# How to Calculate Standard D

#### Kevin Neufeld

##### Board Regular
I have a sample table below. It represents model sizes of products, that are measured in Feet. the sales quantity is the number of each model sized sold. I am wanting to know the Avg Size sold. So I used a weighted average. I also wanted to know the standard deviation of size sold. I have shown below in green, blue and red the end results of the 3 metrics I wanted. Below each I have the steps / formulas used to obtain each. Just wanted to confirm with this group I did this correct.

 Model Size (Feet) Sales Quantity 12 1 15 15 18 40 21 139 24 237 27 179 30 569 33 119 36 1057 39 4 42 764 48 1073 54 98 60 261 66 0 72 65 75 14 78 52 90 45 105 82 135 6 156 0 Total 4820 Formula used for Total =SUM(B2:B23) Avg Size (wgt'd) 41.8 Formula used for - Avg Size (wgt'd) =SUMPRODUCT(\$A\$2:\$A\$23,B2:B23)/SUM(B2:B23) Standard Deviation 15.1 Steps used for Standard Deviation Step 1 =B2*A2+A3*B3+A4*B4+A5*B5+A6*B6+A7*B7+A8*B8+A9*B9+A10*B10+A11*B11+A12*B12+A13*B13+A14*B14+A15*B15+A16*B16+A17*B17+A18*B18+A19*B19+A20*B20+A21*B21+A22*B22+A23*B23 Step 2 =(B2*A2)*A2+(B3*A3)*A3+(B4*A4)*A4+(B5*A5)*A5+(B6*A6)*A6+(B7*A7)*A7+(B8*A8)*A8+(B9*A9)*A9+(B10*A10)*A10+(B11*A11)*A11+(B12*A12)*A12+(B13*A13)*A13+(B14*A14)*A14+(B15*A15)*A15+(B16*A16)*A16+(B17*A17)*A17+(B18*A18)*A18+(B19*A19)*A19 Step 3 =B30/B25 Step 4 =((B25*B31)-(B30^2))/((B25*(B25-1))) Step 5 =SQRT(B33)

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Hello,

I did not check your formulas but I think that your results are correct.
I calculated:
Weighted Average: 41.7846473
Weighted STDEV: 15.12314028
Weighted Variance: 228.709372

Regards,
Bernd

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