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fix stupid error in comments to VARIANCE+STD

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unknown 2002-12-14 04:53:28 +04:00
parent 29624e3212
commit 19f6ad06c0
1 changed files with 12 additions and 8 deletions
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20
sql/item_sum.h
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@ -255,15 +255,15 @@ public:
variance(a) =
= sqrt ( sum (ai - avg(a))^2 / count(a) )
= sqrt ( sum (ai^2 - 2*ai*avg(a) + avg(a)^2) / count(a) )
= sqrt ( (sum(ai^2) - sum(2*ai*avg(a)) + sum(avg(a)^2))/count(a) ) =
= sqrt ( (sum(ai^2) - 2*avg(a)*sum(a) + count(a)*avg(a)^2)/count(a) ) =
= sqrt ( (sum(ai^2) - 2*sum(a)*sum(a)/count(a) + count(a)*sum(a)^2/count(a)^2 )/count(a) ) =
= sqrt ( (sum(ai^2) - 2*sum(a)^2/count(a) + sum(a)^2/count(a) )/count(a) ) =
= sqrt ( (sum(ai^2) - sum(a)^2/count(a))/count(a) )
= sum (ai - avg(a))^2 / count(a) )
= sum (ai^2 - 2*ai*avg(a) + avg(a)^2) / count(a)
= (sum(ai^2) - sum(2*ai*avg(a)) + sum(avg(a)^2))/count(a) =
= (sum(ai^2) - 2*avg(a)*sum(a) + count(a)*avg(a)^2)/count(a) =
= (sum(ai^2) - 2*sum(a)*sum(a)/count(a) + count(a)*sum(a)^2/count(a)^2 )/count(a) =
= (sum(ai^2) - 2*sum(a)^2/count(a) + sum(a)^2/count(a) )/count(a) =
= (sum(ai^2) - sum(a)^2/count(a))/count(a)
*/
*/
class Item_sum_variance : public Item_sum_num
{
@ -294,6 +294,10 @@ public:
double val();
};
/*
standard_deviation(a) = sqrt(variance(a))
*/
class Item_sum_std :public Item_sum_variance
{
public: