The most widely used method to measure the dependence on two variables, is the linear
correlation coefficient *r*. The spectra are normalized and they are considered as vectors
in *R*^{N} with *N*=128.

Let
;
*j*=1,...,128 be the normalized density value for the
stellar spectrum and
;
*j*=1,...,128 be the normalized density
value of the
class standard stellar spectrum with *k*=1,...,6.
For *k*=1,...,6 the standard stellar spectra are *OB*,...,*M* of Figs. 4-9.
The correlation coefficient for the
stellar spectrum for the
class is

(1) |

with being the mean value (over the

The correlation coefficient *r*_{ik} for the
spectrum for the class *k* was calculated with
displacement
pixels to predict a possible displacement from the detection algorithm caused by
the local background.
For these seven correlation coefficients for every class *k*, the maximum value was choosen.
The final classification was given by the maximum value of the coefficient *r*_{i} of all the *r*_{ik}coefficients as

(2) |

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