Using normrnd for generating natural values (without decimal values)

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Sahar khalili 2022-6-14

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https://ww2.mathworks.cn/matlabcentral/answers/1740260-using-normrnd-for-generating-natural-values-without-decimal-values

编辑： Walter Roberson 2022-6-15

I would like to generate data with average of 27 and standard deviation of 1.41, but I would like the data have no decimal values, ex to be like this 12, 24, 27 ... . Could you please help me how I can do so?

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Walter Roberson 2022-6-14

You cannot use the fact that the sum of identically distributed uniform distribution approximates normal. You can generate 54 binary values and sum them. That will have a mean of 27, but the std will be 3.67.

Sahar khalili 2022-6-14

As I mentioned I do not care about the distributation of data, I need 6 numbers that just have the mean of 27 and std deviation of 1.41, and what matters for me is just data has no decimal values.

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Answer 1

Sam Chak 2022-6-14

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https://ww2.mathworks.cn/matlabcentral/answers/1740260-using-normrnd-for-generating-natural-values-without-decimal-values#answer_985615

编辑：Sam Chak 2022-6-14

在 MATLAB Online 中打开

@Sahar khalili

How about this data set {25, 26, 27, 27, 28, 29}?

A = [25, 26, 27, 27, 28, 29]
A = 1×6
    25    26    27    27    28    29
M = mean(A)
M = 27
S = std(A)
S = 1.4142

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Walter Roberson 2022-6-14

编辑：Walter Roberson 2022-6-15

std is sqrt(2). std() involves sqrt(N-1). N=6 so that is division by sqrt(5). In order for that to give sqrt(2) it follows that the numerator must be sqrt(10) and so the sum of exactly 6 squares of values minus mean must be 10.

Can any of the differences from the mean be 4 or more? No that would give a term of 4*4 which exceeds 10.

Can any of the differences be 3? 3*3 = 9 which is less than 10, but that requires that the total sum of squares for the other 5 terms is 1. But you cannot balance a difference of 3 with a single difference of 1 to get the right mean.

Can any of the differences be 2? Yes, make one be -2 and another be +2 and one be -1 and another be +1 and you get a mean balance and sum of squares 2*2 + 2*2 + 1*1 + 1*1 = 10, which works out.

Can exactly one be difference of 2? That contributes 4 to the sum of squares, leaving 6 to be distributed among the remaining 5 values. But under hypothesis none are 2 (squared) and so you have a pigeon-hole problem of distribution of 6 in 5 values that are 0 or 1, and that cannot fit.

Can all the differences be 0 or 1? No, sum of squares for 6 values like that cannot reach 10.

So we conclude that Sam's arrangement (possibly permuted) is the only solution.

Walter Roberson 2022-6-14

在 MATLAB Online 中打开

[x1, x2, x3, x4, x5, x6] = ndgrid(27-4:27+4);
meanmask = (x1 + x2 + x3 + x4 + x5 + x6)/6 == 27;
sx1 = x1(meanmask); sx2 = x2(meanmask); sx3 = x3(meanmask);
sx4 = x4(meanmask); sx5 = x5(meanmask); sx6 = x6(meanmask);
sx = [sx1, sx2, sx3, sx4, sx5, sx6];
st = std(sx, [], 2);
inrange = st >= 1.405 & st < 1.415;
values_that_work = sx(inrange,:);
whos values_that_work
  Name                    Size            Bytes  Class     Attributes

  values_that_work      360x6             17280  double              
unique(sort(values_that_work, 2), 'rows')
ans = 1×6
    25    26    27    27    28    29