Statistical formulas documentation

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mTAB Median Calculation from Income Brackets

Approximate Annual HH Income
Median 71,180
Unweighted Sample Total Count 10,811


Approximate Annual HH Income Weighted
Response (1) Accumulated Response
Less than $15,000 || width="150"|11,714 || 11,714 |- | $15,000 - $24,999 || 46,054 || 57, 768 |- | $25,000 - $34,999 || 83,965 || 141,733 |- | $35,000 - $44,999 || 102,093 || 243,826 |- | $45,000 - $59,999 || 155,721 || '''399,546''' |- | $60,000 - $74,999 || 161,435 || '''560,981''' <--''Median will fall here'' (3) |- | $75,000 - $99,999 || 193,540 || 754,521 |- | $100,000 - $124,999 || 134,706 || 889,227 |- | $125,000 - $149,999 || 59,748 || 948,975 |- | $150,000 - $199,999 || 41,971 || 990,946 |- | $200,000 - $249,999 || 16,391 || 1,007,337 |- | $250,000 or More 32,409 1,039,746
Weighted Subset Total Count 1,039,746
Weighted Sample Total Count 1,255,411


(1) Calculated Accumulated Weighted Response
(2) Divide total (1,039,746) by 2=519,873 519,873
(3) Find first value in Accumulated Response column that is greater than step 2 value
The median will fall between the $60,000-$74,999 bracket
(4) Step 2 amount (519,873) MINUS preceding break accumulated response 399,546 = 120,327
(5) Acc. Response where Meidan will fall 560,981 MINUS preceding break 399,546 = 161,435
(6) Step 4 Divided by Step 5 0.74536
(7) Multiply Step 6 by the range 14,999 ($60,000-$75,999) 11180
(8) Add Step 7 to bottom of range $60,000 || '''71,180''' |} '''mTAB Mean/Weighted Average Calculation from Income Brackets {|width=275px |- |Approximate Annual HH Income |- | Mean/Weighted Average || 83,610 |- | Unweighted Sample Total Count || 10,811 |} {| |- | || (A) || || || (B) || (C) |- | width="225"|Approximate Annual HH Income || || STAT1 || STAT2 || Midpoint |- | Less than $15,000 11,714 1 14,999 7,500 87,857,249
$15,000 - $24,999 46,054 15,000 24,999 20,000 921,059,004
$25,000 - $34,999 83,965 25,000 34,999 30,000 2,518,899,346
$35,000 - $44,999 102,093 35,000 44,999 40,000 4,083,654,266
$45,000 - $59,999 155,721 45,000 59,999 52,500 8,175,254,132
$60,000 - $74,999 161,435 60,000 74,999 67,500 10,896,752,251
$75,000 - $99,999 193,540 75,000 99,999 87,500 16,934,669,636
$100,000 - $124,999 134,706 100,000 124,999 112,500 15,154,345,342
$125,000 - $149,999 59,748 125,000 149,999 137,500 8,125,258,359
$150,000 - $199,999 41,971 150,000 199,999 175,000 7,344,910,850
$200,000 - $249,999 16,391 200,000 249,000 225,000 3,688,025,252
$250,000 or More 32,409 250,000 300,000 275,000 8,912,452,979
Weighted Subset Total Count 1,039,746 86,933,138,666
Weighted Sample Total Count 1,255,411


(1) Find Midpoint of data ranges - Column (B)
(2) Multiply Weighted Counts (A) by Midpoints (B) to generate (C)
(3) Divide the sum of column (C) by the total weighted response at the bottom of column (A)...
86,933,138,666 divided by 1,039,746 = 83,610
You will notice the calculated average matches the mTAB produced average


Standard Deviation Calculation

For categorized questions, each response is assigned 1 or 2 stat weights. If a single weight is assigned, then this is the value used to calculate the standard deviation. If 2 weights are provided, the midpoint is used.


D = Question Mean - Stat Value as described above

SS = Sum of Squares, D*D*Weighted Response Count, for all table responses

Sample = Sum of all Weighted Response Counts for all table responses


Standard Deviation = SQRT(SS/Sample-1));


The calculation is the same for continuous variables except the actual data values are used instead of stat weight.


D = Question Mean - Response Value

SS = Sum of Squares, D*D*Respondent Weight Count for each response

Sample = Sum of all Respondent Weights for each response


Standard Deviation = SQRT(SS/Sample-1));


Comparison of two population means using T-Statistic

When the two populations have equal variances

\(t = \frac{m_1-m_2}{/sqrt{(/frac{(n_1-1)(s_1)^2+(n_2-1)(s_2)^2}{n_1+n_2-2)})(\frac{1}{n_1}+\frac{1}{n_2})}}\)