php formula question

[deras]

i am trying to figure out if some code is properly generating the right standard deviation formula. the numbers look slightly too high.

here is the relevant code

sum(answercount) as sum,sum(answeranswercount) as sqrsum,sum(answercount)/sum(count) as avg

sqrt(($line['sqrsum']-($line['sum']*$line['sum']/$line['count']))/($line['count']-1)))

the formula should be

sqrt((sum of the squares of each $line - (sum of all $line(s))*(sum of all $line(s))/ number of $line(s))/number of $line(s)-1))

so does the above code fit with that standard deviation formula? any opinions are appreciated.

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thanks

[Weedpacket]

I'm not sure, but it looks like there may be a confusion between "square of the sum of all lines" and "sum of the squares of all lines".

std = sqrt(
((avg-x1)^2+(avg-x2)^2+(avg-x3)^2+...+(avg-xn)^2)/n
)

or equivalently

sqrt(
avg^2 - (2*avg*(x1+...+xn) + (x1^2+x2^2+...+xn^2))/n
)