## mariomintchev 2 years ago Can someone explain the STANDARD DEVIATION OF THE DIFFERENCES formula to me?

1. mariomintchev

@jagatuba @Jemurray3 @jhonyy9 @karatechopper @phi @robtobey @satellite73

2. mariomintchev
3. mariomintchev

I am given 2 sets of 10 numbers. How do I plug them into this formula to get the correct answer?

4. jhonyy9

so what numbers you have got for ????

5. mariomintchev

6. jhonyy9

the sum across the values how you can get it ?

7. mariomintchev

im confused about the top part of the formula. ive been getting the wrong answers so i must be doing something wrong.

8. jhonyy9

sorry but this attachement i not can opening ,not id compatibly with my excell programm

9. mariomintchev

10. mariomintchev

11. jhonyy9

can you rewrite me here the values for x and X the mean or average and the value for n ?

12. jhonyy9

@robtobey can you opening this attachement please ?

13. mariomintchev

(column 1) x = 129, 237, 110, 32, 322, 134, 119, 32, 172, 149 total sum for x = 1436 mean for x = 143.6 (column 2) y = 130, 224, 113, 38, 320, 124, 122, 33, 169, 148 total sum for y = 1421 mean for y = 142.1 value for n = 10 df = n-1 (so 9)

14. jhonyy9

have you any idea for this ?

15. mariomintchev

yes but ive been getting the wrong answers. d bar = 1.5 so ive been doing ((129-1.5)^2+(237-1.5)^2+(110-1.5)^2+.......))

16. jhonyy9

so how i see it there will be in numerator sqrt(sum(x-X)^2) divide by sqrt9 so divide by 3 ok ?

17. jhonyy9

can you calcule first this sum ?

18. jhonyy9

how many will be this sum ?

19. mariomintchev

i got 270258.5 for x and 261262.5 for y then i subtracted the 2 to get 8996

20. mariomintchev

then divide by 9 and square root it to get 31.616

21. mariomintchev

but thats wrong

22. jhonyy9

why divide by 9 and not by 3 ?

23. mariomintchev

arent u supposed to do whats in the radical first and then square root it?

24. jhonyy9

why you can separte it sqrt of numerator and sqrt of denominator and vide it after

25. jhonyy9

and divide it after

26. mariomintchev

i get the same answer either way

27. mariomintchev

31.616

28. mariomintchev

29. robtobey

@jhonyy9 It may to late for solving this problem. Attached is a screen capture of part of the xls file. I am running on a mid 2010 IMac with 8Gb of ram to support virtual versions of Windows XP pro, Windows 98SE and in the past Ubuntu Linux. The xls file is not a normal spread sheet data file. All I can provide is a screen capture of what I see.

30. jhonyy9

how can i opening it what program i need using for ?

31. mariomintchev

i provided all the necessary info above ^^^^^

32. jhonyy9

so than you need calcule right this sum and after make the squareroot of them ,yes ?

33. jhonyy9

so and after just divide it by 3

34. robtobey

Ask @mariomintchev what version of Excel did she/he use to generate the spread sheet.

35. mariomintchev

Excel 2010

36. mariomintchev

SCREENSHOT

37. phi

They want the standard deviation of the differences. Your formula tells you how to find the standard deviation of a list of numbers. The first step is find the list of numbers: the list of differences so find x-y where x= 129, 237, 110, 32, 322, 134, 119, 32, 172, 149 y = 130, 224, 113, 38, 320, 124, 122, 33, 169, 148 the differences (call it z) are z= -1 13 -3 -6 2 10 -3 -1 3 1 now use your formula to find the standard deviation

38. mariomintchev

so ((-1-1.5)^2+(13-1.5)^2+(-3-1.5)^2+....)) ???

39. phi

yes, then divide by 9 then take the square root.

40. mariomintchev

ok, i got 5.95 so its close enough. thanks phi, i can always rely on you!

41. phi

I got 5.930 (dd-1.5).^2 6.2500 132.2500 20.2500 56.2500 0.2500 72.2500 20.2500 6.2500 2.2500 0.2500 sum(ans)/9 = 35.1667 sqrt(35.1667)= 5.9302

42. mariomintchev

i must have made a little mistake somewhere when punching in all of those numbers and parenthesis. i understand what i need to do now though. thanks! =o)