) * sigma_1^2 + (N_2 - 1) * sigma_2^2 + (N_1 * N_2)/(N_1 + N_2) * (bar x_1^2 + bar x_2^2 - 2 bar x_1 bar x_2))/(N_1 + N_2 - 1))`
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This material is intended as a summary. Use your textbook for detail explanation.
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