Finding the STDEV of a Group of Number Sets

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EDIT: To clarify, I'm specifically attempting to project the number of points that will be scored by a particular player in the NBA on any given night. So the groups below display the points scored by Player A, Player B, and Player C. Then, when all three players are on my team their combined averages are 73.25. What I'm trying to calculate is the Standard Deviation from the combined points that all three players have scored. In other words, if I put all three players on my team, and the would combime to score an average of 73.25 points, what would the STDEV on that average be based on the number sets that I have?

I'm trying to figure out what the Standard Devation of a group of multiple subsets would be once they are combined.

I have 3 sets of numbers, as follows: Group A:

9
18
27
26
Avg: 20
STDEV: 7.25

Group B:

15
27
32
18
Avg:23
STDEV: 6.82

Group C:

19
48
34
20
Avg: 30.25
STDEV: 11.84

So I have the Standard Deviation for each individual group, but if I were to combine the three groups, for example, the average would be 73.25 (the sum of all of the averages). How would I calculate the Standard Deviation for the total I would get for all of these sets combined?

If I take the STDEV of every number I get 9.91, but the average of 73.25 and STDEV of 9.91 doesn't seem right to me. Is there another way to do it?

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excelstatisticsstandard-deviation
Start by actually calculating the Average. Do you get 73.25? The largest number in your set is 48... – Tim Williams
It isn't clear what you're trying to do. 'Combine' can mean a lot of things and since you added up the averages, that doesn't look like something common, because the average of all of the numbers cannot be 73.25, so what are you trying to do exactly? – Jerry
Apologies, I should have been more specific. I'll clarify in the question now. – fullOfQuestions
Ok, I would just add all the standard deviations. – Jerry

1 Answers

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If I am understanding this correctly groups A,B and C are actually players A,B and C. Given that I would think that it would be significant to say something like player B averages 23 points per game with a standard deviation of 6.8 so that we could project he scores in the 16 to 30 point per game range.
I am further assuming that the four numbers given are for four games and that the 73.25 average is the average number of combined points scored by the three players in the four games. So my thinking is that to get the average standard deviation of the three players combined you would get the standard deviation of the game totals. This would give you a resulting standard deviation of 21 pts. meaning that you could project that the combined points of the three players would be anywhere from 52 to 94 points in a game. (Notice that the first game turns out to be an outlier game for the three players combined and for players A and B individually.)

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